js this

Question 1

What is this in JavaScript?

Answer 1

the current object/context

Question 2

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

Answer 2

doesn’t need to be declared or included as parameter

Question 3

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

Answer 3

call time

Question 4

What does this refer to in the following code snippet?

var character = {
  firstName: 'Mario',
  greet: function () {
    var message = 'It\'s-a-me, ' + this.firstName + '!';
    console.log(message);
  }
};

Answer 4

window

Question 5

Given the above character object, what is the result of the following code snippet? Why?

character.greet();

Answer 5

logs “It’s-a-me, Mario!” to the console. calling the greet method of the character object, so this is the character object

Question 6

Given the above character object, what is the result of the following code snippet? Why?

var hello = character.greet;
hello();

Answer 6

logs “It’s-a-me, undefined!” to the console. calling the hello method of the window object, so this is the window object, and this.firstName is undefined

Question 7

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

Answer 7

the object that contains the function/method defintion

Question 8

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

Answer 8

look to the left of the dot. If nothing there, this is probably the window object