500 lines
20 KiB
JavaScript
500 lines
20 KiB
JavaScript
/**
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* @fileoverview Rule to flag constant comparisons and logical expressions that always/never short circuit
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* @author Jordan Eldredge <https://jordaneldredge.com>
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*/
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"use strict";
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const globals = require("globals");
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const { isNullLiteral, isConstant, isReferenceToGlobalVariable, isLogicalAssignmentOperator } = require("./utils/ast-utils");
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const NUMERIC_OR_STRING_BINARY_OPERATORS = new Set(["+", "-", "*", "/", "%", "|", "^", "&", "**", "<<", ">>", ">>>"]);
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//------------------------------------------------------------------------------
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// Helpers
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//------------------------------------------------------------------------------
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/**
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* Test if an AST node has a statically knowable constant nullishness. Meaning,
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* it will always resolve to a constant value of either: `null`, `undefined`
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* or not `null` _or_ `undefined`. An expression that can vary between those
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* three states at runtime would return `false`.
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* @param {Scope} scope The scope in which the node was found.
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* @param {ASTNode} node The AST node being tested.
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* @returns {boolean} Does `node` have constant nullishness?
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*/
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function hasConstantNullishness(scope, node) {
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switch (node.type) {
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case "ObjectExpression": // Objects are never nullish
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case "ArrayExpression": // Arrays are never nullish
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case "ArrowFunctionExpression": // Functions never nullish
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case "FunctionExpression": // Functions are never nullish
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case "ClassExpression": // Classes are never nullish
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case "NewExpression": // Objects are never nullish
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case "Literal": // Nullish, or non-nullish, literals never change
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case "TemplateLiteral": // A string is never nullish
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case "UpdateExpression": // Numbers are never nullish
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case "BinaryExpression": // Numbers, strings, or booleans are never nullish
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return true;
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case "CallExpression": {
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if (node.callee.type !== "Identifier") {
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return false;
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}
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const functionName = node.callee.name;
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return (functionName === "Boolean" || functionName === "String" || functionName === "Number") &&
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isReferenceToGlobalVariable(scope, node.callee);
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}
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case "AssignmentExpression":
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if (node.operator === "=") {
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return hasConstantNullishness(scope, node.right);
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}
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/*
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* Handling short-circuiting assignment operators would require
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* walking the scope. We won't attempt that (for now...) /
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*/
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if (isLogicalAssignmentOperator(node.operator)) {
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return false;
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}
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/*
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* The remaining assignment expressions all result in a numeric or
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* string (non-nullish) value:
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* "+=", "-=", "*=", "/=", "%=", "<<=", ">>=", ">>>=", "|=", "^=", "&="
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*/
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return true;
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case "UnaryExpression":
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/*
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* "void" Always returns `undefined`
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* "typeof" All types are strings, and thus non-nullish
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* "!" Boolean is never nullish
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* "delete" Returns a boolean, which is never nullish
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* Math operators always return numbers or strings, neither of which
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* are non-nullish "+", "-", "~"
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*/
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return true;
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case "SequenceExpression": {
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const last = node.expressions[node.expressions.length - 1];
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return hasConstantNullishness(scope, last);
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}
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case "Identifier":
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return node.name === "undefined" && isReferenceToGlobalVariable(scope, node);
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case "JSXElement": // ESLint has a policy of not assuming any specific JSX behavior.
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case "JSXFragment":
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return false;
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default:
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return false;
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}
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}
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/**
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* Test if an AST node is a boolean value that never changes. Specifically we
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* test for:
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* 1. Literal booleans (`true` or `false`)
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* 2. Unary `!` expressions with a constant value
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* 3. Constant booleans created via the `Boolean` global function
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* @param {Scope} scope The scope in which the node was found.
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* @param {ASTNode} node The node to test
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* @returns {boolean} Is `node` guaranteed to be a boolean?
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*/
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function isStaticBoolean(scope, node) {
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switch (node.type) {
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case "Literal":
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return typeof node.value === "boolean";
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case "CallExpression":
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return node.callee.type === "Identifier" && node.callee.name === "Boolean" &&
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isReferenceToGlobalVariable(scope, node.callee) &&
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(node.arguments.length === 0 || isConstant(scope, node.arguments[0], true));
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case "UnaryExpression":
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return node.operator === "!" && isConstant(scope, node.argument, true);
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default:
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return false;
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}
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}
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/**
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* Test if an AST node will always give the same result when compared to a
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* boolean value. Note that comparison to boolean values is different than
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* truthiness.
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* https://262.ecma-international.org/5.1/#sec-11.9.3
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*
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* Javascript `==` operator works by converting the boolean to `1` (true) or
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* `+0` (false) and then checks the values `==` equality to that number.
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* @param {Scope} scope The scope in which node was found.
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* @param {ASTNode} node The node to test.
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* @returns {boolean} Will `node` always coerce to the same boolean value?
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*/
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function hasConstantLooseBooleanComparison(scope, node) {
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switch (node.type) {
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case "ObjectExpression":
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case "ClassExpression":
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/**
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* In theory objects like:
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*
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* `{toString: () => a}`
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* `{valueOf: () => a}`
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*
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* Or a classes like:
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*
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* `class { static toString() { return a } }`
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* `class { static valueOf() { return a } }`
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*
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* Are not constant verifiably when `inBooleanPosition` is
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* false, but it's an edge case we've opted not to handle.
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*/
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return true;
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case "ArrayExpression": {
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const nonSpreadElements = node.elements.filter(e =>
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// Elements can be `null` in sparse arrays: `[,,]`;
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e !== null && e.type !== "SpreadElement");
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/*
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* Possible future direction if needed: We could check if the
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* single value would result in variable boolean comparison.
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* For now we will err on the side of caution since `[x]` could
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* evaluate to `[0]` or `[1]`.
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*/
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return node.elements.length === 0 || nonSpreadElements.length > 1;
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}
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case "ArrowFunctionExpression":
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case "FunctionExpression":
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return true;
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case "UnaryExpression":
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if (node.operator === "void" || // Always returns `undefined`
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node.operator === "typeof" // All `typeof` strings, when coerced to number, are not 0 or 1.
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) {
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return true;
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}
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if (node.operator === "!") {
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return isConstant(scope, node.argument, true);
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}
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/*
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* We won't try to reason about +, -, ~, or delete
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* In theory, for the mathematical operators, we could look at the
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* argument and try to determine if it coerces to a constant numeric
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* value.
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*/
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return false;
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case "NewExpression": // Objects might have custom `.valueOf` or `.toString`.
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return false;
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case "CallExpression": {
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if (node.callee.type === "Identifier" &&
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node.callee.name === "Boolean" &&
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isReferenceToGlobalVariable(scope, node.callee)
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) {
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return node.arguments.length === 0 || isConstant(scope, node.arguments[0], true);
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}
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return false;
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}
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case "Literal": // True or false, literals never change
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return true;
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case "Identifier":
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return node.name === "undefined" && isReferenceToGlobalVariable(scope, node);
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case "TemplateLiteral":
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/*
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* In theory we could try to check if the quasi are sufficient to
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* prove that the expression will always be true, but it would be
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* tricky to get right. For example: `000.${foo}000`
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*/
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return node.expressions.length === 0;
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case "AssignmentExpression":
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if (node.operator === "=") {
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return hasConstantLooseBooleanComparison(scope, node.right);
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}
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/*
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* Handling short-circuiting assignment operators would require
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* walking the scope. We won't attempt that (for now...)
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*
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* The remaining assignment expressions all result in a numeric or
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* string (non-nullish) values which could be truthy or falsy:
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* "+=", "-=", "*=", "/=", "%=", "<<=", ">>=", ">>>=", "|=", "^=", "&="
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*/
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return false;
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case "SequenceExpression": {
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const last = node.expressions[node.expressions.length - 1];
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return hasConstantLooseBooleanComparison(scope, last);
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}
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case "JSXElement": // ESLint has a policy of not assuming any specific JSX behavior.
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case "JSXFragment":
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return false;
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default:
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return false;
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}
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}
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/**
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* Test if an AST node will always give the same result when _strictly_ compared
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* to a boolean value. This can happen if the expression can never be boolean, or
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* if it is always the same boolean value.
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* @param {Scope} scope The scope in which the node was found.
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* @param {ASTNode} node The node to test
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* @returns {boolean} Will `node` always give the same result when compared to a
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* static boolean value?
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*/
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function hasConstantStrictBooleanComparison(scope, node) {
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switch (node.type) {
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case "ObjectExpression": // Objects are not booleans
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case "ArrayExpression": // Arrays are not booleans
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case "ArrowFunctionExpression": // Functions are not booleans
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case "FunctionExpression":
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case "ClassExpression": // Classes are not booleans
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case "NewExpression": // Objects are not booleans
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case "TemplateLiteral": // Strings are not booleans
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case "Literal": // True, false, or not boolean, literals never change.
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case "UpdateExpression": // Numbers are not booleans
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return true;
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case "BinaryExpression":
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return NUMERIC_OR_STRING_BINARY_OPERATORS.has(node.operator);
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case "UnaryExpression": {
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if (node.operator === "delete") {
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return false;
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}
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if (node.operator === "!") {
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return isConstant(scope, node.argument, true);
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}
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/*
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* The remaining operators return either strings or numbers, neither
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* of which are boolean.
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*/
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return true;
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}
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case "SequenceExpression": {
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const last = node.expressions[node.expressions.length - 1];
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return hasConstantStrictBooleanComparison(scope, last);
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}
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case "Identifier":
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return node.name === "undefined" && isReferenceToGlobalVariable(scope, node);
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case "AssignmentExpression":
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if (node.operator === "=") {
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return hasConstantStrictBooleanComparison(scope, node.right);
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}
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/*
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* Handling short-circuiting assignment operators would require
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* walking the scope. We won't attempt that (for now...)
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*/
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if (isLogicalAssignmentOperator(node.operator)) {
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return false;
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}
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/*
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* The remaining assignment expressions all result in either a number
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* or a string, neither of which can ever be boolean.
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*/
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return true;
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case "CallExpression": {
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if (node.callee.type !== "Identifier") {
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return false;
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}
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const functionName = node.callee.name;
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if (
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(functionName === "String" || functionName === "Number") &&
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isReferenceToGlobalVariable(scope, node.callee)
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) {
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return true;
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}
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if (functionName === "Boolean" && isReferenceToGlobalVariable(scope, node.callee)) {
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return (
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node.arguments.length === 0 || isConstant(scope, node.arguments[0], true));
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}
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return false;
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}
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case "JSXElement": // ESLint has a policy of not assuming any specific JSX behavior.
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case "JSXFragment":
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return false;
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default:
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return false;
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}
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}
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/**
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* Test if an AST node will always result in a newly constructed object
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* @param {Scope} scope The scope in which the node was found.
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* @param {ASTNode} node The node to test
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* @returns {boolean} Will `node` always be new?
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*/
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function isAlwaysNew(scope, node) {
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switch (node.type) {
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case "ObjectExpression":
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case "ArrayExpression":
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case "ArrowFunctionExpression":
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case "FunctionExpression":
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case "ClassExpression":
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return true;
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case "NewExpression": {
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if (node.callee.type !== "Identifier") {
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return false;
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}
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/*
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* All the built-in constructors are always new, but
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* user-defined constructors could return a sentinel
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* object.
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*
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* Catching these is especially useful for primitive constructures
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* which return boxed values, a surprising gotcha' in JavaScript.
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*/
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return Object.hasOwnProperty.call(globals.builtin, node.callee.name) &&
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isReferenceToGlobalVariable(scope, node.callee);
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}
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case "Literal":
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// Regular expressions are objects, and thus always new
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return typeof node.regex === "object";
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case "SequenceExpression": {
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const last = node.expressions[node.expressions.length - 1];
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return isAlwaysNew(scope, last);
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}
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case "AssignmentExpression":
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if (node.operator === "=") {
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return isAlwaysNew(scope, node.right);
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}
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return false;
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case "ConditionalExpression":
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return isAlwaysNew(scope, node.consequent) && isAlwaysNew(scope, node.alternate);
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case "JSXElement": // ESLint has a policy of not assuming any specific JSX behavior.
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case "JSXFragment":
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return false;
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default:
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return false;
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}
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}
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/**
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* Checks whether or not a node is `null` or `undefined`. Similar to the one
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* found in ast-utils.js, but this one correctly handles the edge case that
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* `undefined` has been redefined.
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* @param {Scope} scope Scope in which the expression was found.
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* @param {ASTNode} node A node to check.
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* @returns {boolean} Whether or not the node is a `null` or `undefined`.
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* @public
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*/
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function isNullOrUndefined(scope, node) {
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return (
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isNullLiteral(node) ||
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(node.type === "Identifier" && node.name === "undefined" && isReferenceToGlobalVariable(scope, node)) ||
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(node.type === "UnaryExpression" && node.operator === "void")
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);
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}
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/**
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* Checks if one operand will cause the result to be constant.
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* @param {Scope} scope Scope in which the expression was found.
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* @param {ASTNode} a One side of the expression
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* @param {ASTNode} b The other side of the expression
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* @param {string} operator The binary expression operator
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* @returns {ASTNode | null} The node which will cause the expression to have a constant result.
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*/
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function findBinaryExpressionConstantOperand(scope, a, b, operator) {
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if (operator === "==" || operator === "!=") {
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if (
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(isNullOrUndefined(scope, a) && hasConstantNullishness(scope, b)) ||
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(isStaticBoolean(scope, a) && hasConstantLooseBooleanComparison(scope, b))
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) {
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return b;
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}
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} else if (operator === "===" || operator === "!==") {
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if (
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(isNullOrUndefined(scope, a) && hasConstantNullishness(scope, b)) ||
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(isStaticBoolean(scope, a) && hasConstantStrictBooleanComparison(scope, b))
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) {
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return b;
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}
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}
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return null;
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}
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//------------------------------------------------------------------------------
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// Rule Definition
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//------------------------------------------------------------------------------
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/** @type {import('../shared/types').Rule} */
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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: "disallow expressions where the operation doesn't affect the value",
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recommended: false,
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url: "https://eslint.org/docs/rules/no-constant-binary-expression"
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},
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schema: [],
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messages: {
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constantBinaryOperand: "Unexpected constant binary expression. Compares constantly with the {{otherSide}}-hand side of the `{{operator}}`.",
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constantShortCircuit: "Unexpected constant {{property}} on the left-hand side of a `{{operator}}` expression.",
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alwaysNew: "Unexpected comparison to newly constructed object. These two values can never be equal.",
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bothAlwaysNew: "Unexpected comparison of two newly constructed objects. These two values can never be equal."
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}
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},
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create(context) {
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return {
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LogicalExpression(node) {
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const { operator, left } = node;
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const scope = context.getScope();
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if ((operator === "&&" || operator === "||") && isConstant(scope, left, true)) {
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context.report({ node: left, messageId: "constantShortCircuit", data: { property: "truthiness", operator } });
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} else if (operator === "??" && hasConstantNullishness(scope, left)) {
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context.report({ node: left, messageId: "constantShortCircuit", data: { property: "nullishness", operator } });
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}
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},
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BinaryExpression(node) {
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const scope = context.getScope();
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const { right, left, operator } = node;
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const rightConstantOperand = findBinaryExpressionConstantOperand(scope, left, right, operator);
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const leftConstantOperand = findBinaryExpressionConstantOperand(scope, right, left, operator);
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if (rightConstantOperand) {
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context.report({ node: rightConstantOperand, messageId: "constantBinaryOperand", data: { operator, otherSide: "left" } });
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} else if (leftConstantOperand) {
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context.report({ node: leftConstantOperand, messageId: "constantBinaryOperand", data: { operator, otherSide: "right" } });
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} else if (operator === "===" || operator === "!==") {
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if (isAlwaysNew(scope, left)) {
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context.report({ node: left, messageId: "alwaysNew" });
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} else if (isAlwaysNew(scope, right)) {
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context.report({ node: right, messageId: "alwaysNew" });
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}
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} else if (operator === "==" || operator === "!=") {
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/*
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* If both sides are "new", then both sides are objects and
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* therefore they will be compared by reference even with `==`
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* equality.
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*/
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if (isAlwaysNew(scope, left) && isAlwaysNew(scope, right)) {
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context.report({ node: left, messageId: "bothAlwaysNew" });
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}
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}
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}
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/*
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* In theory we could handle short-circuiting assignment operators,
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* for some constant values, but that would require walking the
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* scope to find the value of the variable being assigned. This is
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* dependant on https://github.com/eslint/eslint/issues/13776
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*
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* AssignmentExpression() {},
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*/
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};
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}
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};
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