Matcherを使用する

Jest では、マッチャー ("matcher") を使用して、様々な方法で値のテストをすることができます。 このドキュメントでは、よく使われるマッチャーをいくつか紹介します。 For the full list, see the expect API doc.

一般的なマッチャー

値をテストする最も簡単な方法は、厳密に等価であることです。

test('two plus two is four', () => {
  expect(2 + 2).toBe(4);
});

In this code, expect(2 + 2) returns an "expectation" object. マッチャーを利用しなければこれらの "expectation" オブジェクトは大きな効果を発揮しません。 In this code, .toBe(4) is the matcher. Jestが実行されると、失敗したマッチャーをすべて追跡し、素晴らしいエラーメッセージを表示することができます。

toBe uses Object.is to test exact equality. If you want to check the value of an object, use toEqual:

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:

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);
    }
  }
});

真偽値(およびそれらしく思える値)

In tests, you sometimes need to distinguish between undefined, null, and false, but you sometimes do not want to treat these differently. Jestには、(それらのケースで) 求められるものを明確に実装したヘルパーが備わっています。

• 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

例：

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();
});

あなたは、あなたのコードがしたいことに最も正確に対応するマッチャーを使うべきです。

数値

数値の比較するほとんどの方法について、対応するマッチャーがあります。

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.

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.
});

文字列

You can 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/);
});

配列と反復可能なオブジェクト

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');
});

例外

If you want to test whether a particular function throws an error when it's called, use 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 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.

その他

これはほんの一部です。 For a complete list of matchers, check out the reference docs.

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.