AssertionError: expected {} to be a function

Question

I'm getting a strange error while using Mocha with Chai for the first time. The only framework I've used before is Jasmine, if that should help you figure out what I'm doing wrong.

I'm simply trying to make sure my constructor doesn't throw an error. I've been able to reduce the code to this, still getting the error:

class MyLib {
  constructor() {}
}

export default MyLib;

This is the test file:

import chai from 'chai';
let { expect } = chai;

import MyLib from '../index';

describe('mylib-js', () => {
  describe('constructor', () => {
    it('should work', () => {
      expect(new MyLib()).to.not.throw();
    })
  });
});

This is the output from Mocha:

d:\path\to\mylib-js>mocha --require babel-core/register


  mylib-js
    constructor
      1) should work


  0 passing (23ms)
  1 failing

  1) mylib-js constructor should work:
     AssertionError: expected {} to be a function
      at Assertion.an (d:\path\to\mylib-js\node_modules\chai\lib\chai\core\assertions.js:169:10)

      at Assertion.assert (d:\path\to\mylib-js\node_modules\chai\lib\chai\utils\addChainableMethod.js:84:49)
      at Assertion.assertThrows (d:\path\to\mylib-js\node_modules\chai\lib\chai\core\assertions.js:1273:32)
      at Assertion.ctx.(anonymous function) [as throw] (d:\path\to\mylib-js\node_modules\chai\lib\chai\utils\addMethod.js:41:25)
      at Context.<anonymous> (test.js:10:4)
      at callFn (C:\Users\Myuser\AppData\Roaming\npm\node_modules\mocha\lib\runnable.js:315:21)
      at Test.Runnable.run (C:\Users\Myuser\AppData\Roaming\npm\node_modules\mocha\lib\runnable.js:308:7)
      at Runner.runTest (C:\Users\Myuser\AppData\Roaming\npm\node_modules\mocha\lib\runner.js:422:10)
      at C:\Users\Myuser\AppData\Roaming\npm\node_modules\mocha\lib\runner.js:533:12
      at next (C:\Users\Myuser\AppData\Roaming\npm\node_modules\mocha\lib\runner.js:342:14)
      at C:\Users\Myuser\AppData\Roaming\npm\node_modules\mocha\lib\runner.js:352:7
      at next (C:\Users\Myuser\AppData\Roaming\npm\node_modules\mocha\lib\runner.js:284:14)
      at Immediate._onImmediate (C:\Users\Myuser\AppData\Roaming\npm\node_modules\mocha\lib\runner.js:320:5)

I have no idea where it gets {} from in the error output. What is it trying to tell me?

What does it say if you have `export default class MyLib { constructor() {} }`? — Szabolcs Dézsi, Feb 13 '16 at 15:42
@dustmouse: "AssertionError: expected [Function: MyLib] to not throw an error but 'TypeError: Cannot call a class as a function' was thrown" — damd, Feb 13 '16 at 15:45
`new MyLib` is an object not a function. It should be something like `expect( () => new MyLib).not.to.throw()` — Yury Tarabanko, Feb 13 '16 at 15:45
@YuryTarabanko: That makes sense. Add this as an answer and I'll accept! — damd, Feb 13 '16 at 15:46

score 3 · Accepted Answer · answered Feb 13 '16 at 15:47

3

As an exception message says new MyLib is an object not a function.

It should be something like this to work well with throw assertion.

expect( () => new MyLib).not.to.throw()

answered Feb 13 '16 at 15:47

Yury Tarabanko

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AssertionError: expected {} to be a function

1 Answers1