Why Jest's toThrow won't work when create an instance of a ES6 class directly in the constructor?

Question

class TestObject {   constructor(value) {     if (value === null || value === undefined) {       throw new Error('Expect a value!');     }   } }  describe('test the constructor', () => {   test('it works', () => {     expect(() => {       new TestObject();     }).toThrow();   });    test('not work', () => {     expect(new TestObject()).toThrow();   }); }); 

2 Test cases here, one works and the other not.

The failing message for the not work one is as the following:

● test the constructor › not work

Expect a value!

 at new TestObject (tests/client/utils/aaa.test.js:4:11)  at Object.<anonymous> (tests/client/utils/aaa.test.js:17:12)      at Promise (<anonymous>)      at <anonymous>  at process._tickCallback (internal/process/next_tick.js:188:7) 

Why do I need to wrap that call in a function call, we don't need to wrap when the function just return a plain value, or even a promise, we can use async/await to check that in expect() rather than create a function inside expect().

What happened here?

Answer 1

Here

expect(new TestObject()).toThrow(); 

new TestObject() is evaluated first, then expect(...), then ...toThrow(), in accordance with operator precedence. When new TestObject() throws, anything else doesn't matter.

This is why toThrow expects a function that is supposed to throw:

expect(() => {   new TestObject(); }).toThrow(); 

This way it can be wrapped with try..catch internally when being called.

It works similarly in Jasmine toThrow and Chai to.throw assertions as well.