尝试在 JavaScript 中实现 Python 风格的 len()

Question

I've been reading a lot about call() and bind() in JavaScript, specifically the Creating Shorcuts section on this MDN article .

I'm trying to implement the following Python-esque function in JS:

var arr = [1,2,3];
len(arr); // 3

I do realize this is a contrived example, but I'm trying to wrap my head around these methods. Here's how I implemented it:

var len = Function.prototype.call.bind( Array.prototype.slice.length );
len([1,2,3]);

When I run it, I get:

len([1,2,344])
^

TypeError: len is not a function
    at Object.<anonymous> (/private/var/folders/j6/3fs5_k3n17z_0j2xrwj6sphw0000gn/T/CodeRunner/Untitled 11.js:2:1)
    at Module._compile (module.js:435:26)
    at Object.Module._extensions..js (module.js:442:10)
    at Module.load (module.js:356:32)
    at Function.Module._load (module.js:311:12)
    at Function.Module.runMain (module.js:467:10)
    at startup (node.js:136:18)
    at node.js:963:3

What am I missing here to understand how this works?

Answer 1

When you're setting the variable len , in your code, you're actually setting len to a copy of the function Function.prototype.call , with a this (context) of Array.prototype.slice.length .

I think you're misunderstanding the use of bind(). Say I have a function len :

var len = function() {
    return this.length;
};

And you wanted to get the length of an array:

len.call([1,2,3]);

In this case, this in len is the array [1,2,3] , because we used call to supply this . Say we want to permanently bind that array to that function and store it in a variable we can use anytime:

var myArrayLen = len.bind([1,2,3]);

Now, myArrayLen is basically a copy of len, permanently bound with a this of [1,2,3] . If we call it, it returns 3.

In your example, Array.prototype.slice.length is actually the length of the function Array.prototype.slice , which corresponds to the number of arguments that can be passed to the function .

There is no built in length() function, so to follow your (admittedly) contrived example, we just need to make a function that provides the length, add it to the array prototype, and bind it using Function.prototype.apply.bind(); :

Array.prototype.len = function () { return this.length; };
var len = Function.prototype.apply.bind(Array.prototype.len);
len([1,2,3]); // 3