[英]Store the “call”-function of a function in a variable
why does the following not work as expected ? 为什么以下各项无法按预期工作?
> function Double() { return this * 2; }
undefined
> Double
[Function: Double]
> Double.call
[Function: call]
> Double.call(8)
16
> var double = Double.call;
undefined
> double
[Function: call]
> double(8); ////// BAM! Why does this not work ??
TypeError: object is not a function
at repl:1:1
at REPLServer.defaultEval (repl.js:129:27)
at bound (domain.js:271:14)
at REPLServer.runBound [as eval] (domain.js:284:12)
at Interface.<anonymous> (repl.js:277:12)
at Interface.EventEmitter.emit (events.js:101:17)
at Interface._onLine (readline.js:194:10)
at Interface._line (readline.js:523:8)
at Interface._ttyWrite (readline.js:798:14)
at ReadStream.onkeypress (readline.js:98:10)
>
* EDIT * *编辑*
I've created a function ref() to achieve this: 我创建了一个函数ref()来实现这一点:
Function.prototype.ref = function() {
var that = this;
return function(thisArg, args) {
return that.call(thisArg, args);
};
}
Now Double.ref() is a passable function where the first argument is this. 现在Double.ref()是一个可传递函数,其中第一个参数是this。
Because when you do this: 因为当您这样做时:
var double = Double.call;
You lose the context of Double
. 您会失去Double
的上下文。 So then when you invoke double
it expects a context of a function (since that's what Function.prototype.call
requires) and doesn't find it. 因此,当您调用double
它期望一个函数的上下文(因为那是Function.prototype.call
需要的),并且找不到它。
To try and put it more simply, you're trying to invoke Function.prototype.call
on something that isn't a function. 为了更简单地说,您试图在不是Function.prototype.call
上调用Function.prototype.call
。
You could make it work by binding the reference to Double.call
back to Double
: 您可以通过Double.call
的引用绑定到Double
来使其工作:
var double = Double.call.bind(Double);
double(8); // 16
And just to further demonstrate what's happening, you can use any reference to Function.prototype.call
, not just the one you got via Double
: 为了进一步说明正在发生的事情,您可以使用对Function.prototype.call
任何引用,而不仅仅是通过Double
获得的引用:
var double = Function.prototype.call.bind(Double);
double(8); // 16
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