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prefer-for-of.ts
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prefer-for-of.ts
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import type { TSESLint, TSESTree } from '@typescript-eslint/utils';
import { AST_NODE_TYPES } from '@typescript-eslint/utils';
import { createRule, isAssignee } from '../util';
export default createRule({
name: 'prefer-for-of',
meta: {
type: 'suggestion',
docs: {
description:
'Enforce the use of `for-of` loop over the standard `for` loop where possible',
recommended: 'stylistic',
},
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?.type === AST_NODE_TYPES.VariableDeclaration &&
node.kind !== 'const' &&
node.declarations.length === 1
);
}
function isLiteral(
node: TSESTree.Expression | TSESTree.PrivateIdentifier,
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 | TSESTree.PrivateIdentifier,
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?.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 isIndexOnlyUsedWithArray(
body: TSESTree.Statement,
indexVar: TSESLint.Scope.Variable,
arrayExpression: TSESTree.Expression,
): boolean {
const arrayText = context.sourceCode.getText(arrayExpression);
return indexVar.references.every(reference => {
const id = reference.identifier;
const node = id.parent;
return (
!contains(body, id) ||
(node.type === AST_NODE_TYPES.MemberExpression &&
node.object.type !== AST_NODE_TYPES.ThisExpression &&
node.property === id &&
context.sourceCode.getText(node.object) === arrayText &&
!isAssignee(node))
);
});
}
return {
'ForStatement:exit'(node: TSESTree.ForStatement): void {
if (!isSingleVariableDeclaration(node.init)) {
return;
}
const declarator = node.init.declarations[0] as
| TSESTree.VariableDeclarator
| undefined;
if (
!declarator ||
!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.sourceCode.getDeclaredVariables(node.init);
if (
isIncrement(node.update, indexName) &&
isIndexOnlyUsedWithArray(node.body, indexVar, arrayExpression)
) {
context.report({
node,
messageId: 'preferForOf',
});
}
},
};
},
});