Microsoft/TypeScript

commonjs export binding produces invalid code for increment/decrement in PrefixUnaryExpression

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#35,879 opened on Dec 27, 2019

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BugDomain: JS EmitHelp Wanted

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Description

TypeScript Version: 3.7.2

Search Terms: export increment decrement syntax error

Code

// @module: commonjs
let foo = 1;
export { foo };
while (!--foo) { }

Expected behavior:

while (!(exports.foo = --foo)) { }

Actual behavior:

Because the module-transform uses onSubstituteNode, it doesn't use the auto-parenthesizing that's done by the factory API.

while (!exports.foo = --foo) { } // SyntaxError: `!exports.foo` is no valid assignment target

Playground Link: https://www.typescriptlang.org/play/?target=99&module=1&ssl=3&ssc=19&pln=1&pc=1#code/DYUwLgBAZg9jEF4IEYDcAoEAPADjATpAN7RwQC+GA7gBYCWoEAFAIQC0bsMAlBCeUA

Related Issues: #33044

Contributor guide

Research direction
Investigate the commonjs module transformer, specifically the onSubstituteNode callback, and add parenthesization for PrefixUnaryExpression when the operand is an export assignment.
Tech stack
typescript
Domain
compilers
Issue type
Bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-2 days
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
TypeScriptTypeScript compiler internals
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
40