Do you know that Javascript has a set of pre-defined falsy values?

Truthy and Falsy values are the non-boolean values that are coerced to true or false when performing certain operations.

The following are the only six values that are considered as falsy values in JavaScript.

false
undefined
null
""
NaN
0

Anything that is not present in the above list is considered as truthy.
So if you are writing an if condition, you don’t need to check if something is empty or undefined or null. It will automatically be considered as false.



const value = "";
// this condition is enough and no need to check value == ""
if(value) {
  console.log(value); // This code will not be executed
}

const nullValue = null;
// this condition is enough and no need to check value === null
if(nullValue) {
   console.log(nullValue); // This code will not be executed
}

let sum;
// this condition is enough and no need to check value === undefined
if(sum) {
   console.log(sum); // This code will not be executed
}

An easy way of checking if a value is truthy or falsy is by passing it to the Boolean constructor function.



Boolean(NaN) // false
Boolean([]) // true
Boolean({}) // true

You can turn any value into its truthy or falsy boolean value by flipping it twice:



let number1;
console.log(!!number1); // false

const number2 = 10;
console.log(!!number2); // true

const name1 = 'Tim';
console.log(!!name1); // true

const name2 = '';
console.log(!!name2); // false

const nullValue = null;
console.log(!!nullValue); // false

This has some practical applications too.

Applying Truthy and Falsy Values

Suppose, you are parsing a CSV file and each column contains comma-separated names, numbers, and some empty values. There are some blank spaces, so when you parse each line, you might get something like this:



const parsedData = 'David,Mike,Tim,,John, 10,, Jonathan';

And you want to print only values which are valid, so in this case you can do something like this



const parsedData = 'David,Mike,Tim,,John, 10,, Jonathan';
console.log(
 parsedData
  .split(',')
  .map(value => value.trim())
  .filter(value => !!value)
  .join(' ')
); // David Mike Tim John 10 Jonathan

Which can be further simplified as



const parsedData = 'David,Mike,Tim,,John, 10,, Jonathan';
console.log(
 parsedData
  .split(',')
  .map(value => value.trim())
  .filter(Boolean)
  .join(' ')
); // David Mike Tim John 10 Jonathan

If you have an array of different values like



const arr = [10, 20, "raj", 0, [], '', NaN, 3, undefined, 50, null, 89];

Then, you can get valid values as



const arr = [10, 20, 'raj', 0, [], '', NaN, 3, undefined, 50, null, 
89];
console.log(arr.filter(Boolean)); //[ 10, 20, 'raj', [], 3, 50, 89 ]

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JavaScript Basics: Truthy and Falsy values in JavaScript