问题
In Javascript, Function.call() can call Function given a this value and zero or more arguments.
Function.call itself is a function. So in theory, Function.call should be the same (or similarly acting) function as Function.call.call.
In V8, this seems to be the case:
> Function.call === Function.call.call
true
When we call Function.call(), we get an anonymous function
> Function.call()
[Function: anonymous]
However, I can't call .call() on Function.call.
> Function.call.call()
TypeError: undefined is not a function
at repl:1:21
at REPLServer.defaultEval (repl.js:132:27)
at bound (domain.js:291:14)
at REPLServer.runBound [as eval] (domain.js:304:12)
at REPLServer.<anonymous> (repl.js:279:12)
at REPLServer.emit (events.js:107:17)
at REPLServer.Interface._onLine (readline.js:214:10)
at REPLServer.Interface._line (readline.js:553:8)
at REPLServer.Interface._ttyWrite (readline.js:830:14)
at ReadStream.onkeypress (readline.js:109:10)
What is going on here? Function.call is clearly a function - it's not undefined as this error message suggests.
回答1:
Short answer: The error message is very misleading. It is the same error message you get when you do
(undefined)();
Longer answer:
The second .call() is being invoked with a this of Function.call.
Calling it with no parameters causes it to call this with undefined as the this value.
Therefore, you're really doing
Function.call.call(undefined)
which means you're (metaphorically) doing
undefined.call()
which is really just
undefined()
Passing nothing (or undefined) to the this parameter of Function.call.call() is essentially negating the this context of the first Function.call() (which would be just Function itself), causing .call() to be invoked on undefined.
This yields the error message that is produced: undefined is not a function.
来源：https://stackoverflow.com/questions/40884839/why-cant-i-call-function-call