Jest Tutorial For Beginners: Jest Matchers [2/4]
Tags: webdev, javascript, jest, beginners
Published: 2022-06-25 08:23:03

## Introduction
Jest uses "matchers" to let you test values in different ways. 
Most of the information in this part is taken from the <a href="https://jestjs.io/docs/using-matchers">Jest docs</a>.

##Common Matchers
The simplest way to test a value is with exact equality.
```
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 to test exact equality. If you want to check the value of an object, use `toEqual` instead:
```
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.

You can also test for the opposite of a matcher:
```
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);
    }
  }
});
```

###1. Truthiness
In tests, you sometimes need to distinguish between undefined, null, and false. 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:
```
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();
});
```
Note: use matcher that most precisely corresponds to what you want your code to be doing.

###2. Numbers
The jest provides different matchers for comparing numbers.
Example:
```
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 <a href="https://0.30000000000000004.com/">to understand this error click here</a>.
```
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.
});
```

###2. Strings
jest provides the facility to check strings against regular expressions with `toMatch`:
```
test('there is no I in team', () => {
  expect('team').not.toMatch(/I/);
});

test('but there is a "stop" in Christoph', () => {
  expect('Christoph').toMatch(/stop/);
});
```

###3. Arrays and iterables
You can check if an array or iterable contains a particular item using `toContain`:
```
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');
});
```

###4. Exceptions
If you want to test whether a particular function throws an error when it's called, the jest provides the matcher `toThrow`.
```
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 the exact error message or a regexp
  expect(() => compileAndroidCode()).toThrow('you are using the wrong JDK');
  expect(() => compileAndroidCode()).toThrow(/JDK/);
});
```
**Note:** the function that throws an exception needs to be invoked within a wrapping function otherwise the `toThrow` assertion will fail.

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