/
prefer-for-of.ts
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/
prefer-for-of.ts
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import { AST_NODE_TYPES, TSESTree } from '@typescript-eslint/typescript-estree';
import * as util from '../util';
import { Scope } from 'ts-eslint';
export default util.createRule({
name: 'prefer-for-of',
meta: {
type: 'suggestion',
docs: {
description:
'Prefer a ‘for-of’ loop over a standard ‘for’ loop if the index is only used to access the array being iterated.',
category: 'Stylistic Issues',
recommended: false,
tslintName: 'prefer-for-of',
},
messages: {
preferForOf:
'Expected a `for-of` loop instead of a `for` loop with this simple iteration',
},
schema: [],
},
defaultOptions: [],
create(context) {
function isSingleVariableDeclaration(
node: TSESTree.Node | null,
): node is TSESTree.VariableDeclaration {
return (
node !== null &&
node.type === AST_NODE_TYPES.VariableDeclaration &&
node.kind !== 'const' &&
node.declarations.length === 1
);
}
function isLiteral(node: TSESTree.Expression, value: number): boolean {
return node.type === AST_NODE_TYPES.Literal && node.value === value;
}
function isZeroInitialized(node: TSESTree.VariableDeclarator): boolean {
return node.init !== null && isLiteral(node.init, 0);
}
function isMatchingIdentifier(
node: TSESTree.Expression,
name: string,
): boolean {
return node.type === AST_NODE_TYPES.Identifier && node.name === name;
}
function isLessThanLengthExpression(
node: TSESTree.Node | null,
name: string,
): TSESTree.Expression | null {
if (
node !== null &&
node.type === AST_NODE_TYPES.BinaryExpression &&
node.operator === '<' &&
isMatchingIdentifier(node.left, name) &&
node.right.type === AST_NODE_TYPES.MemberExpression &&
isMatchingIdentifier(node.right.property, 'length')
) {
return node.right.object;
}
return null;
}
function isIncrement(node: TSESTree.Node | null, name: string): boolean {
if (!node) {
return false;
}
switch (node.type) {
case AST_NODE_TYPES.UpdateExpression:
// x++ or ++x
return (
node.operator === '++' && isMatchingIdentifier(node.argument, name)
);
case AST_NODE_TYPES.AssignmentExpression:
if (isMatchingIdentifier(node.left, name)) {
if (node.operator === '+=') {
// x += 1
return isLiteral(node.right, 1);
} else if (node.operator === '=') {
// x = x + 1 or x = 1 + x
const expr = node.right;
return (
expr.type === AST_NODE_TYPES.BinaryExpression &&
expr.operator === '+' &&
((isMatchingIdentifier(expr.left, name) &&
isLiteral(expr.right, 1)) ||
(isLiteral(expr.left, 1) &&
isMatchingIdentifier(expr.right, name)))
);
}
}
}
return false;
}
function contains(outer: TSESTree.Node, inner: TSESTree.Node): boolean {
return (
outer.range[0] <= inner.range[0] && outer.range[1] >= inner.range[1]
);
}
function isAssignee(node: TSESTree.Node): boolean {
const parent = node.parent;
if (!parent) {
return false;
}
// a[i] = 1, a[i] += 1, etc.
if (
parent.type === AST_NODE_TYPES.AssignmentExpression &&
parent.left === node
) {
return true;
}
// delete a[i]
if (
parent.type === AST_NODE_TYPES.UnaryExpression &&
parent.operator === 'delete' &&
parent.argument === node
) {
return true;
}
// a[i]++, --a[i], etc.
if (
parent.type === AST_NODE_TYPES.UpdateExpression &&
parent.argument === node
) {
return true;
}
// [a[i]] = [0]
if (parent.type === AST_NODE_TYPES.ArrayPattern) {
return true;
}
// [...a[i]] = [0]
if (parent.type === AST_NODE_TYPES.RestElement) {
return true;
}
// ({ foo: a[i] }) = { foo: 0 }
if (
parent.type === AST_NODE_TYPES.Property &&
parent.parent !== undefined &&
parent.parent.type === AST_NODE_TYPES.ObjectExpression &&
parent.value === node &&
isAssignee(parent.parent)
) {
return true;
}
return false;
}
function isIndexOnlyUsedWithArray(
body: TSESTree.Statement,
indexVar: Scope.Variable,
arrayExpression: TSESTree.Expression,
): boolean {
const sourceCode = context.getSourceCode();
const arrayText = sourceCode.getText(arrayExpression);
return indexVar.references.every(reference => {
const id = reference.identifier;
const node = id.parent;
return (
!contains(body, id) ||
(node !== undefined &&
node.type === AST_NODE_TYPES.MemberExpression &&
node.property === id &&
sourceCode.getText(node.object) === arrayText &&
!isAssignee(node))
);
});
}
return {
'ForStatement:exit'(node: TSESTree.ForStatement) {
if (!isSingleVariableDeclaration(node.init)) {
return;
}
const [declarator] = node.init.declarations;
if (
!isZeroInitialized(declarator) ||
declarator.id.type !== AST_NODE_TYPES.Identifier
) {
return;
}
const indexName = declarator.id.name;
const arrayExpression = isLessThanLengthExpression(
node.test,
indexName,
);
if (!arrayExpression) {
return;
}
const [indexVar] = context.getDeclaredVariables(node.init);
if (
isIncrement(node.update, indexName) &&
isIndexOnlyUsedWithArray(node.body, indexVar, arrayExpression)
) {
context.report({
node,
messageId: 'preferForOf',
});
}
},
};
},
});