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Building iterators

For the following challenges it is essential that you understand the requirements to fully implement the built-in array method. See MDN Array Reference

reading documentation

Let's take a quick look at what MDN says about Array.prototype.forEach

arr.forEach(callback[, thisArg])

It says that forEach takes a callback as the first argument. And it says the callback will receive 3 arguments when called:

  • currentValue
  • index
  • array

It also mentions thisArg which is not an argument to the callback, rather it is an argument to forEach itself. We can also see that thisArg is optional. That means it's not required. For today's work, we will not use it.

building our own

Now we're going to build our own iterators.

Remember to start small and add features later. It's easier to build incrementally than to try to do everything all at once.

  • Create a function myEach which implements Array.prototype.forEach
  • Create a function myMap which implements Array.prototype.map
  • Create a function myReduce which implements Array.prototype.reduce

Note: for myReduce it is suggested that you start by coding a solution to solve it without initialValue first. You can add initialValue later on. See the docs for more details.

BONUS:

  • Create a function myFilter which implements Array.prototype.filter
  • Create a function mySome which implements Array.prototype.some
  • Create a function myEvery which implements Array.prototype.every

getting started

  1. Fork this repo, and clone it into your dev folder on your local machine. Make sure you are in dev before cloning.

  2. Start at myEach.js. There is already some starter code there; begin by filling in the function body.

  3. Use the included test suite to help you test your solutions.

tests

Tests have been provided for you to test your solutions. At this point don't worry about the test code. You don't even need to read it.
Focus on building your solution and just use the tests as a system of hints and a way to confirm your solutions.

test setup

cd into this repositories directory. Then run npm install. (You should not be in the test directory). This installs the dependencies and testing framework we need.

running tests

Run the tests individually as you work on each challenge. For example to run the tests for myMap, in your terminal:

mocha test/test-myMap.js

This will test against your solution in myMap.js.

Sample output:

The first portion of the output shows RED for all failing tests and GREEN for successes. Successful tests also show a ✓ (check-mark). You can see we had 3 passing tests in the below output.

$ mocha test/test-myMap.js


 myMap
   1) takes and calls a callback function
      results:  []
   2) passes each item in the array to the callback
      results:  []
   3) passes each index in the array to the callback
   ✓ passes the entire array to the callback
      results:  undefined
   4) returns an array
      results:  undefined
   5) returns an array with the same number of elements
      results:  undefined
   6) returns an array constructed from the return values of the callback
      results:  [ 'a', 'b', 'c', 'd' ]
   ✓ doesn't alter the original array
      results:  []
   ✓ works with arrays of length 0
      results:  []
   7) works with arrays of length 1

Also it's important to note that the three passing tests above are actually passing only by happenstance. As soon as you implement a little more of the myMap function they'll stop passing. For example doesn't alter the original array is passing; primarily because an empty function doesn't alter arrays, right?. You need to get all the tests to pass.

Further on down you will see more detailed test failure messages.

7) myMap works with arrays of length 1:

  AssertionError: expected 0 to equal 1
  + expected - actual

  +1
  -0

  at Context.testArrayL1 (test/test-myMap.js:115:38)

An assertion is a statement that asserts or says this "MUST BE TRUE". If the statement turns out to be false, then the assertion fails and the test fails.
Assertions are the real tests inside the tests.

In the above we can see that at line 115 in the tests it expected for 0 to equal 1.

Here's another one:

2) myMap passes each item in the array to the callback:
   AssertionError: expected [] to have the same members as [ 'a', 'b', 'c', 'd' ]
    at Context.testEachItem (test/test-myMap.js:37:36)

What do you think this means?

At line 37 in the test file there was an expectation that an array would have the elements ['a', 'b', 'c', 'd']. But instead it got an empty array.


## another way to test/use your code

You can write additional code in `index.js` that uses your code in the other files.
This is the best place for you to write your own code to use your new functions.  

You'll see that we already `require`d the other files for you.  Don't worry about how this works. Just know that it does and that you can use those functions in `index.js`.