This not working in javascript function

As an object method

When a function is called as a method of an object, its this is set to the object the method is called on.

In the following example, when o.f[] is invoked, inside the function this is bound to the o object.

const o = {
  prop: 37,
  f[] {
    return this.prop;
  }
};

console.log[o.f[]]; // 37

Note that this behavior is not at all affected by how or where the function was defined. In the previous example, we defined the function inline as the f member during the definition of o. However, we could have just as easily defined the function first and later attached it to o.f. Doing so results in the same behavior:

const o = { prop: 37 };

function independent[] {
  return this.prop;
}

o.f = independent;

console.log[o.f[]]; // 37

This demonstrates that it matters only that the function was invoked from the f member of o.

Similarly, the this binding is only affected by the most immediate member reference. In the following example, when we invoke the function, we call it as a method g of the object o.b. This time during execution, this inside the function will refer to o.b. The fact that the object is itself a member of o has no consequence; the most immediate reference is all that matters.

o.b = { g: independent, prop: 42 };
console.log[o.b.g[]]; // 42

this on the object's prototype chain

The same notion holds true for methods defined somewhere on the object's prototype chain. If the method is on an object's prototype chain, this refers to the object the method was called on, as if the method were on the object.

const o = {
  f[] {
    return this.a + this.b;
  },
};
const p = Object.create[o];
p.a = 1;
p.b = 4;

console.log[p.f[]]; // 5

In this example, the object assigned to the variable p doesn't have its own f property, it inherits it from its prototype. But it doesn't matter that the lookup for f eventually finds a member with that name on o; the lookup began as a reference to p.f, so this inside the function takes the value of the object referred to as p. That is, since f is called as a method of p, its this refers to p. This is an interesting feature of JavaScript's prototype inheritance.

this with a getter or setter

Again, the same notion holds true when a function is invoked from a getter or a setter. A function used as getter or setter has its this bound to the object from which the property is being set or gotten.

function sum[] {
  return this.a + this.b + this.c;
}

const o = {
  a: 1,
  b: 2,
  c: 3,
  get average[] {
    return [this.a + this.b + this.c] / 3;
  }
};

Object.defineProperty[o, 'sum', {
  get: sum,
  enumerable: true,
  configurable: true,
}];

console.log[o.average, o.sum]; // 2, 6

As a constructor

When a function is used as a constructor [with the new keyword], its this is bound to the new object being constructed.

Note: While the default for a constructor is to return the object referenced by this, it can instead return some other object [if the return value isn't an object, then the this object is returned].

/*
 * Constructors work like this:
 *
 * function MyConstructor[] {
 *   // Actual function body code goes here.
 *   // Create properties on `this` as
 *   // desired by assigning to them, for example,
 *   this.fum = "nom";
 *   // et cetera...
 *
 *   // If the function has a return statement that
 *   // returns an object, that object will be the
 *   // result of the `new` expression. Otherwise,
 *   // the result of the expression is the object
 *   // currently bound to `this`
 *   // [i.e., the common case most usually seen].
 * }
 */

function C[] {
  this.a = 37;
}

let o = new C[];
console.log[o.a]; // 37

function C2[] {
  this.a = 37;
  return { a: 38 };
}

o = new C2[];
console.log[o.a]; // 38

In the last example [C2], because an object was returned during construction, the new object that this was bound to gets discarded. [This essentially makes the statement this.a = 37; dead code. It's not exactly dead because it gets executed, but it can be eliminated with no outside effects.]

As a DOM event handler

When a function is used as an event handler, its this is set to the element on which the listener is placed [some browsers do not follow this convention for listeners added dynamically with methods other than addEventListener[]].

// When called as a listener, turns the related element blue
function bluify[e] {
  // Always true
  console.log[this === e.currentTarget];
  // true when currentTarget and target are the same object
  console.log[this === e.target];
  this.style.backgroundColor = '#A5D9F3';
}

// Get a list of every element in the document
const elements = document.getElementsByTagName['*'];

// Add bluify as a click listener so when the
// element is clicked on, it turns blue
for [const element of elements] {
  element.addEventListener['click', bluify, false];
}

In an inline event handler

When the code is called from an inline on-event handler, its this is set to the DOM element on which the listener is placed:

  Show this

The above alert shows button. Note however that only the outer code has its this set this way:

  Show inner this

In this case, the inner function's this isn't set so it returns the global/window object [i.e. the default object in non–strict mode where this isn't set by the call].

this in classes

Just like with regular functions, the value of this within methods depends on how they are called. Sometimes it is useful to override this behavior so that this within classes always refers to the class instance. To achieve this, bind the class methods in the constructor:

class Car {
  constructor[] {
    // Bind sayBye but not sayHi to show the difference
    this.sayBye = this.sayBye.bind[this];
  }
  sayHi[] {
    console.log[`Hello from ${this.name}`];
  }
  sayBye[] {
    console.log[`Bye from ${this.name}`];
  }
  get name[] {
    return 'Ferrari';
  }
}

class Bird {
  get name[] {
    return 'Tweety';
  }
}

const car = new Car[];
const bird = new Bird[];

// The value of 'this' in methods depends on their caller
car.sayHi[]; // Hello from Ferrari
bird.sayHi = car.sayHi;
bird.sayHi[]; // Hello from Tweety

// For bound methods, 'this' doesn't depend on the caller
bird.sayBye = car.sayBye;
bird.sayBye[];  // Bye from Ferrari

Note: Classes are always strict mode code. Calling methods with an undefined this will throw an error.

See also