JavaScript OOP

Question 1

What is a method?

Answer 1

A method is a function which is a property of an object. There are two kinds of methods: instance methods which are built-in tasks performed by an object instance, or static methods which are tasks that are called directly on an object constructor.

Question 2

How can you tell the difference between a method definition and a method call?

Answer 2

A method definition will have a colon (:) following the name followed by an anonymous function for that method and a method call will have a set of parentheses with some or none parameters following the name of the method.

Question 3

Describe method definition syntax (structure).

Answer 3

The method name followed by colon, then anonymous function called with parameters, then curly braces for what the function will do, including a return. This method is defined inside of an object as a property.

Question 4

Describe method call syntax (structure).

Answer 4

The object name followed by a period (‘.’) followed by the name of the method with a set of parentheses ( ‘()’ ) filled with either some or no parameters.

Question 5

How is a method different from any other function?

Answer 5

A method is a function which is a property of an object and must be called with that object.

Question 6

What is the defining characteristic of Object-Oriented Programming?

Answer 6

That objects can contain both data (as properties) and behavior (as methods).

Question 7

What are the four “principles” of Object-Oriented Programming?

Answer 7

Abstraction, encapsulation, inheritance, and polymorphism

Question 8

What is “abstraction”?

Answer 8

Making complex or specific details into a more abstract object or idea that focuses on the bigger picture instead of details.

Question 9

What does API stand for?

Answer 9

Application programming interface.

Question 10

What is the purpose of an API?

Answer 10

To give programmers a way to interact with a system in a simplified, consistent fashion (abstraction).

Question 11

What is this in JavaScript?

Answer 11

this is an implicit parameter of all JavaScript functions.
the value of this is determined at call time.
the value of this can be recognized as “the object to the left of the dot” when the function is called (as a method).
if there is no value to the left of the dot when the function is called, then by default, this will be the global window object.
if you cannot see the function being called, then you do not know what the value of this will be.

Question 12

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

Answer 12

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

Question 13

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

Answer 13

The value of this is determined at the call time of a function

Question 14

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 14

This refers to the character object when it will be called, but since the code snippet is a definition, it refers to nothing and defaults to the window object.

Question 15

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

Answer 15

‘It’s-a-me, Mario!’
This is because the greet method of the character object is being called. The greet method concatenates this.firstName, which would be the value of the firstName property of the character object since it is what is left of the dot when the function is called, with other strings.

Question 16

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

Answer 16

‘It’s-a-me, undefined!’
This is because the hello function is being called not as a method of an object, so this defaults to the window object which does not have a firstName property of character object, so this.firstName === undefined.

Question 17

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

Answer 17

You can’t because the value of this is determined at call time.

Question 18

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

Answer 18

the value of this can be recognized as “the object to the left of the dot” when the function is called (as a method).
if there is no value to the left of the dot when the function is called, then by default, this will be the global window object.
if you cannot see the function being called, then you do not know what the value of this will be.

Question 19

What kind of inheritance does the JavaScript programming language use?

Answer 19

prototype-based (or prototypal) inheritance.

Question 20

What is a prototype in JavaScript?

Answer 20

An object that contains properties and (predominantly) methods that can be used by other objects.

Question 21

How is it possible to call methods on strings, arrays, and numbers even though those methods don’t actually exist on strings, arrays, and numbers?

Answer 21

Those methods are defined on a “prototype” object and strings, arrays, and numbers simply borrow those methods when they’re needed. They inherit it from the prototype chain.

Question 22

If an object does not have it’s own property or method by a given key, where does JavaScript look for it?

Answer 22

The prototype object of the object and keeps going up through prototypes until it finds the property or method.

Question 23

What does the new operator do?

Answer 23

The new operator lets developers create an instance of a user-defined object type or of one of the built-in object types that has a constructor function.

Question 24

What property of JavaScript functions can store shared behavior for instances created with new?

Answer 24

Their prototypes

Question 25

What does the instanceof operator do?

Answer 25

The instanceof operator tests to see if the prototype property of a constructor appears anywhere in the prototype chain of an object. The return value is a boolean value.

Question 26

What is a “callback” function?

Answer 26

A function passed into another function as an argument and only invoked when the outer function is called. They’re often used to continue code execution after an asynchronous operation has completed.

Question 27

Besides adding an event listener callback function to an element or the document, what is one way to delay the execution of a JavaScript function until some point in the future?

Answer 27

The setTimeout() global function

Question 28

How can you set up a function to be called repeatedly without using a loop?

Answer 28

With the setInterval() global function which needs the clearInterval() global function to stop it.

Question 29

What is the default time delay if you omit the delay parameter from setTimeout() or setInterval()?

Answer 29

0ms or immediately.

Question 30

What do setTimeout() and setInterval() return?

Answer 30

The returned timeoutID is a positive integer value which identifies the timer created by the call to setTimeout(). This value can be passed to clearTimeout() to cancel the timeout. The returned intervalID is a numeric, non-zero value which identifies the timer created by the call to setInterval(); this value can be passed to clearInterval() to cancel the interval.
They both share the same pool of IDs and clearInterval() and clearTimeout() can technically be used interchangeably.