• JavaScript Functions "Ins & Outs"

By the end of this talk, developers should be able to:

• Write callbacks to pass to array methods
• Write functions to emulate array methods
• Write functions using array methods to add functionality.

1. Fork and clone this repository.
2. Create a new branch, training, for your work.
3. Checkout the training branch.
4. Install dependencies with npm install.

A callback is a function that we pass as an argument to another function or method. This other function invokes the callback as a necessary part of accomplishing its task. The JavaScript Array Iteration Methods require callback functions.

We often use predicate functions, functions returning either true or false, as callbacks.

We frequently use arrow (sometimes referred to as fat arrow) functions as callbacks. This is convenient when the callback is anonymous.

Although the following is true of arrow functions:

> typeof () => {}
'function'

> () => {} instanceof Function
true

There are a few caveats.

Arrow functions:

• cannot use arguments (trying generates: ReferenceError: arguments is not defined).
• cannot be used as a Constructor (new does not bind this, no prototype property).
• always have a lexically bound this (we'll learn more about that later).

We'll explore the array methods that allow us to test and transform arrays more simply and consistently, Iteration methods, and we'll model some of these JavaScript Array methods as functions.

We'll check our work in node or using bin/array-iteration-methods.js.

The forEach method iterates over all of the elements in an array. Unlike a for loop, it cannot be stopped (all elements are processed). forEach returns undefined.

From the MDN documentation:

There is no way to stop or break a forEach() loop other than by throwing an
exception. If you need such behavior, the forEach() method is the wrong
tool, use a plain loop instead. If you are testing the array elements for a
predicate and need a Boolean return value, you can use every() or some()
instead. If available, the new methods find() or findIndex() can be used for
early termination upon true predicates as well.

This means that forEach is a poor choice for an operation on an array that may terminate early.

The reduce method returns a single value from operating on all the values in the array. It "reduces" many to one. The original array does not change.

The key to using reduce properly is to methodically walk-through the "How reduce works" section at the above link.

Because reduce must access all of the elements of the array, we'll implement it in terms of forEach.

The map method returns a new array the same size as the existing array. The elements of the new array are set to the return value of the callback passed to map invoked with the corresponding element from the original array as its argument (e.g. newArray[i] = callback(array[i])). The array map is called upon is not mutated.

The filter method returns a new array containing elements from the original array for which the callback returns true. The length of the new array may be 0, the callback returned false for every element, or equal to the length of the original array, the callback returned true for every element in the original array.

Callbacks passed to filter should be predicate functions.

The some method return true if the callback returns true for any element of the array.

Callbacks passed to some should be predicate functions.

The every method checks to see if all elements of the array meet some test. The function used for this should only return true or false. This type of function is often called a predicate.

Callbacks passed to every should be predicate functions.

Can we build every using some?

The find method returns the first element in the array for which the callback returns true.

Could we have built some using find?

The findIndex method returns the index of the first element in the array for which the callback returns true.

Can we build findIndex using find?

The reduceRight method ifs functionally equivalent to using reduce on the array returned by the reverse JavaScript Array method.

Hint: Start by writing forEachRight (similar to forEach but iterating from the last element (at index length-1) through the first (at index 0).

How would reduceRight differ from reduce?

• List
• Array

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