Defer to the built-in typeof if support for symbols exists.

[jdalton]

This PR addresses https://phabricator.babeljs.io/T6875 by deferring to the built-in typeof operator if support for symbols exists.

[codecov-io]

Current coverage is 84.87%

Merging #3218 into master will decrease coverage by -0.31% as of d9bf5da

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[gnarf]

Forgive me for lacking context here.

Does this mean every use of typeof will call Symbol() or does this result get cached and the resulting function is used?

[jdalton]

@gnarf
I'm not a pro at Babel's helpers, I based this PR off of the extends helper, but the idea is that this is an expression that will be assigned to an identifier. So the end result would look something like

var _typeof = (typeof Symbol === "function" && typeof Symbol() === "symbol")
  ? function (obj) { return typeof obj; }
  : function (obj) { return obj && typeof Symbol === "function" && obj.constructor === Symbol ? "symbol" : typeof obj; };

So modern engines would get a _typeof helper that's only

function (obj) { return typeof obj; }

[zloirock]

I can explain the problem. Without external helpers / runtime, in each file with typeof will be created a symbol. Someone can transpile node_modules too - we will have thousands of symbols even without the use of symbols. Almost Symbol polyfills based on accessors in Object.prototype. Thousands useless accessors in Object.prototype can reduce performance in old engines.

Ok, how we can solve this problem?

We can add mark to the Symbol polyfill:

(typeof Symbol === "function" && !Symbol._shim) ? foo : bar;

Or we can cache the result of this detection:

(typeof Symbol === "function" && !('_shim' in Symbol ? Symbol._shim : (Symbol._shim = typeof Symbol() != "symbol"))) ? foo : bar;

[jdalton]

Changed the check to

(typeof Symbol === "function" && typeof Symbol.iterator === "symbol")

[zloirock]

It makes sense. LGTM.

[ljharb]

Every engine with native Symbols implements Symbol.iterator without exception, so this LGTM also!

[jdalton]

@ljharb Yep, I did a quick scan of @kangax's compat table before I amended the commit \o/

[hzoo]

Looks good - thanks!