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---
id: using-matchers
title: Using Matchers
---
Jest uses "matchers" to let you test values in different ways. This document will introduce some commonly used matchers. For the full list, see the [`expect` API doc](ExpectAPI.md).
## Common Matchers
The simplest way to test a value is with exact equality.
```js
test('two plus two is four', () => {
expect(2 + 2).toBe(4);
});
```
In this code, `expect(2 + 2)` returns an "expectation" object. You typically won't do much with these expectation objects except call matchers on them. In this code, `.toBe(4)` is the matcher. When Jest runs, it tracks all the failing matchers so that it can print out nice error messages for you.
`toBe` uses `Object.is` to test exact equality. If you want to check the value of an object, use `toEqual`:
```js
test('object assignment', () => {
const data = {one: 1};
data['two'] = 2;
expect(data).toEqual({one: 1, two: 2});
});
```
`toEqual` recursively checks every field of an object or array.
:::tip
`toEqual` ignores object keys with `undefined` properties, `undefined` array items, array sparseness, or object type mismatch. To take these into account use `toStrictEqual` instead.
:::
You can also test for the opposite of a matcher using `not`:
```js
test('adding positive numbers is not zero', () => {
for (let a = 1; a < 10; a++) {
for (let b = 1; b < 10; b++) {
expect(a + b).not.toBe(0);
}
}
});
```
## Truthiness
In tests, you sometimes need to distinguish between `undefined`, `null`, and `false`, but you sometimes do not want to treat these differently. Jest contains helpers that let you be explicit about what you want.
- `toBeNull` matches only `null`
- `toBeUndefined` matches only `undefined`
- `toBeDefined` is the opposite of `toBeUndefined`
- `toBeTruthy` matches anything that an `if` statement treats as true
- `toBeFalsy` matches anything that an `if` statement treats as false
For example:
```js
test('null', () => {
const n = null;
expect(n).toBeNull();
expect(n).toBeDefined();
expect(n).not.toBeUndefined();
expect(n).not.toBeTruthy();
expect(n).toBeFalsy();
});
test('zero', () => {
const z = 0;
expect(z).not.toBeNull();
expect(z).toBeDefined();
expect(z).not.toBeUndefined();
expect(z).not.toBeTruthy();
expect(z).toBeFalsy();
});
```
You should use the matcher that most precisely corresponds to what you want your code to be doing.
## Numbers
Most ways of comparing numbers have matcher equivalents.
```js
test('two plus two', () => {
const value = 2 + 2;
expect(value).toBeGreaterThan(3);
expect(value).toBeGreaterThanOrEqual(3.5);
expect(value).toBeLessThan(5);
expect(value).toBeLessThanOrEqual(4.5);
// toBe and toEqual are equivalent for numbers
expect(value).toBe(4);
expect(value).toEqual(4);
});
```
For floating point equality, use `toBeCloseTo` instead of `toEqual`, because you don't want a test to depend on a tiny rounding error.
```js
test('adding floating point numbers', () => {
const value = 0.1 + 0.2;
//expect(value).toBe(0.3); This won't work because of rounding error
expect(value).toBeCloseTo(0.3); // This works.
});
```
## Strings
You can check strings against regular expressions with `toMatch`:
```js
test('there is no I in team', () => {
expect('team').not.toMatch(/I/);
});
test('but there is a "stop" in Christoph', () => {
expect('Christoph').toMatch(/stop/);
});
```
## Arrays and iterables
You can check if an array or iterable contains a particular item using `toContain`:
```js
const shoppingList = [
'diapers',
'kleenex',
'trash bags',
'paper towels',
'milk',
];
test('the shopping list has milk on it', () => {
expect(shoppingList).toContain('milk');
expect(new Set(shoppingList)).toContain('milk');
});
```
## Exceptions
If you want to test whether a particular function throws an error when it's called, use `toThrow`.
```js
function compileAndroidCode() {
throw new Error('you are using the wrong JDK!');
}
test('compiling android goes as expected', () => {
expect(() => compileAndroidCode()).toThrow();
expect(() => compileAndroidCode()).toThrow(Error);
// You can also use a string that must be contained in the error message or a regexp
expect(() => compileAndroidCode()).toThrow('you are using the wrong JDK');
expect(() => compileAndroidCode()).toThrow(/JDK/);
// Or you can match an exact error message using a regexp like below
expect(() => compileAndroidCode()).toThrow(/^you are using the wrong JDK$/); // Test fails
expect(() => compileAndroidCode()).toThrow(/^you are using the wrong JDK!$/); // Test pass
});
```
:::tip
The function that throws an exception needs to be invoked within a wrapping function otherwise the `toThrow` assertion will fail.
:::
## And More
This is just a taste. For a complete list of matchers, check out the [reference docs](ExpectAPI.md).
Once you've learned about the matchers that are available, a good next step is to check out how Jest lets you [test asynchronous code](TestingAsyncCode.md).
|
