JS 'this'

Question 1

What is ‘this’ in JavaScript?

Answer 1

‘this’ is an implicit parameter of all JavaScript functions.

Question 2

What does it mean to say that ‘this’ is an “implicit parameter”?

Answer 2

It is available in a function’s code block even though it was never included in the function’s parameters list or declared with ‘var’.

Question 3

<i>When</i> is the value of this determined in a function; call time or definition time?

Answer 3

call time

Question 4

<b>What does 'this' refer to in the following code snippet?</b>
var character = {
  firstName: 'Mario',
  greet: function ( ) {
     var message = 'It\'s-a-me, ' + this.firstName + '!';
     console.log(message);
  }
};

Answer 4

Do not know yet. The value of ‘this’ can only be determined during call time (not during a function definition).

Question 5

Given the above character object, what is the result of the following code snippet? Why?
character.greet( );

Answer 5

Result = “It’s-a-me, Mario!” because ‘this’ refers to the object being called. The object firstName property has the value of Mario.

Question 6

Given the above character object, what is the result of the following code snippet? Why?
var hello = character.greet;
hello();

Answer 6

Result = “It’s-a-me, undefined!” because ‘this’ refers to the window (hello is not a property of an object, therefore default object would be the window object). Window object does not have the property firstName so therefore, ‘this.firstName” has the value of undefined.

Question 7

How can you tell what the value of ‘this’ will be for a particular function or method definition?

Answer 7

You can’t. Value of ‘this’ is determined at call time.

Question 8

How can you tell what the value of ‘this’ is for a particular function or method call?

Answer 8

By looking to left of the dot (the object).