Testing for instanceof using Jasmine

Question

I'm new to Jasmine and testing in general. One block of my code checks whether my library has been instantiated using the new operator:

 //if 'this' isn't an instance of mylib...  if (!(this instanceof mylib)) {      //return a new instance      return new mylib();     } 

How can I test this using Jasmine?

Answer 1

Jasmine >=3.5.0

Jasmine provides the toBeInstanceOf matcher.

it("matches any value", () => {   expect(3).toBeInstanceOf(Number); }); 

Jasmine >2.3.0

To check if something is an instanceof [Object] Jasmine provides jasmine.any:

it("matches any value", function() {   expect({}).toEqual(jasmine.any(Object));   expect(12).toEqual(jasmine.any(Number)); }); 

Answer 2

I do prefer the more readable/intuitive (in my opinion) use with the instanceof operator.

class Parent {} class Child extends Parent {}  let c = new Child();  expect(c instanceof Child).toBeTruthy(); expect(c instanceof Parent).toBeTruthy(); 

For the sake of completeness you can also use the prototype constructor property in some cases.

expect(my_var_1.constructor).toBe(Array); expect(my_var_2.constructor).toBe(Object); expect(my_var_3.constructor).toBe(Error);  // ... 

BEWARE that this won't work if you need to check whether an object inherited from another or not.

class Parent {} class Child extends Parent {}  let c = new Child();  console.log(c.constructor === Child); // prints "true" console.log(c.constructor === Parent); // prints "false" 

If you need inheritance support definitely use the instanceof operator or the jasmine.any() function like Roger suggested.

Object.prototype.constructor reference.