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
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