Truthy and Falsy in JavaScript & How to Toggle Between?

Tips and Tricks to improve the way you write JavaScript.

Photo by Aarohi Achwal from Parenting Firstcry

What is Falsy?

In simple terms, it’s any value that evaluates to false, nullable, or empty.

• False => Falsy
• Null => Falsy
• Undefined => Falsy
• Empty String => Falsy
• Length of an array with no items inside => Falsy
• Length of an object without keys (Object.keys(obj).length === 0) => Falsy
• Zero => Falsy

What this means is that if you have a variable:

const name = '';

And a condition where you want to check the variable:

if (CHECK IF NAME IS EMPTY) {
  throw new Error('Missing name');
}

You do not need to compare it to an empty string or null or undefined:

// Unnecessary
if (name === '' || name === undefined || name === null) { ... }

Or convert it to a boolean to validate if it’s Falsy (as you’d in other strongly typed languages).

Instead, consider it as a Falsy value (like a boolean) and just put ! prefix and JavaScript will evaluate it for you:

// Simpler
if (!name) {
  throw new Error('Missing name');
}

What is Truthy? Well, the opposite of Falsy:

• True
• Number greater or less than zero
• String with at least one character
• Array or an Object with at least one item
• Variable with a value, e.g. (‘A’, 1, {}, [])

Variety of Outcomes

// name = '';
!name === !false // True

// name = undefined;
!name === !false // True

// name = null;
!name === !false // True

// name = [];
!name.length === !false // True

// name = 0
!name === !false // True

Likewise, if the outcome is the opposite, it will evaluate to a Truthy value.

// sport = 'Football';
!sport === !true // True

// sport = '    ';
!sport === !true // True

// sport = ['Football'];
!sport.length === !true // True

// sport = 1
!sport === !true // True

A common way to toggle between truthy and falsy values is to use the logical operators:

• And (&&)
• Or (||)

&&

When setting up a condition between values, the && operator will capture a value that evaluates to False.
If either value in a condition is Falsy, the output will be false:

true && false // False
false && true // False 
false && false // False

Only if both values are Truthy, the && operator will evaluate to true:

true && true // True

Keep in mind that && will evaluate all values in a condition until it finds the first Falsy value. Otherwise, it will take the last value.

true && 'Hello' && 100; // 100 (Truthy value)

Validate user input

If Either condition fails, the statement will be evaluated as False.

const email = 'hello';

if (email.includes('@') && email.includes('.com') {
  console.log('Email is valid!');
} else {
  console.log('Email is invalid!');
}

Display dynamic content

In Web frameworks like React, you can display HTML elements (or components) only when a certain condition is satisfied.

<div>
  { conditionIsMet && <h1>Hello World</h1> }
</div>

Avoid nullable values

A common mistake that developers make is to attempt to read a value from a property whose parent is null/undefined.

const array = [];
console.log(array.length); ✔️

const newArray = null;
console.log(newArray.length); ❌ // Exception

To solve this, you first make sure that the root property is not null/undefined and then try to read the contents of a given property.

if (newArray !== null && newArray.length === 0) {
  console.log('newArray is empty!');
}

||

The logical OR (||) operator works the opposite of the && operator. It looks for any value in a condition that results in Truthy and takes that value.

true || true // True
true || false // True
false || true // True

Only if both values are Falsy, the outcome will be false:

false || false // False

|| will evaluate all values in a condition until it finds the first Truthy value. Otherwise, it will take the last value.

false || undefiend || null // Null

Searching for a selection of items

The || operator can be used to filter items that meet one condition or another.

const clothes = [
  {
    type: 'Shirt',
    color: 'Black',
    size: 'XL',
    price: 0
  },
  {
    type: 'Shirt',
    color: 'White',
    size: 'XL',
    price: 100
  },
  {
    type: 'Skirt',
    color: 'Red',
    size: 'L',
    price: 50
  }
];

const favClothes = clothes
  .filter(item => item.color === 'Black' || item.color === 'White');

console.log('favClothes :>> ', favClothes);
/*
  [
    { type: 'Shirt', color: 'Black', size: 'XL' },
    { type: 'Shirt', color: 'White', size: 'XL' }
  ]
*/

Set default values

If one expression turns out Falsy, you can use the || operator to set up a default value:

function findHero(hero) {
  const defaultHero = 'Goku';
  const myHero = hero || defaultHero;

  return heroes.find(h => h.name === myHero);
}

Multiple Operators

When you have multiple && & || operators in a single statement, the rule of precedence applies. The first two conditions will be evaluated and then the outcome of that against the third value;

console.log(true && null || false ); // False
// #1 true && null (where null is Falsy) === Null
// #2 null || false === False

console.log(false || 100 && undefined ); // Undefined
// #1 false || 100 (where 100 is Truthy) === 100
// #2 100 && undefined (truthy && falsy) === Undefined

console.log(null || 0 && '' || true); // True
console.log(null || 0 && '' && false); // 0

The ES2020 introduced a new operator, the Nullish Coalescing, also known as the Elvis operator.

const possiblyNull = null;

const result = possiblyNull ?? 'Default Value';

console.log(result); // 'Default Value'

const possiblyUndefined = undefined;

const result = possiblyUndefined ?? 'Default Value';

console.log(result); // 'Default Value'

It works similarly to the || operator. However, while || applies to all falsy values, the Nullish Coalescing operator is limited to only null and undefined values.
If used against other types, it will just return the first value:

false ?? true // False
true ?? false // True

Another way to avoid nullable properties is to use the Optional Chaining operator.

Instead of repeating yourself:

if (newArray !== null && newArray.length === 0) {
  console.log('newArray is empty!');
}

You can apply the Optional Chaining:

if (newArray?.length === 0) {
  console.log('newArray is empty!');
}

What this will do is:

• Evaluate if newArray is nullable
  If that’s the case, the condition will be evaluated as null/false which will close the statement (go to the else case).
• Check the length of newArray (after already evaluating newArray)

And it’s much cleaner.
Also works for as many properties as you have:

const name = currentUser?.username?.name?.firstName;

You can combine the Optional Chaining with other operators as well:

const name = user?.username ?? 'Default Username';
console.log(name);

The ‘Default Username’ will print if:

• user is null or undefined
• user.username is null or undefined

Alternatively, assuming the user property is already defined (has value), you can assign a default value to user.username using the Object Destructuring:

const { username = 'Default Username' } = user;
console.log(name);

Where username will equal to ‘Default Username’ only if it’s undefined or null.

A common inline way to toggle between truthy and falsy values is to use the Ternary Operator. It works like:

• If the condition evaluates to true (a)
• The outcome will be the first option (b)
• Otherwise, the outcome will be the latter option (c)

const isGreaterThan = 10 > 1 ? true : false;

console.log(isGreaterThan); // True

//

const isLesserThan = 10 < 1 ? true : false;

console.log(isLesserThan); // False

The latest addition to JavaScript is a set of Logical Assignment operators. Think of everything you’ve learned so far combined with an ability to assign values on the fly.

So instead of doing:

function setHero(myHero) {
  myHero = myHero ?? 'Goku';
  return myHero;
}

const result = setHero();
console.log(result) // 'Goku'

You’d do:

function setHero(myHero) {
  myHero ??= 'Goku';
  return myHero;
}

const result = setHero();
console.log(result) // 'Goku'

The motivation behind this is to reduce the extra step of needing to check values and then assign. Now all of that comes at once.

For other Falsy scenarios, you can use the good old || operator:

function setSalary(mySalary) {
  const minimum_salary = 1000;
  mySalary ||= minimum_salary;
  return mySalary;
}

const result = setSalary();
console.log(result) // 1000

This also applies to more complex scenarios:

const cybertruck = { 
  topSpeedInKmh: 180, 
  maximumBatteryLife: null, 
  towingCapacity: undefined
};

const defaultTopSpeed = 100;
const defaultBatteryCapacity = '122.4 kWh'
const defaultTowingCapacity = '11 000 pounds'

cybertruck.topSpeedInKmh ||= defaultMaxSpeed; // takes first truthy value
cybertruck.maximumBatteryLife ??= defaultBatteryCapacity; // avoids null
cybertruck.towingCapacity &&= defaultTowingCapacity; // takes falsy value

console.log(cybertruck.topSpeedInKmh); // 180
console.log(cybertruck.maximumBatteryLife); // '122.4 kWh'
console.log(cybertruck.towingCapacity); // undefined

Implicit Evaluation

If a value evaluates to a truthy or a falsy value, you do not need to explicitly say that the outcome is either true or false.

#1

# Explicit

if (value === true) { ... }

# Implicit

if (value) { ... }

#2

# Explicit

const isGreaterThan = 10 > 1 ? true : false;

# Implicit

const isGreaterThan = 10 > 1;

#3

# Explicit

const freeItem = clothes.find(item => item.price === 0);

# Implicit

const freeItem = clothes.find(item => !item.price);
// { type: 'Shirt', color: 'Black', size: 'XL', price: 0 }

Convert to Boolean

You can also explicitly convert a value into a boolean using a pair of exclamation marks.

const greeting = 'Hello';
const truthy = !!greeting; // True

const empty = '';
const falsy = !!empty; // False

Alternatively, you can use the Boolean() constructor:

const greeting = 'Hello';
const empty = '';

console.log(Boolean(greeting)); // True
console.log(Boolean(empty)); // False

Read More of my blogs on JavaScript:

Node.js: The Big Picture

Extending Core classes in JavaScript, TypeScript & C# .NET

Many ways of using Destructuring in Modern JavaScript