# JavaScript: Truthy and Falsy Values Explained

> false, 0, empty strings, null, undefined, NaN, and document.all are JavaScript's falsy values; every other value, including empty arrays, is truthy

**URL:** https://sentry.io/answers/truthy-and-falsy-values-in-javascript/

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## The problem

Let's say we have an online store that has recently introduced a loyalty points system. We store users as JSON objects, and only newly created users have the `loyaltyPoints` field. Consider the following code:

```javascript
const user1 = {
  name: "Jane Doe",
  email: "jane.doe@example.com"
}; // old user

const user2 = {
  name: "John Smith",
  email: "john.smith@example.com",
  loyaltyPoints: 0
}; // new user

function displayLoyaltyPoints(user) {
  if (user.loyaltyPoints) {
    console.log(`User has ${user.loyaltyPoints} loyalty points`);
  } else {
    console.log("Field 'loyaltyPoints' not defined for user.");
  }
}

displayLoyaltyPoints(user1);
displayLoyaltyPoints(user2);
```

Based on a cursory reading of this code, we might expect the function `displayLoyaltyPoints` to follow the `else` path for our first user and the `if` path for our second user. However, if we run this code, we'll see that it follows the `else` path in both cases, even though `loyaltyPoints` is defined for the second user.

Why does this happen and how can we fix our code?

## The solution

When non-boolean values are used in a boolean context, such as the condition of an `if` statement, they will be [coerced into either `true` or `false`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Boolean#boolean_coercion). Values that are coerced into `true` are called [truthy](https://developer.mozilla.org/en-US/docs/Glossary/Truthy) and values that are coerced into `false` are called [falsy](https://developer.mozilla.org/en-US/docs/Glossary/Falsy).

JavaScript contains the following falsy values:

* `false`
* `0`, `-0` and `0n`
* `""`, `''`  (empty strings)
* `null`, `undefined` and `NaN`
* `document.all`

All other values are truthy.

Because `0` is a falsy value, the `if` condition in our `displayLoyaltyPoints()` function will evaluate to `false` when provided with a user whose `loyaltyPoints` property is set to `0`. The `if` condition will also evaluate to false if the `loyaltyPoints` field does not exist (making it `undefined`).

We can fix our code by explicitly checking that `user.loyaltyPoints` does not equal `undefined` instead of evaluating the value's truthiness.

```javascript
function displayLoyaltyPoints(user) {
  if (user.loyaltyPoints !== undefined) {
    console.log(`User has ${user.loyaltyPoints} loyalty points`);
  } else {
    console.log("Field 'loyaltyPoints' not defined for user.");
  }
}
```

Some values can be truthy without also being [loosely equal](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Equality) to true or falsy without being loosely equal to false. For example, both `if` statements in the code below will evaluate to `true`:

```javascript
if ([]) {
  console.log("[] is truthy");
}

if ([] == false) {
  console.log("[] is loosely equal to false");
}
```

To avoid the pitfalls associated with boolean coercion, constructions such as `if (variableName)` should only be used when `variableName` is known to be a boolean value. For variables of other types, an explicit comparison is preferred. For example, `if (variableName > 0)` or `if (variableName != undefined)`.

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*Source: [sentry.io/answers/truthy-and-falsy-values-in-javascript/](https://sentry.io/answers/truthy-and-falsy-values-in-javascript/)*