---
id: 672d26917a8ab3220c038a42
title: How Do Comparisons Work with Null and Undefined Data Types?
challengeType: 19
dashedName: how-do-comparisons-work-with-null-and-undefined-data-types
---

# --interactive--

In JavaScript, `null` and `undefined` are two distinct data types that represent the absence of a value, but they behave differently in comparisons. Understanding how these types interact in various comparison scenarios is crucial for writing robust and bug-free code.

Let's start with the `undefined` type. A variable is `undefined` when it has been declared but hasn't been assigned a value. It's the default value of uninitialized variables and function parameters that weren't provided an argument.

The `null` type, on the other hand, is an assignment value that represents a deliberate non-value. It's often used to indicate that a variable intentionally has no value.

When comparing `null` and `undefined` using the equality operator (`==`), JavaScript performs type coercion. This means it tries to convert the operands to the same type before making the comparison. In this case, `null` and `undefined` are considered equal:

:::interactive_editor

```js
console.log(null == undefined); // true
```

:::

However, when using the strict equality operator (`===`), which checks both value and type without performing type coercion, `null` and `undefined` are not equal:

:::interactive_editor

```js
console.log(null === undefined); // false
```

:::

This difference is important to keep in mind when writing conditional statements or performing equality checks in your code. When comparing `null` or `undefined` with other values using the equality operator (`==`), the behavior can be unexpected. For example:

:::interactive_editor

```js
console.log(null == 0);  // false
console.log(null == ''); // false
console.log(undefined == 0); // false
console.log(undefined == ''); // false
```

:::

These comparisons return `false` because `null` and `undefined` are only equal to each other (and themselves) when using the equality operator. The behavior of `null` in other comparisons is particularly tricky:

:::interactive_editor

```js
console.log(null > 0);  // false
console.log(null == 0); // false
console.log(null >= 0); // true
```

:::

`undefined`, on the other hand, always converts to `NaN` in numeric contexts, which makes all numeric comparisons with `undefined` return `false`:

:::interactive_editor

```js
console.log(undefined > 0);  // false
console.log(undefined < 0);  // false
console.log(undefined == 0); // false
```

:::

Given these nuances, it's generally recommended to use the strict equality operator when comparing values, especially when dealing with `null` and `undefined`. This approach helps avoid unexpected type coercion and makes your code's behavior more predictable.

In summary, while `null` and `undefined` are both used to represent the absence of a value, they behave differently in comparisons. Understanding these differences is key to writing clear and error-free JavaScript code.

# --questions--

## --text--

What is the result of the expression: `null === undefined`?

## --answers--

`true`

### --feedback--

Think about what the strict equality operator (`===`) checks for.

---

`false`

---

`undefined`

### --feedback--

Think about what the strict equality operator (`===`) checks for.

---

`null`

### --feedback--

Think about what the strict equality operator (`===`) checks for.

## --video-solution--

2

## --text--

In JavaScript, what is the result of the comparison: `null >= 0`?

## --answers--

`true`

---

`false`

### --feedback--

Consider how `null` behaves in numeric comparisons.

---

`undefined`

### --feedback--

Consider how `null` behaves in numeric comparisons.

---

`Error`

### --feedback--

Consider how `null` behaves in numeric comparisons.

## --video-solution--

1

## --text--

Which of the following statements about `undefined` is correct?

## --answers--

`undefined == null` evaluates to `false`.

### --feedback--

Think about how `undefined` behaves in different types of comparisons.

---

`undefined === null` evaluates to `true`.

### --feedback--

Think about how `undefined` behaves in different types of comparisons.

---

`undefined < 0` evaluates to `true`.

### --feedback--

Think about how `undefined` behaves in different types of comparisons.

---

`undefined == 0` evaluates to `false`.

## --video-solution--

4
