jshint/jshint

undeclared variable in es6 comprehension

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#1656 opened on May 4, 2014

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P4help wantedmoz

Description

[for (i of [1,2,3]) i]

Jshint will warn that i is undeclared. Should it? This makes the warning go away:

var i;
[for (i of [1,2,3]) i]

but seems dubious. For example, in the traceur-generated code, the two "i"s are different "i"s. The "i" in the comprehension is scoped to the comprehension.

Contributor guide

Tech stack
javascript
Domain
toolingdeveloper experience
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
2
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
Basic JavaScriptUnderstanding of ES6 comprehensions
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
45
Research direction
The issue reports that jshint incorrectly flags 'i' as undeclared in `[for (i of [1,2,3]) i]`. The fix involves modifying jshint's scoping rules to recognize that variables in array comprehensions are scoped to the comprehension, not the outer scope. Look at jshint's source code in `src/jshint.js` or `src/lex.js` to see how variable declarations are handled. Comments in the issue suggest that the comprehension scoping is similar to function scope. Implement a check that treats comprehension variables as properly declared within their scope.