chakra-core/ChakraCore

Hoisting a var declaration past a lexical declaration of the same name should be a SyntaxError

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#357 opened on Feb 19, 2016

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 (4 comments) (0 reactions) (0 assignees)JavaScript (9,000 stars) (1,374 forks)batch import
BugSeverity: 2help wanted

Description

Second bullet point of block static semantics and similarly for function body static semantics.

E.g.

let x;
{
    var x; // error (already works)
    let y;
    var y; // should be an error (doesn't work)
    {
        var y; // should also be an error (doesn't work)
    }
}

This applies for var declarations hoisting past all lexical declarations, i.e. let, const, class, function (and generators and async functions).

Contributor guide

Tech stack
javascriptcpp
Domain
backend
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
4
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
3-5 days
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
JavaScriptC++parser knowledgeECMAScript spec
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
30
Research direction
Study the ECMAScript spec's static semantics for blocks and function bodies, specifically the second bullet point. Examine ChakraCore's parser implementation to understand how early errors for variable declarations are currently handled. Look for where `var` declarations are checked against lexical declarations, and add the missing error when a `var` hoists past a `let`, `const`, `class`, or function declaration. The relevant files are likely in the parser directory, e.g., 'lib/Parser/Parse.cpp' or similar. Review existing tests for similar errors to understand the pattern.