How does Undersore's _.now work?

问题:

It doesn't look like it is written in JavaScript.

if you type _now in the console, you only get

function now() { [native code] }

You usually only get that when you try to look at some built-in method where the inner-workings are invisible to the browser.

setTimeout
=>function setTimeout() { [native code] }

Has _.now done something with "native code" of the JavaScript engine?

回答1:

By default _.now is just Date.now, except in environments that do not support it. Where Date.now isn't supported _.now will use this implementation instead (same goes for lodash)

_.now = function() {
   return (new Date()).getTime()
};

As your browser supports Date.now, _.now is just a proxy to the native implementation

---

Note: you can also make any of your functions appear as native in console by calling using Function.prototype.bind

function foo() {console.log('bar');}
var bar = foo.bind(null);

console.log(bar);
// => function () { [native code] }

回答2:

Take a look at the underscore source code:

_.now = Date.now || function() {
  return new Date().getTime();
};

This means that it will use Date.now() if it exists, which is an internal function. Otherwise it will use new Date().getTime(), which is supported by all JavaScript engines.

回答3:

It returns an integer timestamp for the current time. Useful for implementing timing/animation functions.

_.now(); => 1392066795351