Assertions

toBe vs toEqual vs toStrictEqual

toBe is a primitive comparison.

• use for string.

toEqual is a deep comparison.

• use for array and objects.

Two identical objects can have identical toEqual but different toStrictEqual or toBe

• toStrictEqual tests reference
• toBe tests Object.is()
  • MDN Link
• toEqual tests values

Rule: avoid toBe and prefer toStrictEqual

const a = { id: "1" };

expect(a).toEqual({ id: "1" }); // Pass
expect(a).toStrictEqual({ id: "1" }); // Fail

// `b` points to memory location of `a`
const b = a;

expect(a).toEqual(b); // Pass
expect(a).toStrictEqual(b); // Pass

Special Assertions

// assertion to be implemented later
it.todo()

// run specified assertion exclusively
describe.only()
it.only()

// positional injector parameter
// %p is inserted to each assertion description
describe.each([1, 2, 3])("each element %p exists", (el) => {
  test(`returns ${el}`, () => {
    expect(el).toBe(true);
  })
})

toHaveBeenCalledWith

expect(someObject).toHaveBeenCalledWith(
  expect.objectContaining({
   "action": "PublicationPage", 
   "category": "PublicationPage", 
   "label": "7",
   "name": "n/a"
  })
)

Object field comparison

Matches a subset of the properties of an object:

// toMatchObject
expect(someObject).toMatchObject({
    active: true,
    data: expect.any(Array),
    ID: expect.any(String)
});


// works with nested fields
expect.objectContaining({
  field: "specific value",
  id: expect.anything(),
  data: expect.objectContaining({
    nestedField: expect.any(Array),
  }),
  ID: expect.any(String)
})

// field-level comparison
expect(someObj.fieldOne).toEqual(true)
expect(someObj.fieldTwo).toEqual(expect.any(Array))
expect(someObj.fieldThree).toEqual(expect.any(String))