"var let" should be allowed in non-strict mode [$15]

[DanielRosenwasser]

Repro:

var let = 10;

See jquery/esprima#1048

There is a $15 open bounty on this issue. Add to the bounty at Bountysource.

[nzakas]

I believe this happens because let token is marked as a keyword, so this check fails:

https://github.com/eslint/espree/blob/master/espree.js#L3596

I think this might be as simple as removing "let" from this statement:

https://github.com/eslint/espree/blob/master/lib/syntax.js#L154

@btmills @xjamundx @FredKSchott do any of you have time to look at fixing this?

[xjamundx]

Early next week if no one claims it first.

[xjamundx]

Assuming var const = 10 should also work?

[DanielRosenwasser]

@xjamundx no, const is a future reserved word. I can send out a PR. My hesitation in doing so was really due to the issue that this behavior was originally supposed to be for compatibility with the SpiderMonkey AST API.

[DanielRosenwasser]

Actually, there are some implications to that original suggestion that make this a little more involved:

• The tokenizer now no longer categorizes let as a keyword even though this may be ideal for most purposes.
• More places must be checked for let as an identifier. See below.

I'll do some more work on this, but given that I'm largely unfamiliar with the code, I might end up handing this back off to you 😄

---

---

13.2.1.1 Static Semantics: Early Errors

LexicalDeclaration : LetOrConst BindingList ;

• It is a Syntax Error if the BoundNames of BindingList contains "let".

---

13.6.4.1 Static Semantics: Early Errors

IterationStatement :

for ( ForDeclaration in Expression ) Statement

for ( ForDeclaration of AssignmentExpression ) Statement

• It is a Syntax Error if the BoundNames of ForDeclaration contains "let".

[nzakas]

OK, so probably makes sense to keep let as a keyword and just add an exception that looks for an identifier or let in var declarations outside of loops.

[xjamundx]

And by "early next week"....I'll try again in the coming week.

[nzakas]

👍

[DanielRosenwasser]

The thing is that something like let + 100 is valid, so it's not always a keyword.

[nzakas]

I wonder if Acorn handles this correctly. Might be useful to see what they're doing.

[xjamundx]

Cool. Will check on how they're doing it!

[xjamundx]

With the latest version of acorn 2.1.1:

> require('./dist/acorn').parse('var let = y', {ecmaVersion: '6'})
SyntaxError: Unexpected token (1:4)
    at Parser.pp.raise (/Users/jamuferguson/dev/acorn/dist/acorn.js:975:13)
    at Parser.pp.unexpected (/Users/jamuferguson/dev/acorn/dist/acorn.js:1524:8)
    at Parser.pp.parseBindingAtom (/Users/jamuferguson/dev/acorn/dist/acorn.js:1164:12)
    at Parser.pp.parseVarId (/Users/jamuferguson/dev/acorn/dist/acorn.js:2041:18)
    at Parser.pp.parseVar (/Users/jamuferguson/dev/acorn/dist/acorn.js:2024:10)
    at Parser.pp.parseVarStatement (/Users/jamuferguson/dev/acorn/dist/acorn.js:1913:8)
    at Parser.pp.parseStatement (/Users/jamuferguson/dev/acorn/dist/acorn.js:1713:19)
    at Parser.pp.parseTopLevel (/Users/jamuferguson/dev/acorn/dist/acorn.js:1653:21)
    at Parser.parse (/Users/jamuferguson/dev/acorn/dist/acorn.js:1623:17)
    at Object.parse (/Users/jamuferguson/dev/acorn/dist/acorn.js:937:44)

[xjamundx]

I sort of gave up this when thinking harder about the complexity. May take another stab if I get motivated today though after looking at some of the other espree isues...

[xjamundx]

Going to throw up a PR with broken tests, just to keep me motivated....

[nzakas]

Acorn issue: acornjs/acorn#359

[DanielRosenwasser]

As a heads up, this was fixed in esprima.

[michaelficarra]

I just got bit by this with

(let[a] = b);

This should be valid, but is giving me the error "Parsing error: Unexpected token let".

[nzakas]

Yup, it's an upstream issue with Acorn.

[RReverser]

IIUC, this issue can be closed now.

[mysticatea]

I confirmed it.

"use strict";

const espree = require("espree");
const ast = espree.parse(
    `var let = 0`,
    {
        ecmaVersion: 6,
        sourceType: "script"
    }
);

console.log(JSON.stringify(ast, null, 4));

{
    "type": "Program",
    "start": 0,
    "end": 11,
    "body": [
        {
            "type": "VariableDeclaration",
            "start": 0,
            "end": 11,
            "declarations": [
                {
                    "type": "VariableDeclarator",
                    "start": 4,
                    "end": 11,
                    "id": {
                        "type": "Identifier",
                        "start": 4,
                        "end": 7,
                        "name": "let"
                    },
                    "init": {
                        "type": "Literal",
                        "start": 10,
                        "end": 11,
                        "value": 0,
                        "raw": "0"
                    }
                }
            ],
            "kind": "var"
        }
    ],
    "sourceType": "script"
}

[michaelficarra]

@mysticatea how about the program I mentioned?

[RReverser]

@michaelficarra It works http://astexplorer.net/#/5L9D8euj22. Been fixed in acornjs/acorn@8f46eb5 couple of months ago.

[nzakas]

I'll add a test case just to be sure.

[RReverser]

I guess money goes to @marijnh in such case 😄

[nzakas]

Yup. @marijnh, once this is closed, you can claim the bounty on this issue.