Falsy Values in JavaScript

Description

A falsy value is something which evaluates to FALSE, for instance when checking a variable. There are only six falsey values in JavaScript: undefined, null, NaN, 0, "" (empty string), and false of course.

Checking for falsy values on variables

It is possible to check for a falsy value in a variable with a simple conditional:

if (!variable) {
  // When the variable has a falsy value the condition is true.
}

General Examples

var string = ""; // <-- falsy

var filledString = "some string in here"; // <-- truthy

var zero = 0; // <-- falsy

var numberGreaterThanZero // <-- truthy

var emptyArray = []; // <-- truthy, we'll explore more about this next

var emptyObject = {}; // <-- truthy

Fun With Arrays

if ([] == false) // <-- truthy, will run code in if-block

if ([]) // <-- truthy, will also run code in if-block

if ([] == true) // <-- falsy, will NOT run code in if-block

if (![]) // <-- falsy, will also NOT run code in if-block

Caveat

Be aware of the data type when evaluating a value in a Boolean context. If the data type of the value is meant to be a number, the truthy/falsy evalution can result in an unexpected outcome:

const match = { teamA: 0, teamB: 1 }
if (match.teamA)
  // The following won't run due to the falsy evaluation
  console.log('Team A: ' + match.teamA);
}

An alternative to the use case above is to evaluate the value using typeof:

const match = { teamA: 0, teamB: 1 }
if (typeof match.teamA === 'number')
  console.log('Team A: ' + match.teamA);
}

More Information

• truthy | Blog Post - Truthy & Falsey
• Falsy | Glossary | MDN
• Truthy and Falsy: When All is Not Equal in JavaScript