655 lines
19 KiB
JavaScript
Executable File
655 lines
19 KiB
JavaScript
Executable File
/**
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* @fileoverview A rule to disallow unnecessary assignments`.
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* @author Yosuke Ota
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*/
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"use strict";
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const { findVariable } = require("@eslint-community/eslint-utils");
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//------------------------------------------------------------------------------
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// Types
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//------------------------------------------------------------------------------
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/** @typedef {import("estree").Node} ASTNode */
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/** @typedef {import("estree").Pattern} Pattern */
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/** @typedef {import("estree").Identifier} Identifier */
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/** @typedef {import("estree").VariableDeclarator} VariableDeclarator */
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/** @typedef {import("estree").AssignmentExpression} AssignmentExpression */
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/** @typedef {import("estree").UpdateExpression} UpdateExpression */
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/** @typedef {import("estree").Expression} Expression */
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/** @typedef {import("eslint-scope").Scope} Scope */
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/** @typedef {import("eslint-scope").Variable} Variable */
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/** @typedef {import("../linter/code-path-analysis/code-path")} CodePath */
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/** @typedef {import("../linter/code-path-analysis/code-path-segment")} CodePathSegment */
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//------------------------------------------------------------------------------
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// Helpers
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//------------------------------------------------------------------------------
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/**
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* Extract identifier from the given pattern node used on the left-hand side of the assignment.
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* @param {Pattern} pattern The pattern node to extract identifier
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* @returns {Iterable<Identifier>} The extracted identifier
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*/
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function* extractIdentifiersFromPattern(pattern) {
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switch (pattern.type) {
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case "Identifier":
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yield pattern;
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return;
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case "ObjectPattern":
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for (const property of pattern.properties) {
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yield* extractIdentifiersFromPattern(
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property.type === "Property" ? property.value : property,
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);
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}
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return;
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case "ArrayPattern":
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for (const element of pattern.elements) {
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if (!element) {
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continue;
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}
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yield* extractIdentifiersFromPattern(element);
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}
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return;
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case "RestElement":
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yield* extractIdentifiersFromPattern(pattern.argument);
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return;
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case "AssignmentPattern":
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yield* extractIdentifiersFromPattern(pattern.left);
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// no default
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}
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}
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/**
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* Checks whether the given identifier node is evaluated after the assignment identifier.
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* @param {AssignmentInfo} assignment The assignment info.
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* @param {Identifier} identifier The identifier to check.
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* @returns {boolean} `true` if the given identifier node is evaluated after the assignment identifier.
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*/
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function isIdentifierEvaluatedAfterAssignment(assignment, identifier) {
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if (identifier.range[0] < assignment.identifier.range[1]) {
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return false;
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}
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if (
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assignment.expression &&
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assignment.expression.range[0] <= identifier.range[0] &&
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identifier.range[1] <= assignment.expression.range[1]
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) {
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/*
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* The identifier node is in an expression that is evaluated before the assignment.
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* e.g. x = id;
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* ^^ identifier to check
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* ^ assignment identifier
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*/
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return false;
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}
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/*
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* e.g.
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* x = 42; id;
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* ^^ identifier to check
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* ^ assignment identifier
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* let { x, y = id } = obj;
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* ^^ identifier to check
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* ^ assignment identifier
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*/
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return true;
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}
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/**
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* Checks whether the given identifier node is used between the assigned identifier and the equal sign.
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*
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* e.g. let { x, y = x } = obj;
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* ^ identifier to check
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* ^ assigned identifier
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* @param {AssignmentInfo} assignment The assignment info.
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* @param {Identifier} identifier The identifier to check.
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* @returns {boolean} `true` if the given identifier node is used between the assigned identifier and the equal sign.
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*/
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function isIdentifierUsedBetweenAssignedAndEqualSign(assignment, identifier) {
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if (!assignment.expression) {
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return false;
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}
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return (
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assignment.identifier.range[1] <= identifier.range[0] &&
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identifier.range[1] <= assignment.expression.range[0]
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);
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}
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//------------------------------------------------------------------------------
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// Rule Definition
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//------------------------------------------------------------------------------
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/** @type {import('../types').Rule.RuleModule} */
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module.exports = {
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meta: {
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type: "problem",
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docs: {
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description:
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"Disallow variable assignments when the value is not used",
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recommended: false,
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url: "https://eslint.org/docs/latest/rules/no-useless-assignment",
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},
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schema: [],
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messages: {
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unnecessaryAssignment:
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"This assigned value is not used in subsequent statements.",
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},
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},
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create(context) {
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const sourceCode = context.sourceCode;
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/**
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* @typedef {Object} ScopeStack
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* @property {CodePath} codePath The code path of this scope stack.
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* @property {Scope} scope The scope of this scope stack.
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* @property {ScopeStack} upper The upper scope stack.
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* @property {Record<string, ScopeStackSegmentInfo>} segments The map of ScopeStackSegmentInfo.
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* @property {Set<CodePathSegment>} currentSegments The current CodePathSegments.
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* @property {Map<Variable, AssignmentInfo[]>} assignments The map of list of AssignmentInfo for each variable.
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* @property {Array} tryStatementBlocks The array of TryStatement block nodes in this scope stack.
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*/
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/**
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* @typedef {Object} ScopeStackSegmentInfo
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* @property {CodePathSegment} segment The code path segment.
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* @property {Identifier|null} first The first identifier that appears within the segment.
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* @property {Identifier|null} last The last identifier that appears within the segment.
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* `first` and `last` are used to determine whether an identifier exists within the segment position range.
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* Since it is used as a range of segments, we should originally hold all nodes, not just identifiers,
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* but since the only nodes to be judged are identifiers, it is sufficient to have a range of identifiers.
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*/
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/**
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* @typedef {Object} AssignmentInfo
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* @property {Variable} variable The variable that is assigned.
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* @property {Identifier} identifier The identifier that is assigned.
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* @property {VariableDeclarator|AssignmentExpression|UpdateExpression} node The node where the variable was updated.
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* @property {Expression|null} expression The expression that is evaluated before the assignment.
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* @property {CodePathSegment[]} segments The code path segments where the assignment was made.
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*/
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/** @type {ScopeStack} */
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let scopeStack = null;
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/** @type {Set<Scope>} */
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const codePathStartScopes = new Set();
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/**
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* Gets the scope of code path start from given scope
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* @param {Scope} scope The initial scope
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* @returns {Scope} The scope of code path start
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* @throws {Error} Unexpected error
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*/
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function getCodePathStartScope(scope) {
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let target = scope;
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while (target) {
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if (codePathStartScopes.has(target)) {
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return target;
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}
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target = target.upper;
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}
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// Should be unreachable
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return null;
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}
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/**
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* Verify the given scope stack.
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* @param {ScopeStack} target The scope stack to verify.
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* @returns {void}
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*/
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function verify(target) {
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/**
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* Checks whether the given identifier is used in the segment.
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* @param {CodePathSegment} segment The code path segment.
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* @param {Identifier} identifier The identifier to check.
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* @returns {boolean} `true` if the identifier is used in the segment.
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*/
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function isIdentifierUsedInSegment(segment, identifier) {
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const segmentInfo = target.segments[segment.id];
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return (
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segmentInfo.first &&
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segmentInfo.last &&
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segmentInfo.first.range[0] <= identifier.range[0] &&
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identifier.range[1] <= segmentInfo.last.range[1]
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);
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}
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/**
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* Verifies whether the given assignment info is an used assignment.
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* Report if it is an unused assignment.
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* @param {AssignmentInfo} targetAssignment The assignment info to verify.
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* @param {AssignmentInfo[]} allAssignments The list of all assignment info for variables.
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* @returns {void}
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*/
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function verifyAssignmentIsUsed(targetAssignment, allAssignments) {
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// Skip assignment if it is in a try block.
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const isAssignmentInTryBlock = target.tryStatementBlocks.some(
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tryBlock =>
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tryBlock.range[0] <=
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targetAssignment.identifier.range[0] &&
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targetAssignment.identifier.range[1] <=
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tryBlock.range[1],
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);
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if (isAssignmentInTryBlock) {
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return;
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}
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/**
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* @typedef {Object} SubsequentSegmentData
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* @property {CodePathSegment} segment The code path segment
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* @property {AssignmentInfo} [assignment] The first occurrence of the assignment within the segment.
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* There is no need to check if the variable is used after this assignment,
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* as the value it was assigned will be used.
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*/
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/**
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* Information used in `getSubsequentSegments()`.
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* To avoid unnecessary iterations, cache information that has already been iterated over,
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* and if additional iterations are needed, start iterating from the retained position.
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*/
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const subsequentSegmentData = {
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/**
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* Cache of subsequent segment information list that have already been iterated.
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* @type {SubsequentSegmentData[]}
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*/
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results: [],
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/**
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* Subsequent segments that have already been iterated on. Used to avoid infinite loops.
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* @type {Set<CodePathSegment>}
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*/
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subsequentSegments: new Set(),
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/**
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* Unexplored code path segment.
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* If additional iterations are needed, consume this information and iterate.
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* @type {CodePathSegment[]}
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*/
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queueSegments: targetAssignment.segments.flatMap(
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segment => segment.nextSegments,
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),
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};
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/**
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* Gets the subsequent segments from the segment of
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* the assignment currently being validated (targetAssignment).
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* @returns {Iterable<SubsequentSegmentData>} the subsequent segments
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*/
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function* getSubsequentSegments() {
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yield* subsequentSegmentData.results;
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while (subsequentSegmentData.queueSegments.length > 0) {
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const nextSegment =
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subsequentSegmentData.queueSegments.shift();
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if (
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subsequentSegmentData.subsequentSegments.has(
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nextSegment,
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)
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) {
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continue;
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}
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subsequentSegmentData.subsequentSegments.add(
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nextSegment,
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);
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const assignmentInSegment = allAssignments.find(
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otherAssignment =>
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otherAssignment.segments.includes(
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nextSegment,
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) &&
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!isIdentifierUsedBetweenAssignedAndEqualSign(
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otherAssignment,
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targetAssignment.identifier,
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),
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);
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if (!assignmentInSegment) {
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/*
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* Stores the next segment to explore.
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* If `assignmentInSegment` exists,
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* we are guarding it because we don't need to explore the next segment.
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*/
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subsequentSegmentData.queueSegments.push(
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...nextSegment.nextSegments,
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);
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}
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/** @type {SubsequentSegmentData} */
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const result = {
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segment: nextSegment,
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assignment: assignmentInSegment,
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};
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subsequentSegmentData.results.push(result);
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yield result;
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}
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}
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if (
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targetAssignment.variable.references.some(
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ref => ref.identifier.type !== "Identifier",
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)
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) {
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/**
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* Skip checking for a variable that has at least one non-identifier reference.
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* It's generated by plugins and cannot be handled reliably in the core rule.
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*/
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return;
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}
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const readReferences =
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targetAssignment.variable.references.filter(reference =>
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reference.isRead(),
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);
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if (!readReferences.length) {
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/*
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* It is not just an unnecessary assignment, but an unnecessary (unused) variable
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* and thus should not be reported by this rule because it is reported by `no-unused-vars`.
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*/
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return;
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}
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/**
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* Other assignment on the current segment and after current assignment.
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*/
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const otherAssignmentAfterTargetAssignment =
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allAssignments.find(assignment => {
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if (
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assignment === targetAssignment ||
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(assignment.segments.length &&
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assignment.segments.every(
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segment =>
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!targetAssignment.segments.includes(
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segment,
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),
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))
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) {
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return false;
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}
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if (
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isIdentifierEvaluatedAfterAssignment(
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targetAssignment,
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assignment.identifier,
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)
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) {
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return true;
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}
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if (
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assignment.expression &&
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assignment.expression.range[0] <=
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targetAssignment.identifier.range[0] &&
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targetAssignment.identifier.range[1] <=
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assignment.expression.range[1]
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) {
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/*
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* The target assignment is in an expression that is evaluated before the assignment.
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* e.g. x=(x=1);
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* ^^^ targetAssignment
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* ^^^^^^^ assignment
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*/
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return true;
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}
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return false;
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});
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for (const reference of readReferences) {
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/*
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* If the scope of the reference is outside the current code path scope,
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* we cannot track whether this assignment is not used.
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* For example, it can also be called asynchronously.
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*/
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if (
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target.scope !== getCodePathStartScope(reference.from)
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) {
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return;
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}
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// Checks if it is used in the same segment as the target assignment.
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if (
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isIdentifierEvaluatedAfterAssignment(
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targetAssignment,
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reference.identifier,
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) &&
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(isIdentifierUsedBetweenAssignedAndEqualSign(
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targetAssignment,
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reference.identifier,
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) ||
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targetAssignment.segments.some(segment =>
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isIdentifierUsedInSegment(
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segment,
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reference.identifier,
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),
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))
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) {
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if (
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otherAssignmentAfterTargetAssignment &&
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isIdentifierEvaluatedAfterAssignment(
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otherAssignmentAfterTargetAssignment,
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reference.identifier,
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)
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) {
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// There was another assignment before the reference. Therefore, it has not been used yet.
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continue;
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}
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// Uses in statements after the written identifier.
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return;
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}
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if (otherAssignmentAfterTargetAssignment) {
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/*
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* The assignment was followed by another assignment in the same segment.
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* Therefore, there is no need to check the next segment.
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*/
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continue;
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}
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// Check subsequent segments.
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for (const subsequentSegment of getSubsequentSegments()) {
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if (
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isIdentifierUsedInSegment(
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subsequentSegment.segment,
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reference.identifier,
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)
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) {
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if (
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subsequentSegment.assignment &&
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isIdentifierEvaluatedAfterAssignment(
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subsequentSegment.assignment,
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reference.identifier,
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)
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) {
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// There was another assignment before the reference. Therefore, it has not been used yet.
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continue;
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}
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// It is used
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return;
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}
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}
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}
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context.report({
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node: targetAssignment.identifier,
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messageId: "unnecessaryAssignment",
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});
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}
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// Verify that each assignment in the code path is used.
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for (const assignments of target.assignments.values()) {
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assignments.sort(
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(a, b) => a.identifier.range[0] - b.identifier.range[0],
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);
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for (const assignment of assignments) {
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verifyAssignmentIsUsed(assignment, assignments);
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}
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}
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}
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return {
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onCodePathStart(codePath, node) {
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const scope = sourceCode.getScope(node);
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scopeStack = {
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upper: scopeStack,
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codePath,
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scope,
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segments: Object.create(null),
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currentSegments: new Set(),
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assignments: new Map(),
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tryStatementBlocks: [],
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};
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codePathStartScopes.add(scopeStack.scope);
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},
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onCodePathEnd() {
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verify(scopeStack);
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scopeStack = scopeStack.upper;
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},
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onCodePathSegmentStart(segment) {
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const segmentInfo = { segment, first: null, last: null };
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scopeStack.segments[segment.id] = segmentInfo;
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scopeStack.currentSegments.add(segment);
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},
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onCodePathSegmentEnd(segment) {
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scopeStack.currentSegments.delete(segment);
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},
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TryStatement(node) {
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scopeStack.tryStatementBlocks.push(node.block);
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},
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Identifier(node) {
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for (const segment of scopeStack.currentSegments) {
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const segmentInfo = scopeStack.segments[segment.id];
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if (!segmentInfo.first) {
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segmentInfo.first = node;
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}
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segmentInfo.last = node;
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}
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},
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":matches(VariableDeclarator[init!=null], AssignmentExpression, UpdateExpression):exit"(
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node,
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) {
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if (scopeStack.currentSegments.size === 0) {
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// Ignore unreachable segments
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return;
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}
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const assignments = scopeStack.assignments;
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let pattern;
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let expression = null;
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if (node.type === "VariableDeclarator") {
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pattern = node.id;
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expression = node.init;
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} else if (node.type === "AssignmentExpression") {
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pattern = node.left;
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expression = node.right;
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} else {
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// UpdateExpression
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pattern = node.argument;
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}
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for (const identifier of extractIdentifiersFromPattern(
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pattern,
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)) {
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const scope = sourceCode.getScope(identifier);
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/** @type {Variable} */
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const variable = findVariable(scope, identifier);
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if (!variable) {
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continue;
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}
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// We don't know where global variables are used.
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if (
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variable.scope.type === "global" &&
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variable.defs.length === 0
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) {
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continue;
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}
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|
|
/*
|
|
* If the scope of the variable is outside the current code path scope,
|
|
* we cannot track whether this assignment is not used.
|
|
*/
|
|
if (
|
|
scopeStack.scope !==
|
|
getCodePathStartScope(variable.scope)
|
|
) {
|
|
continue;
|
|
}
|
|
|
|
// Variables marked by `markVariableAsUsed()` or
|
|
// exported by "exported" block comment.
|
|
if (variable.eslintUsed) {
|
|
continue;
|
|
}
|
|
|
|
// Variables exported by ESM export syntax
|
|
if (variable.scope.type === "module") {
|
|
if (
|
|
variable.defs.some(
|
|
def =>
|
|
(def.type === "Variable" &&
|
|
def.parent.parent.type ===
|
|
"ExportNamedDeclaration") ||
|
|
(def.type === "FunctionName" &&
|
|
(def.node.parent.type ===
|
|
"ExportNamedDeclaration" ||
|
|
def.node.parent.type ===
|
|
"ExportDefaultDeclaration")) ||
|
|
(def.type === "ClassName" &&
|
|
(def.node.parent.type ===
|
|
"ExportNamedDeclaration" ||
|
|
def.node.parent.type ===
|
|
"ExportDefaultDeclaration")),
|
|
)
|
|
) {
|
|
continue;
|
|
}
|
|
if (
|
|
variable.references.some(
|
|
reference =>
|
|
reference.identifier.parent.type ===
|
|
"ExportSpecifier",
|
|
)
|
|
) {
|
|
// It have `export { ... }` reference.
|
|
continue;
|
|
}
|
|
}
|
|
|
|
let list = assignments.get(variable);
|
|
|
|
if (!list) {
|
|
list = [];
|
|
assignments.set(variable, list);
|
|
}
|
|
list.push({
|
|
variable,
|
|
identifier,
|
|
node,
|
|
expression,
|
|
segments: [...scopeStack.currentSegments],
|
|
});
|
|
}
|
|
},
|
|
};
|
|
},
|
|
};
|