关于JavaScript中的函数变量

Question

I am new to javascript so please understand if the question is a bit naive. I have heard that functions are also objects in javascript . So that means functions can also have properties like objects. So I tried this :

var foo=function(){
        var v1=1;
        console.log(foo.v1);
    };
    foo();

The output of this is undefined . I dont understand what is happening. So when I declare the variable v1 in the function foo ,according to the result v1 is not a property of the function-object foo .If it is not the former then what is it a property of ? Could some one explain to me what is happening ?

Answer 1

You're right in Javascript function is a object. You can have attribute in a function object, examples are link length etc.

according to the result v1 is not a property of the function-object foo.

v1 is just a variable defined in function foo, it is not a attribute.

To add a attribute you could use foo.v1 = "1" to add attribute v1 to object foo.

If you use console.log(v1) instead of console.log(foo.v1). You'll see output 1. Here you're accessing a local variable inside a function.

You might think var foo is already a object why can't I access it inside the function? This because these two foo variables are in different scopes. You might want to learn more about function scope

Answer 2

Object properties and variables are not the same thing. Functions can have both local variables (this includes function arguments) and properties:

function foo () {
    v1 = 1; // this is a local variable
    return [
        v1,    // local var above
        foo.v2 // object property
    ]
}

foo(); // returns [1,undefined]

foo.v2 = 2;
foo(); // returns [1,2]

So you see, functions are indeed objects. But variables inside the function have nothing to do about the fact that functions are objects.

Side note: the fact that functions are first-class objects and that functions can be defined anonymously (without a name) are two different features. In C for example, functions are also first-class objects - that is, you can assign them to function pointers. But in C you can't declare functions without a name.

function bar() {}

b = bar; // this demonstrates that functions are first-class objects
c = function(){}; // this demonstrates that functions can be anonymous