javascript-this

Question 1

What is this in JavaScript?

Answer 1

“this” is the the object to the left of the dot when the function is called(as a method).

Question 2

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

Answer 2

it means that it’s available in the function code block even it’s not included in the parameter list or declared as a variable.

Question 3

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

Answer 3

the value of “this” is determined at 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

the default window object

Question 5

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

Answer 5

the message in the greet method of character object will show because the greet method of the character object is called.

Question 6

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

Answer 6

the result would only show the string part, It’s-a-me, and this.firstName would be undefined!
the firstName is undefined because no object is appointed when the hello function is called.

Question 7

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

Answer 7

you don’t know the value yet

Question 8

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

Answer 8

you can look for the object to the left of the dot when a function or method is called.