Javascript

Question 1

What does javascript do?

Answer 1

Browser receives html to render. Gets some script code, which executes JS. The script changes what we see on the screen

Question 2

Ajax requests

Answer 2

Allows a website to communicate with a server asynchronously. I.e. we don’t have to re-render page url to see updates

Question 3

Node

Answer 3

Allows us to use javascript on the backend.

Question 4

object in Js

Answer 4

ruby equivalent of a key value hash

Question 5

Constructor function

Answer 5

defines what our object looks like

Question 6

.prototype

Answer 6

use it to define methods on a constructor form

Question 7

new keyword

Answer 7

constant___proto === Obj(Like class on ruby).prototype

Question 8

node

Answer 8

repl like pry for js

Question 9

__proto__ vs .prototype

Answer 9

__proto is defined on the instance prototype on the object itself

Question 10

closure

Answer 10

a function that uses a variable defined in before it

Question 11

How JS Scope is defined

Answer 11

1 - Arguments
2 - Variables defined in the function
3 - Any variables that were already declared when function was defined (“free variables”)

Closure can also change variables defined before it

Question 12

What is a callback

Answer 12

A function that is passed into another function as an argument with the intention of being called back at a later time

Common with asynchronous functions. Doesn’t require all functions to finish.

setTimeout

Question 13

setTimeout

Answer 13

Defined globally

let callback = function() {
console.log("It has been 5 seconds!");
}

let timeToWait = 5000

global.setTimeout(callback, timeToWait);

global.setTimeout(function() {
console.log(“It has been 5 seconds!”);
}, 5000) ->ES5

global. setTimeout(() => console.log(“It has been 5 seconds”), 5000);
- > ES6 preferred syntax

Question 14

callback vs. callback()

Answer 14

() passes in the return value not the invocation of the function

Question 15

Expressions

Answer 15

line of code that returns a value. Needs a semi-colon at the end

Question 16

Statement

Answer 16

Any line of code. Statements that aren’t expression do not require semi-colons

Question 17

{ curly braces }

Answer 17

Curly braces are used to delineate code blocks such as in function definitions, loops, and if blocks. Curly braces can also be used to define JavaScript objects.q

Question 18

NaN

Answer 18

Not a number. Property of the global object and is not a JS data type. NaN is the result of any illegal numerical operation

Question 19

What are the Falsey values in JS

Answer 19

zeros, empty strings, undefined, null, and NaN

Question 20

What is a primitive type

Answer 20

A primitive type is data that is not an object and cannot have methods.

Numbers are primitives and do not have object methods. So to do ruby like operation we need to call the Math object and pass in the number

Question 21

What are JS Objects

Answer 21

They can store both state (ruby instance variables) and behavior(ruby methods)

Objects are just a collection of associated properties and values. They are like hashes

In JavaScript, state and behavior of the object are treated the same way. Both are accessible as properties of the object.

In Ruby, state and behavior are separated into instance variables and methods which are accessed differently (ie. @ivar, vs self.method_name).

Question 22

Declaration

Answer 22

In JavaScript, in order to use a variable or constant, we must declare it by prepending the variable’s name with var, let, or const. Declaring a variable is the only way to introduce it to the environment.

Question 23

Return Values

Answer 23

Everything in javascript has a return value, even variable declarations and initializations. undefined is the default return value.

Question 24

Var

Answer 24

var is not the preferred means of declaring a variable in ES6. let or const are preferred.

Question 25

let

Answer 25

We can use let to declare block-scoped variables.

Question 26

Blocks

Answer 26

if statements, while llops, switch statements, for loops

Question 27

const

Answer 27

wConstants should be used for constructs that will not be re-declared or re-assigned. Properties:

• block-scoped like let
• Unlike Ruby constants, JavaScript will actually enforce constants by raising an error if you try to reassign them.
• Trying to redeclare a constant with a var or let by the same name will also raise an error.\

Constants are not immutable. Only the binding is immutable. For example, if we set a constant equal to an object, we can still modify that object:

Constants cannot be reassigned but new constants of the same names can be declared in nested scopes.

Question 28

Declarations

Answer 28

Scoped to if blocks

Question 29

Globals

Answer 29

If you leave off a declaration when initializing a variable, it will become a global. Never do this.

Question 30

Function

Answer 30

Functions in JavaScript are just a special type of JS objects. In other words, you can do anything to a function that you can do to an object.

In Javascript, referencing the function name by itself will only return a pointer to the function; to execute it, you must invoke the function with arguments. If no arguments are required, this looks like () appended to the function name.