Forem: Esraa Refaat

Javascript Questions

Esraa Refaat — Fri, 25 Jun 2021 02:20:59 +0000

Is this a pure function?

function sum(a, b) {
return a + b;
}

A: Yes
B: No

Answer: A

A pure function is a function that always returns the same result, if the same arguments are passed.

The sum function always returns the same result. If we pass 1 and 2, it will always return 3 without side effects. If we pass 5 and 10, it will always return 15, and so on. This is the definition of a pure function.

Javascript Questions

Esraa Refaat — Wed, 23 Jun 2021 23:43:36 +0000

let a = 3;
let b = new Number(3);
let c = 3;

console.log(a == b);
console.log(a === b);
console.log(b === c);

A: true false true
B: false false true
C: true false false
D: false true true

Answer: C

new Number() is a built-in function constructor. Although it looks like a number, it's not really a number: it has a bunch of extra features and is an object.

When we use the == operator, it only checks whether it has the same value. They both have the value of 3, so it returns true.

However, when we use the === operator, both value and type should be the same. It's not: new Number() is not a number, it's an object. Both return false.

Javascript operators (part 1)

Esraa Refaat — Wed, 23 Jun 2021 01:55:00 +0000

Operators in Javascript has three categories:

1- unary operators
One operand
X

2- binary operators

Two operands
X Y

3- ternary operators
Three operands
X Y Z

unary operators :
increment
X++
decrement
X--
Keep in mind
prefix ++X
postfix X++

binary operators :

arthimatic operators : X+Y X-Y X*Y X/Y X%Y
assignment operators :
X+=y ➡️ X=X+Y
(-=)(*=)(+=)(/=)(%=)
comparison operators :
(==) ( > ) ( < )

Javascript Questions

Esraa Refaat — Tue, 22 Jun 2021 14:33:46 +0000

What's the output?

let c = { greeting: 'Hey!' };
let d;

d = c;
c.greeting = 'Hello';
console.log(d.greeting);

A: Hello
B: Hey!
C: undefined
D: ReferenceError
E: TypeError

Answer: A

In JavaScript, all objects interact by reference when setting them equal to each other.

First, variable c holds a value to an object. Later, we assign d with the same reference that c has to the object.

When you change one object, you change all of them.

Javascript Questions

Esraa Refaat — Mon, 21 Jun 2021 13:05:48 +0000

💡 for (var i = 0; i < 3; i++)
{
setTimeout(() =>
console.log(i), 1);
}

for (let i = 0; i < 3; i++) {
setTimeout(() => console.log(i), 1);
}

A: 0 1 2 and 0 1 2
B: 0 1 2 and 3 3 3
C: 3 3 3 and 0 1 2

Answer: C
Because of the event queue in JavaScript, the setTimeout callback function is called after the loop has been executed. Since the variable i in the first loop was declared using the var keyword, this value was global. During the loop, we incremented the value of i by 1 each time, using the unary operator ++. By the time the setTimeout callback function was invoked, i was equal to 3 in the first example.

In the second loop, the variable i was declared using the let keyword: variables declared with the let (and const) keyword are block-scoped (a block is anything between { }). During each iteration, i will have a new value, and each value is scoped inside the loop.

Javascript variables and data types

Esraa Refaat — Mon, 21 Jun 2021 02:54:19 +0000

Variable: containers that hold values
var x = 10 ;
var x is variable
10 is value
variable is loosely typed
Js data types are :
1-undefined
2-number
3-string
4-boolean
5-null
Any variable has undefined data type as initial value until you assign value to it

You can know the type of the variable by using typeof
console.log(typeof x)