Using a for loop in a Mocha Test

Question

I'm new to mocha, and I wanted to try using a for loop to create test cases. I want to test a function that I've made that takes an input of standard 12 hour time, and outputs it into 24 hour military time. This is what it looks like.

exports.main = function(time) {
var hr = parseInt(time.substr(0,2));
var period = time.substr(8,10);
if (period == 'AM' && hr == 12) {
    hr = '0';
}
if (period == 'PM' && hr < 12) {
           hr += 12;
}
    hr.toString();
    if (hr < 10) {
        hr = '0' + hr;
    }
 return time = time.replace(/^\d{2}/g, hr).substr(0,8);

}

To test my function in mocha, I have two arrays, one array holds the standard times and the other holds the corresponding expected output. I want to iterate through them and produce a test case for each iteration and test my function.

test_array = ["12:00:00AM", "01:00:00AM", "02:00:00AM", "03:00:00AM", "04:00:00AM",
"05:00:00AM", "06:00:00AM", "07:00:00AM", "08:00:00AM", "09:00:00AM",
"10:00:00AM", "11:00:00AM", "12:00:00PM", "01:00:00PM", "02:00:00PM",
"03:00:00PM", "04:00:00PM", "05:00:00PM", "06:00:00PM", "07:00:00PM",
"08:00:00PM", "09:00:00PM", "10:00:00PM", "11:00:00PM"];

against = ["00:00:00", "01:00:00", "02:00:00", "03:00:00", "04:00:00",
"05:00:00", "06:00:00", "07:00:00", "08:00:00", "09:00:00", "10:00:00",
"11:00:00", "12:00:00", "13:00:00", "14:00:00", "15:00:00", "16:00:00",
"17:00:00", "18:00:00", "19:00:00", "20:00:00", "21:00:00", "22:00:00",
"23:00:00"]

This is what my test script looks like:

var converter = require('../modules/time.js');
describe('Time Converter', function() {
 describe('main()', function() {
  for(i = 0; i < 24; i++) {
   it(test_array[i]  + ' should convert to ' + against[i], function() {
   var test = converter.main(test_array[i]);
   assert.equal(test, against[i]);
   });
  }
 });
});

The following is the results of the tests:

0 passing (23ms)
24 failing

1) Time Converter main() 12:00:00AM should convert to 00:00:00:
 TypeError: Cannot read property 'substr' of undefined
  at Object.exports.main (app/modules/time.js:43:27)
  at Context.<anonymous> (app/test/test.js:35:26)

2) - 24) has the same result:

24) Time Converter main() 11:00:00PM should convert to 23:00:00:
 TypeError: Cannot read property 'substr' of undefined
  at Object.exports.main (app/modules/time.js:43:27)
  at Context.<anonymous> (app/test/test.js:35:26)

However when I change the for loop to

for(i = 0; i < 23; i++)

All the tests pass except, naturally it doesn't test the last test case.

Time Converter
main()
  ✓ 12:00:00AM should convert to 00:00:00
  ✓ 01:00:00AM should convert to 01:00:00
  ✓ 02:00:00AM should convert to 02:00:00
  ✓ 03:00:00AM should convert to 03:00:00
  ✓ 04:00:00AM should convert to 04:00:00
  ✓ 05:00:00AM should convert to 05:00:00
  ✓ 06:00:00AM should convert to 06:00:00
  ✓ 07:00:00AM should convert to 07:00:00
  ✓ 08:00:00AM should convert to 08:00:00
  ✓ 09:00:00AM should convert to 09:00:00
  ✓ 10:00:00AM should convert to 10:00:00
  ✓ 11:00:00AM should convert to 11:00:00
  ✓ 12:00:00PM should convert to 12:00:00
  ✓ 01:00:00PM should convert to 13:00:00
  ✓ 02:00:00PM should convert to 14:00:00
  ✓ 03:00:00PM should convert to 15:00:00
  ✓ 04:00:00PM should convert to 16:00:00
  ✓ 05:00:00PM should convert to 17:00:00
  ✓ 06:00:00PM should convert to 18:00:00
  ✓ 07:00:00PM should convert to 19:00:00
  ✓ 08:00:00PM should convert to 20:00:00
  ✓ 09:00:00PM should convert to 21:00:00
  ✓ 10:00:00PM should convert to 22:00:00
  23 passing (14ms)

However when I test the last test case by itself, it passes.

✓ 11:00:00PMshould convert to 23:00:00

So, I'm confused as to why all the tests don't work if the for loop iterates through the whole array, but works if I iterate up until the last index. Can someone clear this up for me?

Answer 1

Your code may be sync, but mocha calls it it async. Means: your it descriptions are parsed sync, mocha stores this information and runs every test in it's own context. This is async. To make this work, you have to create a closure with a function:

var converter = require('../modules/time.js');
// outside the loop:
function itShouldTestArray(i) {
   // i is now within the function scope and won't change anymore
   it(test_array[i]  + ' should convert to ' + against[i], function() {
   var test = converter.main(test_array[i]);
   assert.equal(test, against[i]);
}
describe('Time Converter', function() {
 describe('main()', function() {
  for(i = 0; i < 24; i++) {
   itShouldTestArray(i);
   });
  }
 });
});

Answer 2

So, I'm confused as to why all the tests don't work if the for loop iterates through the whole array, but works if I iterate up until the last index. Can someone clear this up for me?

This is probably a very-hard-to-debug-problem caused by the asynchronous testing conducted by Mocha. It seems like that changing i from 23 to 24 somehow results in your arrays being deleted before the tests are conducted, confer the error message:

 TypeError: Cannot read property 'substr' of undefined

You can avoid this deletion of the arrays before testing time by assigning their values inside a closure:

var converter = require('../modules/time.js');

describe('Time Converter', function() {
  describe('main()', function() {

       test_array = // insert array inside closure;
       against = //insert array inside closure;

       callback = function () {

        var test = converter.main (test_array[i]);
        assert.equal(test, against[i]);
       }

       for(i = 0; i < 24; i++) {

          it(test_array[i]  + ' should convert to ' + against[i], callback);
        }
      })
    })

This way each callback passed to it() will have access to the arrays without any possibility of them being deleted or altered before testing time.