More JavaScript Concepts Module#26

Question 1

What are the 3 kinds of scope?

Answer 1

Global, Block, Function

Question 2

What is closure

Answer 2

Source: MDN
A closure is the combination of a function bundled together (enclosed) with references to its surrounding state (the lexical environment). In other words, a closure gives you access to an outer function’s scope from an inner function. In JavaScript, closures are created every time a function is created, at function creation time.

Question 3

What happens if a variable is used without being declared first while in non-strict mode?

Answer 3

The variable gets attached to the global scope which can be a gnarly source of bugs.

Question 4

Describe “hoisting” in JavaScript.

Answer 4

Before executing the code a person writes, the JavaScript engine reorders it according to some rules. For example, functions are moved to the top of their scope.

This is why this is legal to write:
dosomething()
function dosomething( ){
console.log("did something") }

The function is “hoisted” above the call before execution.

Question 5

Which mode should we default to when writing our JS?

Answer 5

Strict mode

Question 6

Why are immediately invoked function expressions useful?

Answer 6

Because they don’t pollute the global object.

This is how you should be writing an IIFE:
( function( ) {
//do something now
} )( )

Arrow function format:
( ( ) => { // do something now } ) ( )

We basically have a function defined inside parentheses, and then we append () to execute that function: (/* function */)( ).

Question 7

When you see this error: TypeError: console.log(…) is not a function what’s “usually” the problem?

Answer 7

JavaScript sees a second opening parenteses, and thinks you’re trying to invoke console.log(‘test’), doing console.log(‘test’)().

Solution is to add a semi ; to the beginning of the second line of the code. Example:

console.log('test')
;(function( ) {
 /* */
})( )

Question 8

How would you convert another primitive value to a string?

Answer 8

By invoking the toString( ) method on the value that you are targeting.

Question 9

There are several other ways to convert boolean values to strings. Consider these real quick»»»»»

Answer 9

var truthNess = true
var falseNess = false

String(truthNess) //”true”
truthNess.toString( ) //”true”
String(falseNess) //”false”
falseNess.toString( ) //”false”

Question 10

Casting from Date to String

Answer 10

String(new Date('2019-01-22'))
//"Tue Jan 22 2019 01:00:00 GMT+0100 (Central European Standard Time)"

(new Date('2019-01-22')).toString()
//"Tue Jan 22 2019 01:00:00 GMT+0100 (Central European Standard Time)"

Question 11

How do you convert a String into a number?

Answer 11

By calling the Number( ) global function. See below.

Number(“1”) //1
Number(“0”) //0

Question 12

Cast a boolean into a number. Think binary.

Answer 12

Number(true) //1

Number(false) //0

Question 13

Cast a date into a number primitive

Answer 13

Number(null) //0
Number(undefined) //NaN
Number(NaN) //NaN

Question 14

ANY value can be converted into a Boolean

Answer 14

Here we go using the Boolean( ) global function:
Boolean(false) //false
Boolean(0) //false
Boolean(NaN) //false
Boolean("") //false
Boolean(null) //false
Boolean(undefined) //false

Question 15

What is the value of this while in strict mode?

Answer 15

Outside any object, this in strict mode is always undefined. Remember functions are objects too.

Question 16

How do you enable strict mode in your JS file?

Answer 16

To invoke strict mode for an entire script, put the exact statement “use strict”; (or ‘use strict’;) before any other statements.

// Whole-script strict mode syntax

'use strict';
var v = "Hi! I'm a strict mode script!";

Strict mode for modules:

function strict() {
    // because this is a module, I'm strict by default
}
export default strict;

Question 17

Refresher! What is a callback?

Answer 17

A callback is a function passed as an argument to another function. This technique allows a function to call another function.A callback function can run after another function has finished.

Consider the below:

function myDisplayer(some) {
  document.getElementById("demo").innerHTML = some;
}

function myCalculator(num1, num2, myCallback) {
  let sum = num1 + num2;
  myCallback(sum);
}

myCalculator(5, 5, myDisplayer);

The function is being is executing a callback which has been passed as a parameter and then the callback is passing the output of the function as a parameter.

Question 18

What are the rules for automatic semi-colon insertion? This is !important

Answer 18

1. When the next line starts with code that breaks the current one (code can spawn on multiple lines)
2. When the next line starts with a }, closing the current block
3. When the end of the source code file is reached
4. When there is a return statement on its own line
5. When there is a break statement on its own line
6. When there is a throw statement on its own line
7. When there is a continue statement on its own line

Question 19

Under what circumstances is a switch statement appropriate?

Answer 19

Something that would require numerous if/else chains. The syntax is below for review. This is exactly the way it functions in PHP.

const a = 2
switch(a) {
  case 1:
    //handle case a is 1
    break
  case 2:
    //handle case a is 2
    break
  case 3:
    //handle case a is 3
    break
  default:
    //handle all other cases
    break
}