What about checking tail call optimization support? Imho, we need a possibility to verify engine supports it. `eval`, `Function` and `Reflect.parse` wont work here. So `Reflect.supports` looks more meaningful in this case.
On Mon, Mar 23, 2015 at 3:31 PM, Rick Waldron <[email protected]> wrote: > > > On Sun, Mar 22, 2015 at 4:47 PM Getify Solutions <[email protected]> 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: > > Components.utils.import("resource://gre/modules/reflect.jsm"); > > Then try this: > > function isSyntaxSupported(syntax) { > try { > Reflect.parse(syntax); > 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" > > > > Rick > > _______________________________________________ > es-discuss mailing list > [email protected] > https://mail.mozilla.org/listinfo/es-discuss > >
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