We are going to discuss an important javascript topic for interviews that can help you also understand a basic of Javascript.

Truthy and Falsy values

In javascript when you declare a variable with some value except 0 Javascript should consider this as a true value and if the value is 0 Javascript will consider this as false.

const score = 20;

if(score){
console.log("It is true")
}
else{
console.log("It is false")
}
// Output : It is true
const duck = 0;

if(duck){
console.log("It is true")
}
else{
console.log("It is false")
}
// Output : It is false

So Javascript will consider 0 as false and other values as true.

If you declare a string with its length < 0 then javascript will consider this as true, otherwise if you declare an empty string it will be considered as false.

const name1 = "Alvee";

if(name1){
console.log("It is true")
}
else{
console.log("It is false")
}
// Output : It is true
const name2 = "";

if(name2){
console.log("It is true")
}
else{
console.log("It is false")
}
// Output : It is false

If you don't define a variable Javascript will consider it as false.

let name;

if(name){
console.log("It is true")
}
else{
console.log("It is false")
}
// Output : It is false

If you define a variable as Null Javascript will consider it as false.

let name = null;

if(name){
console.log("It is true")
}
else{
console.log("It is false")
}
// Output : It is false

If you also define a variable with NaN Javascript will consider it as false.

let value = NaN ;

if(value){
console.log("It is true")
}
else{
console.log("It is false")
}
// Output : It is false

We saw that the empty string was false, but if you declare an empty array or object it will be considered as true value.

const array = [];

if(array){
console.log("It is true")
}
else{
console.log("It is false")
}
// Output : It is true

const object = {};

if(object){
console.log("It is true")
}
else{
console.log("It is false")
}
// Output : It is true

Besides all these, if you declare a value as false, there what can Javascript do!

const value = false;

if(value){
console.log("It is true")
}
else{
console.log("It is false")
}
// Output : It is false

So we can say that if you declare a variable with the value of undefined, null, NaN, 0, "" the output will be false.

The following values are always falsy:

• false
• 0 (zero)
• '' or "" (empty string)
• null
• undefined
• NaN

Everything else is truthy. That includes:

• '0' (a string containing a single zero)
• 'false' (a string containing the text “false”)
• {} (an empty object)
• function(){} (an “empty” function)

you can also check out this article I found informative