jest unit test 基本比對

Common Matchers

toBe

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

toBe 算是精準比對，所以比對 obj 裡面的值

toEqual

它可以比對 obj or array

---

Truthiness

• 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

Numbers

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);
});

or 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.
});

Strings

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:

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.

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
});