Supporting feature tests directly

Rick Waldron waldron.rick at
Mon Mar 23 13:31:11 UTC 2015

On Sun, Mar 22, 2015 at 4:47 PM Getify Solutions <getify at> wrote:

> > So why not just add a sandbox, and ... means to catch error
> Other than the `import` / `export` thing I mentioned, for the exact reason
> why `eval(..)` and `new Function(..)` are not preferred (which roughly do
> the same thing)… A feature test that requires the entire
> parse/compile/execute cycle is, even a little bit, slower than a feature
> test that requires only the parser to answer the question at hand.
> Since these tests are likely to happen in the critical path (page load),
> their speed is pretty important to be as optimal as possible.
> ------
> I don't want or need a general means to try out a whole program to see if
> it compiles or not. Don't let the `eval`-looking form of the proposal
> mislead as to intention. Intention is only to, feature-by-feature,
> determine feature support where simple tests for identifiers is
> insufficient.
> For example, this is *not* intended to be possible:
> ```js
> let x;
> Reflect.supports( "let x;" ); // false -- dupe declaration!
> ```
> That kind of test would require running in the context of the current
> lexical env, and would imply an entirely different level of integration
> with the program than intended. I don't need static errors like preventing
> duplicate declaration or anything of that nature. Even stuff like what
> `strict mode` would enforce are outside of the "scope" of what's being
> proposed.
> Only want to know if, in general, `let x;` could parse by the current
> engine. That's why `Reflect.supports( Symbol.letDecl )` would be an
> entirely sufficient option.

The SpiderMonkey/Firefox Reflect.parse is non-standard, but may be a useful
place to start.

First, "import" the "reflect.jsm" component module:


Then try this:

  function isSyntaxSupported(syntax) {
    try {
      return true;
    } catch (_) {
      return false;

    "import foo from 'bar';",   // valid
    "export var a = 1;",        // valid
    "export default class {}",  // valid
    "export class List {}",     // valid
    "async function foo() {}",  // invalid
    "let (x = 1) { x; }",       // invalid
    "module Name {}",           // invalid
  ].forEach(function(syntax) {
    console.log("`%s` is %ssupported", syntax, isSyntaxSupported(syntax) ?
"" : "un");

Firefox 38.0a2 (2015-03-23):

"`import foo from 'bar';` is supported"
"`export var a = 1;` is supported"
"`export default class {}` is unsupported"
"`export class List {}` is unsupported"
"`async function foo() {}` is unsupported"
"`let (x = 1) { x; }` is supported"
"`module Name {}` is unsupported"

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