node.jsjestjs

How to assert data type with Jest

I'm using Jest to test my Node application.

Is it possible for me to expect/assert a value to be a Date Object?

expect(typeof result).toEqual(typeof Date())

Was my attempt, but naturally returns [Object]. And so this would pass {} too.

Solution

  • for Jest with newer version > 16.0.0:

    There is a new matcher called toBeInstanceOf. You can use the matcher to compare instances of a value.

    Example:

    expect(result).toBeInstanceOf(Date)

    for Jest with version < 16.0.0:

    Use instanceof to prove whether the result variable is a Date Object or Not.

    Example:

    expect(result instanceof Date).toBe(true)

    Another example to match common types:

    boolean

    expect(typeof target).toBe("boolean")

    number

    expect(typeof target).toBe("number")

    string

    expect(typeof target).toBe("string")

    array

    expect(Array.isArray(target)).toBe(true)

    object

    expect(target && typeof target === 'object').toBe(true)

    null

    expect(target === null).toBe(true)

    undefined

    expect(target === undefined).toBe(true)

    function

    expect(typeof target).toBe('function')

    Promise or async function

    expect(!!target && typeof target.then === 'function').toBe(true)

    Another example to match further types:

    float (decimal number. i.e. 3.14, 137.03, etc.)

    expect(Number(target) === target && target % 1 !== 0).toBe(true)

    Promise or async function that return an Error

    await expect(asyncFunction()).rejects.toThrow(errorMessage)

    References: