Parser rejects async used as a binding name or called as a function

async is only a reserved word in specific contexts; as a plain identifier it is valid, and V8/Node accept it. The parser currently rejects two such cases:

// 1. calling a function named `async`
function async(cb) { return cb; }
async(1);            // SyntaxError: expected token 'identifier', got '1'

// 2. binding named `async`
const async = 5;     // SyntaxError: abrupt end
async;

These already work, so async is accepted as a property/method name but not as a binding or a call target:

const o = { async: 1 }; o.async;      // ok
const o = { async() {} }; o.async();  // ok

This isn't purely academic: the widely used asynckit package (a transitive dependency of axios, form-data, …) calls a function it names async, so loading axios fails with the error above.

Possibly a regression or an uncovered case from #2275 / #2936, which added async-as-identifier support.

Environment: boa_engine main at commit 39cd112.

Hi! I'd like to work on this issue.

Diagnosis

In ECMAScript, async is a contextual keyword rather than a reserved word. Currently, when boa_parser encounters async followed by (, it eagerly assumes an AsyncArrowFunction parameter list. Because of this, argument expressions like async(1) fail with a SyntaxError (expecting formal parameter identifiers) instead of resolving via CoverCallExpressionAndAsyncArrowHead into a normal CallExpression.

Proposed Fix

Update boa_parser's expression parsing so async(...) correctly falls back to a CallExpression if it is not followed by [no LineTerminator] =>.
Add regression tests covering contextual identifier calls like async(...), variable declarations (const async = ...), and valid/invalid async arrow head disambiguation.

Could you please assign this to me?

Navigation Menu

Repository navigation

Parser rejects `async` used as a binding name or called as a function #5551

Diagnosis

Proposed Fix

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

Parser rejects async used as a binding name or called as a function #5551

Description

Activity

Basharkhan7776 commented on Oct 4, 2026

Diagnosis

Proposed Fix

Metadata

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

Issue actions

Parser rejects `async` used as a binding name or called as a function #5551