如何从函数内部为全局函数分配属性

Question

Assuming functions in javascript are actually objects, how can I assign properties to this function object within the function itself.

Below are not working as 'this' keyword is referring to the window object, so 'prop' will be assigned to the window global object not to the function object.

function test() {
    this.prop = value;
}

Why 'this' keyword inside global functions is referring to the window object and not referring to the function object itself?

Edit:

My question is different from the duplicate question as I am asking how to assign a property to a global function inside that function, not about the scope of variables.

Answer 1

Reference the function by name:

 function test(value) { test.prop = value; } test('hello'); console.log(test.prop); // => "hello" 

Answer 2

Short answer: invoke the function via call and pass the intended this as the first argument.

function test(value) {
  this.prop = value
}

//  'this' v     v value
test.call(test, 10)
console.log(test.prop) // 10

The purpose of call is to explicitly set the this of the invoked function.

Explanation:

For non-strict mode, when the function is invoked without a caller, eg, test(10) , this is implicitly set to the global object, in this case window . In strict mode, it will be undefined .

function test() {
  return this
}

console.log(test()) // window (or undefined in strict mode)

Inside of a function, this refers to the caller of the function.

const caller = {
    test: function () { return this }
}

console.log(caller.test()) // caller, i.e., { test: function() {...} }

This is true with 'classes' as well (functions invoked with new ).

function MyConstructor() {
  this.test = function() {
    return this
  }
}

const instance = new MyConstructor()
console.log(instance.test()) // instance of MyConstructor

Depending on your use case, it may be preferable to use this form:

const myObj = {
  test: function(value) {
    this.prop = value
  }
}

// 'prop' will be set on `myObj` instead of the function.
myObj.test(10)
console.log(myObj) // { prop: 10, test: function(value) {...} }

or a class-like construct:

function MyConstructor(value) {
  this.prop = value
}

const instance = new MyConstructor(10)
console.log(instance) // MyConstructor { prop: 10 }
console.log(instance.prop) // 10