Truthy and Falsy values in Javascript

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