Change Request: provide option to allow parsing specific ES3 reserved words

[ljharb]

ESLint version

v8.2.0

What problem do you want to solve?

#15017 was great in spirit - I agreed with it - but my agreement assumed that

ES3 code using reserved words will throw a runtime error

was the case. I can't speak for char, because I don't use that anywhere - but I do use .native and int as identifiers/property names, and these work perfectly fine in everyone's favorite ES3 engine, IE 6 (and presumably in many of the others). I use .native in the runtime code path of resolve, which has extensive usage, so I'm pretty confident it doesn't actually throw a runtime error anywhere that matters.

This parsing error means I can't upgrade to eslint 8 on resolve or es-abstract (so far; i've only tested a dozen or two out of my ~350 packages) without making an undesirable and unnecessary code change.

What do you think is the correct solution?

Add a parserOption that allows specific reserved words to parse as identifiers or property names anyways (it could apply to any ecmaVersion). This would be a generic escape hatch, that would allow anyone to lint code that works on what most implementations are: not precisely matching any arbitrary spec edition.

I tried adding parserOptions.allowReserved: true, but this doesn't seem to do it.

Participation

• I am willing to submit a pull request for this change.

Additional comments

Sorry for not realizing this would be the consequence sooner :-)

[nzakas]

I’m not sure we want to add an option that is only relevant when ecmaVersion is 3. That seems like a recipe for confusion.

Reverting to previous behavior is an option, but would be a breaking change and would also not fulfill the goal of the change: to provide a strict ES3 parser.

@ljharb since it seems like you are doing some regular IE6 testing, can you get us a list of reserved words that do throw an error?

[nzakas]

TSC Summary:* In v8.0, we changed the behavior of ecmaVersion:3 to forbid reserved words. IE 6 actually allows some (or all) reserved words to be used. This request is to provide a looser ES3 variant for such purposes.

TSC Question: How do we want to address this?

[ljharb]

My thinking was that it would apply to every ecmaVersion, to ensure it was generic and wouldn’t need attending to in the future.

I’ll certainly check IE 6-8, and a few other ES3 browsers, and report back with a list.

As for the goal: what value is there In a strict ES3 parser if no accessible engine actually implements it? My hope would be for a strict “matches implementations” parser, for as many implementation tranches as were practical.

[ljharb]

I ran this jsbin: https://jsbin.com/zihosuh/edit?js,output populated with this info: https://mathiasbynens.be/notes/reserved-keywords via this link: https://output.jsbin.com/zihosuh and got these results:

IE 6

es2: int (var) es2: int (prop) es2: int (fn) es2: byte (var) es2: byte (prop) es2: byte (fn) es2: char (var) es2: char (prop) es2: char (fn) es2: goto (var) es2: goto (prop) es2: goto (fn) es2: long (var) es2: long (prop) es2: long (fn) es2: final (var) es2: final (prop) es2: final (fn) es2: float (var) es2: float (prop) es2: float (fn) es2: short (var) es2: short (prop) es2: short (fn) es2: double (var) es2: double (prop) es2: double (fn) es2: native (var) es2: native (prop) es2: native (fn) es2: public (var) es2: public (prop) es2: public (fn) es2: static (var) es2: static (prop) es2: static (fn) es2: throws (var) es2: throws (prop) es2: throws (fn) es2: boolean (var) es2: boolean (prop) es2: boolean (fn) es2: package (var) es2: package (prop) es2: package (fn) es2: private (var) es2: private (prop) es2: private (fn) es2: abstract (var) es2: abstract (prop) es2: abstract (fn) es2: volatile (var) es2: volatile (prop) es2: volatile (fn) es2: interface (var) es2: interface (prop) es2: interface (fn) es2: protected (var) es2: protected (prop) es2: protected (fn) es2: transient (var) es2: transient (prop) es2: transient (fn) es2: implements (var) es2: implements (prop) es2: implements (fn) es2: synchronized (var) es2: synchronized (prop) es2: synchronized (fn) es5: eval (var)

IE 7

es2: int (var) es2: int (prop) es2: int (fn) es2: byte (var) es2: byte (prop) es2: byte (fn) es2: char (var) es2: char (prop) es2: char (fn) es2: goto (var) es2: goto (prop) es2: goto (fn) es2: long (var) es2: long (prop) es2: long (fn) es2: final (var) es2: final (prop) es2: final (fn) es2: float (var) es2: float (prop) es2: float (fn) es2: short (var) es2: short (prop) es2: short (fn) es2: double (var) es2: double (prop) es2: double (fn) es2: native (var) es2: native (prop) es2: native (fn) es2: public (var) es2: public (prop) es2: public (fn) es2: static (var) es2: static (prop) es2: static (fn) es2: throws (var) es2: throws (prop) es2: throws (fn) es2: boolean (var) es2: boolean (prop) es2: boolean (fn) es2: package (var) es2: package (prop) es2: package (fn) es2: private (var) es2: private (prop) es2: private (fn) es2: abstract (var) es2: abstract (prop) es2: abstract (fn) es2: volatile (var) es2: volatile (prop) es2: volatile (fn) es2: interface (var) es2: interface (prop) es2: interface (fn) es2: protected (var) es2: protected (prop) es2: protected (fn) es2: transient (var) es2: transient (prop) es2: transient (fn) es2: implements (var) es2: implements (prop) es2: implements (fn) es2: synchronized (var) es2: synchronized (prop) es2: synchronized (fn) es5: eval (var)

IE 8

es2: int (var) es2: int (prop) es2: int (fn) es2: byte (var) es2: byte (prop) es2: byte (fn) es2: char (var) es2: char (prop) es2: char (fn) es2: goto (var) es2: goto (prop) es2: goto (fn) es2: long (var) es2: long (prop) es2: long (fn) es2: final (var) es2: final (prop) es2: final (fn) es2: float (var) es2: float (prop) es2: float (fn) es2: short (var) es2: short (prop) es2: short (fn) es2: double (var) es2: double (prop) es2: double (fn) es2: native (var) es2: native (prop) es2: native (fn) es2: public (var) es2: public (prop) es2: public (fn) es2: static (var) es2: static (prop) es2: static (fn) es2: throws (var) es2: throws (prop) es2: throws (fn) es2: boolean (var) es2: boolean (prop) es2: boolean (fn) es2: package (var) es2: package (prop) es2: package (fn) es2: private (var) es2: private (prop) es2: private (fn) es2: abstract (var) es2: abstract (prop) es2: abstract (fn) es2: volatile (var) es2: volatile (prop) es2: volatile (fn) es2: interface (var) es2: interface (prop) es2: interface (fn) es2: protected (var) es2: protected (prop) es2: protected (fn) es2: transient (var) es2: transient (prop) es2: transient (fn) es2: implements (var) es2: implements (prop) es2: implements (fn) es2: synchronized (var) es2: synchronized (prop) es2: synchronized (fn) es5: eval (var)

FF 3

es1: do (prop) es1: do (fn) es1: if (prop) es1: if (fn) es1: in (prop) es1: in (fn) es1: for (prop) es1: for (fn) es1: new (prop) es1: new (fn) es1: try (prop) es1: try (fn) es1: var (prop) es1: var (fn) es1: case (prop) es1: case (fn) es1: else (prop) es1: else (fn) es1: enum (var) es1: enum (prop) es1: enum (fn) es1: null (prop) es1: null (fn) es1: this (prop) es1: this (fn) es1: true (prop) es1: true (fn) es1: void (prop) es1: void (fn) es1: with (prop) es1: with (fn) es1: break (prop) es1: break (fn) es1: catch (prop) es1: catch (fn) es1: class (var) es1: class (prop) es1: class (fn) es1: const (prop) es1: const (fn) es1: false (prop) es1: false (fn) es1: super (var) es1: super (prop) es1: super (fn) es1: throw (prop) es1: throw (fn) es1: while (prop) es1: while (fn) es1: delete (prop) es1: delete (fn) es1: export (prop) es1: export (fn) es1: import (prop) es1: import (fn) es1: return (prop) es1: return (fn) es1: switch (prop) es1: switch (fn) es1: typeof (prop) es1: typeof (fn) es1: default (prop) es1: default (fn) es1: extends (var) es1: extends (prop) es1: extends (fn) es1: finally (prop) es1: finally (fn) es1: continue (prop) es1: continue (fn) es1: debugger (prop) es1: debugger (fn) es1: function (prop) es1: function (fn) es2: do (prop) es2: do (fn) es2: if (prop) es2: if (fn) es2: in (prop) es2: in (fn) es2: for (prop) es2: for (fn) es2: int (var) es2: int (prop) es2: int (fn) es2: new (prop) es2: new (fn) es2: try (prop) es2: try (fn) es2: var (prop) es2: var (fn) es2: byte (var) es2: byte (prop) es2: byte (fn) es2: case (prop) es2: case (fn) es2: char (var) es2: char (prop) es2: char (fn) es2: else (prop) es2: else (fn) es2: enum (var) es2: enum (prop) es2: enum (fn) es2: goto (var) es2: goto (prop) es2: goto (fn) es2: long (var) es2: long (prop) es2: long (fn) es2: null (prop) es2: null (fn) es2: this (prop) es2: this (fn) es2: true (prop) es2: true (fn) es2: void (prop) es2: void (fn) es2: with (prop) es2: with (fn) es2: break (prop) es2: break (fn) es2: catch (prop) es2: catch (fn) es2: class (var) es2: class (prop) es2: class (fn) es2: const (prop) es2: const (fn) es2: false (prop) es2: false (fn) es2: final (var) es2: final (prop) es2: final (fn) es2: float (var) es2: float (prop) es2: float (fn) es2: short (var) es2: short (prop) es2: short (fn) es2: super (var) es2: super (prop) es2: super (fn) es2: throw (prop) es2: throw (fn) es2: while (prop) es2: while (fn) es2: delete (prop) es2: delete (fn) es2: double (var) es2: double (prop) es2: double (fn) es2: export (prop) es2: export (fn) es2: import (prop) es2: import (fn) es2: native (var) es2: native (prop) es2: native (fn) es2: public (var) es2: public (prop) es2: public (fn) es2: return (prop) es2: return (fn) es2: static (var) es2: static (prop) es2: static (fn) es2: switch (prop) es2: switch (fn) es2: throws (var) es2: throws (prop) es2: throws (fn) es2: typeof (prop) es2: typeof (fn) es2: boolean (var) es2: boolean (prop) es2: boolean (fn) es2: default (prop) es2: default (fn) es2: extends (var) es2: extends (prop) es2: extends (fn) es2: finally (prop) es2: finally (fn) es2: package (var) es2: package (prop) es2: package (fn) es2: private (var) es2: private (prop) es2: private (fn) es2: abstract (var) es2: abstract (prop) es2: abstract (fn) es2: continue (prop) es2: continue (fn) es2: debugger (prop) es2: debugger (fn) es2: function (prop) es2: function (fn) es2: volatile (var) es2: volatile (prop) es2: volatile (fn) es2: interface (var) es2: interface (prop) es2: interface (fn) es2: protected (var) es2: protected (prop) es2: protected (fn) es2: transient (var) es2: transient (prop) es2: transient (fn) es2: implements (var) es2: implements (prop) es2: implements (fn) es2: instanceof (prop) es2: instanceof (fn) es2: synchronized (var) es2: synchronized (prop) es2: synchronized (fn) es5: do (prop) es5: do (fn) es5: if (prop) es5: if (fn) es5: in (prop) es5: in (fn) es5: for (prop) es5: for (fn) es5: new (prop) es5: new (fn) es5: try (prop) es5: try (fn) es5: var (prop) es5: var (fn) es5: case (prop) es5: case (fn) es5: else (prop) es5: else (fn) es5: enum (var) es5: enum (prop) es5: enum (fn) es5: eval (var)

Safari 4

es2: int (var) es2: int (prop) es2: int (fn) es2: byte (var) es2: byte (prop) es2: byte (fn) es2: char (var) es2: char (prop) es2: char (fn) es2: goto (var) es2: goto (prop) es2: goto (fn) es2: long (var) es2: long (prop) es2: long (fn) es2: final (var) es2: final (prop) es2: final (fn) es2: float (var) es2: float (prop) es2: float (fn) es2: short (var) es2: short (prop) es2: short (fn) es2: double (var) es2: double (prop) es2: double (fn) es2: native (var) es2: native (prop) es2: native (fn) es2: public (var) es2: public (prop) es2: public (fn) es2: static (var) es2: static (prop) es2: static (fn) es2: throws (var) es2: throws (prop) es2: throws (fn) es2: boolean (var) es2: boolean (prop) es2: boolean (fn) es2: package (var) es2: package (prop) es2: package (fn) es2: private (var) es2: private (prop) es2: private (fn) es2: abstract (var) es2: abstract (prop) es2: abstract (fn) es2: volatile (var) es2: volatile (prop) es2: volatile (fn) es2: interface (var) es2: interface (prop) es2: interface (fn) es2: protected (var) es2: protected (prop) es2: protected (fn) es2: transient (var) es2: transient (prop) es2: transient (fn) es2: implements (var) es2: implements (prop) es2: implements (fn) es2: synchronized (var) es2: synchronized (prop) es2: synchronized (fn) es5: eval (var)

In other words, all of these are things that are reserved, and should be failing, but actually do work - and I think it's reasonable for eslint to parse them. The spec is only fiction unless implementations follow it - when every implementation does something different from the spec, TC39 changes the spec to match reality.

[btmills]

In the 2021-11-18 TSC meeting, we agreed to add an allowReserved option that can be used with ecmaVersion: 3. Espree will be responsible for checking that constraint.

[nzakas]

Following up on @ljharb’s Discord message.

The changes actually need to happen in Espree. You’d want to check that if allowReserved is present that ecmaVersion is 3 in this function (otherwise throw error):
https://github.com/eslint/espree/blob/ed95791c0d9334383ddf59ae0d0968842801c171/lib/options.js#L90

Down below you can set the value of allowReserved:

https://github.com/eslint/espree/blob/ed95791c0d9334383ddf59ae0d0968842801c171/lib/options.js#L95

And a couple of tests here:

https://github.com/eslint/espree/blob/main/tests/lib/options.js

[nzakas]

Oh and don’t forget to update the README with the new option.

[ljharb]

Thanks! Filed eslint/espree#522, please review :-)