How to assert data type with Jest

Question

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 {}.

Thanks!

Answer 1

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) 

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for Jest with version < 16.0.0:

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

Example:

expect(result instanceof Date).toBe(true) 

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Another example to match primitive types:

boolean, number, string & function:

expect(typeof target).toBe("boolean") expect(typeof target).toBe("number") expect(typeof target).toBe("string") expect(typeof target).toBe('function') 

array & object:

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

null & undefined:

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

Promise or async function:

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

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References:

• https://jestjs.io/docs/expect#tobeinstanceofclass
• https://github.com/facebook/jest/issues/3457
• https://futurestud.io/tutorials/detect-if-value-is-a-promise-in-node-js-and-javascript

Answer 2

Jest supports toBeInstanceOf. See their docs, but here is what they have for example at the time of this answer:

class A {}  expect(new A()).toBeInstanceOf(A); expect(() => {}).toBeInstanceOf(Function); expect(new A()).toBeInstanceOf(Function); // throws