存储对`call`函数的引用

Question

I noticed something curious earlier today. I can't seem to store a reference to the call property of a function, then execute it. Example:

var log = console.log;
log.call(console, 'This works'); 

var logCall = console.log.call;
logCall(console, 'This does not');

To me, this seems like perfectly legal Javascript, but the second invocation always gives me the error that undefined is not a function . Feel free to play around with it here , you'll get the same results.

So why does Javascript prevent me from calling call in this manner?

EDIT: I finally got it straight in my head after reading SimpleJ's answer. So I'm going to update this with how you can get the above to work:

var log = console.log;
log.call(console, 'This works'); 

var logCall = console.log.call;
logCall.call(console.log, console, 'This works now too');

The problem was that console.log was receiving the proper this value, but console.log.call wasn't given a proper this value. So as you can see, I basically had to execute console.log.call.call . Obviously you'd never really use code like this, I was just curious.

Answer 1

You need to keep the binding to console. Try this:

var logCall = console.log.call.bind(console.log);
// example: logCall(console, "foobar");

or

var log = console.log.bind(console);
// example: log("foobar");

For a bound reference to log .

Edit: jsfiddle: http://jsfiddle.net/67mfQ/2/

Answer 2

This is my favorite code in JavaScript:

var bind = Function.bind;
var call = Function.call;

var bindable = bind.bind(bind);
var callable = bindable(call);

You can use the bindable function to grab a reference to f.bind . Similarly you can use the callable function to grab a reference to f.call as follows:

var log = callable(console.log, console);

Now all you need to do is call the log function like any other function:

log("Hello World!");

That's all folks.