JavaScript Technical Q&A

Instead of re-writing my notes, I decided to ask myself questions and try to answer them. I hope this is helpful :)

Q. What is the difference between =, == and ===?

• = Assignment operator: assign a variable a value

const name = 'Eva';

• == Loose equality comparison: only values must match

"4" == 4 //true 

• === Strict equality comparison: data type and value BOTH must match

"4" === 4 //false 
4 === 4 //true - same data type (number) and same value (four)

Q. What is the difference between var, let, and const?

• var: globally scoped or function scoped VS let & const: blocked scoped

Here’s a link for more information on var, let & const from freeCodeCamp:)

https://www.freecodecamp.org/news/var-let-and-const-whats-the-difference/#:~:text=var%20declarations%20are%20globally%20scoped%20or%20function%20scoped%20while%20let,the%20top%20of%20their%20scope.

Q. What is the difference between null and undefined?

• null: A variable has no “real” value

It contains nothing and is equivalent to a false Boolean

• undefined: A variable that has not been assigned a value

let number; 
//the variable has been declared, but not assigned a value
//the value is undefined by default

Q. What are the similarities between null and undefined?

They both have no value and have an empty state

Q. What’s the difference between a function declaration, a function expression and an arrow function?

• Function declaration: They are hoisted → Can be invoked even if they are defined later in the source code

sayHello('Eva');
function sayHello(name) {
   console.log(`Hello ${name}!`);
}

• Function expression: They are not hoisted → Cannot be invoked before they are defined

let sayHello = function(name) {
    console.log(`Hello ${name}!`);
}
sayHello('Eva');

• Arrow function: They are not hoisted either and the value of this is equal to window object (global scope)

let sayHello = (name) => {
    console.log(`Hello ${name}!`);
}
sayHello('Eva');

Q. What are the falsey values in JavaScript?

• There are 6 inherently falsey values in JavaScript

Q. What is && , || , ?? , !!

• && AND: Both values have to be true

//If the 1st value is false, it returns the 1st value
0 || null //returns '0'
LOGIC: If the 1st value is false, there's no need to check the other value since both need to be true so return early
//If the 1st value is true, it returns the 2nd value
'hi' && 'hello' //returns 'hello'
LOGIC: If the 1st value is true, I still need to check if the other value is true too since both need to be true

• || OR: Either one of the values have to be true

//If the 1st value is true, it returns the 1st value. 
'hi' || 'hello' //returns 'hi'
//If the 1st value is false, it returns the 2nd value:
0 || null //returns null
LOGIC: I need either or -- so if the 1st value is true, return immediately - no need to check the other value

• ?? Nullish Coalescing: Error handling for values that are null or undefined

//If the 1st value is null or defined, use the 2nd value
null ?? 'hi' //returns 'hi'
undefined ?? 'hi' //returns 'hi'
0 ?? 'hi' //returns 0

! Not/Bang Operator: turns a true expression into a false expression (and vice versa)

console.log(!null) //true -> null is falsey, but is now truthy
console.log(!5) //false --> 3 is truthy, but is now falsey

• !!: Forces an expression into its actual boolean value of true or false

console.log(!!null) //false 
console.log(!!5) //true 

Q. What is implicit and explicit conversion?

• Implicit conversion: indirectly change a variable’s type

//Change type by reassignment
let x = 15; //was a number
x = 'hello'; //is now a string
//Add a string to a number
"4" + 3 = 43 
//Loose comparison of string and number
"13" == 13 //true

• Explicit conversion: directly change a variable’s type

a) Convert number to a string

//Convert number to a string
let number = 12.5634;
let string1 = number.toString(); //"12.5634"
let string2 = number.toFixed(2); //"12.56"

b) Convert string to a number

//Convert string to a number
let string = "365.3986";
let number1 = parseInt(string); //365
let number2 = parseFloat(string); //365.3986