.toStrictEqual(value)

Use .toStrictEqual to test that objects have the same types as well as structure.

Differences from .toEqual:

• Keys with undefined properties are checked. e.g. {a: undefined, b: 2} does not match {b: 2} when using .toStrictEqual.
• Array sparseness is checked. e.g. [, 1] does not match [undefined, 1] when using .toStrictEqual.
• Object types are checked to be equal. e.g. A class instance with fields a and b will not equal a literal object with fields a and b.

class LaCroix {
  constructor(flavor) {
    this.flavor = flavor;
  }
}
describe('the La Croix cans on my desk', () => {
  test('are not semantically the same', () => {
    expect(new LaCroix('lemon')).toEqual({flavor: 'lemon'});
    expect(new LaCroix('lemon')).not.toStrictEqual({flavor: 'lemon'});
  });
});