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tools: avoid let in for loops
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This adds a new ESLint tool to check for let
declarations within the for, forIn, forOf expressions.

Fixes: #9045
Ref: #9553
Ref: #8873
PR-URL: #9049
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Teddy Katz <teddy.katz@gmail.com>
Reviewed-By: Prince John Wesley <princejohnwesley@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
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jalafel authored and Myles Borins committed Nov 14, 2016
1 parent d9c3364 commit c8dccf2
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1 change: 1 addition & 0 deletions lib/.eslintrc
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rules:
# Custom rules in tools/eslint-rules
require-buffer: 2
no-let-in-for-declaration: 2
46 changes: 46 additions & 0 deletions tools/eslint-rules/no-let-in-for-declaration.js
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/**
* @fileoverview Prohibit the use of `let` as the loop variable
* in the initialization of for, and the left-hand
* iterator in forIn and forOf loops.
*
* @author Jessica Quynh Tran
*/

'use strict';

//------------------------------------------------------------------------------
// Rule Definition
//------------------------------------------------------------------------------

module.exports = {
create(context) {

const msg = 'Use of `let` as the loop variable in a for-loop is ' +
'not recommended. Please use `var` instead.';

/**
* Report function to test if the for-loop is declared using `let`.
*/
function testForLoop(node) {
if (node.init && node.init.kind === 'let') {
context.report(node.init, msg);
}
}

/**
* Report function to test if the for-in or for-of loop
* is declared using `let`.
*/
function testForInOfLoop(node) {
if (node.left && node.left.kind === 'let') {
context.report(node.left, msg);
}
}

return {
'ForStatement': testForLoop,
'ForInStatement': testForInOfLoop,
'ForOfStatement': testForInOfLoop
};
}
};

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