Javascript: Predict the value of this argument

this is determined by the calling context, the way in which function is called.

Calling standalone functions directly

When a function is executed as a simple object function directly, this refers to the global object.

function foo() {
   console.log('Hello World!');
   console.log(this);
}

foo();

Calling functions as property of an object reference

Here we have created an object obj, and there is a property on it which is a function. The function is being called as a property on the object. Here this refers to the object obj itself.

var obj = {};

obj.foo = function() {
   console.log('Hello World!');
   console.log(this);
}

obj.foo();

Calling standalone functions using 'new' keyword

When a function is called using the new keyword, this always refer to the newly created object, an empty object.

function foo() {
   console.log('Hello World!');
   console.log(this);
}

new foo();