在哪里可以找到 Javascript `typeof` 运算符的源代码？

Question

I'm looking to see how the typeof operator in JavaScript knows that an object is a function.

To be precise, I'm wondering how I can take the function 'body' and reverse engineer it to determine what arguments it expects. The arguments property seemed close, but is only available within the evaluated function.

Neither uneval() or toSource() appear to do quite what I want, in addition to being obsolete.

Answer 1

The specification shows that:

Objects that do not implement [[Call]] are object s

Objects that do implement [[Call]] are function s

( [[Call]] is an "internal property" of objects - it's not exposed directly to interactable running JavaScript)

So anything that can be () d is a function . This will even include Proxies, since proxies can be called as if they were functions.

For the other question:

I'm wondering how I can take the function 'body' and reverse engineer it to determine what arguments it expects

The simplest method would be to turn the function into a string and use a regex to parse the parameter list:

 function sum(a, b) { return a + b; } const argList = String(sum).match(/\((.*)\)/)[1]; console.log(argList.split(/,\s*/)); // depending on how complicated the list is // you may need to make the above pattern more elaborate