Why is a callback and a variable assignment needed here, instead of testing the function straightaway?

Question

Hi community,
I would appreciate today receiving some insight, because I dealt with this challenge for some while, and although I could finally solve it, I can't quite wrap my head around why, or learn from it, but I'd like to.
First this is the base code:
```javascript
function subtraction (number1, number2) {
    if (typeof number1 !== 'number' || typeof number2 !== 'number') {
      throw Error('subtraction only works with numbers!')
    }
    return number1 - number2
  }  
``
This code is in somewherever` module (the path doesn't really count because it's just a challenge anyway).
Now the challenge's code is:
```javascript
var expect = require('chai').expect
describe('subtraction', function () {
  var subtraction = require('../WHEREVER')  
  it('only works with numbers', function () {
    // YOUR CODE HERE
  })
})
```
And the idea of the challenge is that we can test (and prove) that the function will throw an error (our customized error, specifically) when passed any NaN as an argument.
Now, I was trying to do it this way (among others):
```javascript
describe('subtraction', function () {
  var subtraction = require('../game_logic/subtraction.js').subtraction  
  it('only works with numbers', function () {
    // YOUR CODE HERE
    var num = 45
    var letters = 'Hello'
    expect(subtraction(num, letters)).to.throw('subtraction only works with numbers!')
  })
})
// Also tried other options, like:
//  var num = 45
//  var letters = 'Hello'
//  var result = subtraction(num, letters)
//  expect(result).to.throw('subtraction only works with numbers!')
```
But all of these would return an error in the Mocha tests stating:
TypeError: subtraction is not a function.
Now, when I tried to do it with a callback with the function inside, being assigned to a variable, more like we did in the last video, it worked:
```javascript
var expect = require('chai').expect
describe('subtraction', function () {
  var subtraction = require('../WHEREVER')  
  it('only works with numbers', function () {
    // YOUR CODE HERE
    var num = 45
    var letters = 'Hello'
var result = function() {
  subtraction(num, letters)
}
expect(result).to.throw('subtraction only works with numbers!');
  })
})
```
The truth is I don't fully understand why this behavior? Isn't passing the function giving it the return value (or in this case the error thrown instead) to be tested? Just fighting to understand that bit.
Thanks in advance for any insight, suggestion, idea, correction, or alike about it!

Steven Parker · Accepted Answer

The difference is in what you are passing to the expect function, and when the function being tested gets called.  For example, in this code:
js
    expect(subtraction(num, letters)).to.throw('subtraction only works with numbers!')

The function subtraction will be called first, and after it runs, the result value it returns will be supplied as the argument to expect.  Any exception thrown will apply to the outer context (the "it" context) before expect is called.
On the other hand, in this code:
js
    expect(result).to.throw('subtraction only works with numbers!');

The function result is itself passed as the argument to expect, which will call it from within its internal context and check that it throws the exception.
If you didn't want to create the intermediate function and variables, you could also do this:
js
    // YOUR CODE HERE
    expect(subtraction.bind(null, 45, 'Hello')).to.throw('subtraction only works with numbers!');

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Why is a callback and a variable assignment needed here, instead of testing the function straightaway?

1 Answer

Steven Parker

Steven Parker

nico dev

nico dev