for-in cannot be parsed

[neumatho]

The following for-in statement cannot be parsed in Meriyah, but works fine in Acorn. The Chrome browser can also parse it when entering it in the console window.

for(var a = /a/ in []);

-Thomas

[3cp]

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Invalid_for-in_initializer

Should be only an error in strict mode.

[3cp]

Found some trouble, it seems there are some js features (for example let) implicitly turn on strict mode.

In node:

> for(var a = /a/ in []);
undefined

> for(let a = /a/ in []);
for(let a = /a/ in []);
        ^^^^^^^

Uncaught SyntaxError: for-in loop variable declaration may not have an initializer.
> 

[3cp]

I could find the case in ECMA2020 spec about let turning on strict mode. Maybe it's a nodejs/v8 deviation from spec? @KFlash

10.2.1 Strict Mode Code
An ECMAScript Script syntactic unit may be processed using either unrestricted or strict mode syntax and semantics. Code is interpreted as strict mode code in the following situations:

Global code is strict mode code if it begins with a Directive Prologue that contains a Use Strict Directive.
Module code is always strict mode code.
All parts of a ClassDeclaration or a ClassExpression are strict mode code.
Eval code is strict mode code if it begins with a Directive Prologue that contains a Use Strict Directive or if the call to eval is a direct eval that is contained in strict mode code.
Function code is strict mode code if the associated FunctionDeclaration, FunctionExpression, GeneratorDeclaration, GeneratorExpression, AsyncFunctionDeclaration, AsyncFunctionExpression, AsyncGeneratorDeclaration, AsyncGeneratorExpression, MethodDefinition, ArrowFunction, or AsyncArrowFunction is contained in strict mode code or if the code that produces the value of the function's [[ECMAScriptCode]] internal slot begins with a Directive Prologue that contains a Use Strict Directive.
Function code that is supplied as the arguments to the built-in Function, Generator, AsyncFunction, and AsyncGenerator constructors is strict mode code if the last argument is a String that when processed is a FunctionBody that begins with a Directive Prologue that contains a Use Strict Directive.

[3cp]

Add ref: while for-in error only applies in strict mode, for-of error is universal.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Invalid_for-of_initializer