Override read only object within a jasmine unit test

Question

Anyone know how to override properties and functions in a read only objects like window or [htmlelement].style?

Example function to need to test:

public static getCSSTransitionEvent(element: HTMLElement): string {
    let transitions = {
        'transition': 'transitionend',
        'OTransition': 'oTransitionEnd',
        'MozTransition': 'transitionend',
        'WebkitTransition': 'webkitTransitionEnd'
    };

    for (let transition in transitions) {
        if (element.style[transition] !== undefined ) {
            return transitions[transition];
        }
    }
    return;
}

How can I override the transition property in element.style to tests that the function returns undefined at the bottom?

Another example, how do I test this if statement

function isCryptoAvailable() {
    if (typeof (window.crypto) !== 'undefined' && typeof (window.crypto.getRandomValues) !== 'undefined') {
        return true
    }
    else {
        return false
    }
}

Answer 1

This is done with property descriptors. Read-only properties have writable descriptor property set to false. As long as they are configurable, they can be redefined with Object.defineProperty like:

 const cryptoDescriptor = Object.getOwnPropertyDescriptor(window, 'crypto');

 afterEach(() => {
   if (origCryptoDescriptor) {
     delete window.crypto;
     Object.defineProperty(window, 'crypto', cryptoDescriptor);
   }
 });

 it('...', () => {
   expect(origCryptoDescriptor).toBeTruthy();
   expect(origCryptoDescriptor.configurable).toBeTruthy();

   const cryptoMock = ...;
   delete window.crypto;
   window.crypto = cryptoMock;
   ...
 });

The descriptor should be restored in afterEach because it will be executed even if test fails. And test will fail if a property is non-configurable. This is possible in some browsers, so a test should be excluded from suite in browsers that are known to fail it.

The same concerns a function that involves non-globals like HTMLElement object, but if it is possible to mock this object entirely instead of mocking its properties, this is the preferable strategy.