By the end of this talk, developers should be able to:
training
, for your work.training
branch.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 with array iteration methods. This is convenient when the callback is
simple and anonymous.
Arrow functions bodies that are a single expression have an added benefit, an
implicit return. This means that arrow function bodies without {}
return the
value of the expression without needing to use return
.
Although the following is true of arrow functions:
> typeof () => {}
'function'
> typeof (() => true)
'function'
> () => {} instanceof Function
true
> (() => true) instanceof Function
true
>
There are a few caveats.
Arrow functions:
arguments
(trying generates: ReferenceError: arguments is not defined
).new
does not bind this
, no
prototype
property).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. Building models of these methods helps us understand their functionality.
We'll check our work in node
or using the scripts in bin/
.
There are two main groups of array iteration methods:
forEach
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 array operation that may
terminate early.
map
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.
Because map
must access all of the elements of the array, we'll implement our
model of it in terms of forEach
.
After we build our version of map
, we'll test and compare it against the built
in version.
reduce
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.
Because reduce
must access all of the elements of the array, our
implementation uses forEach
.
The key to using reduce
properly is to methodically walk-through the
"How reduce works" section at the above link.
After we annotate our version of reduce
, we'll test and compare it against the
built in version.
filter
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.
Array.prototype.length
tells us the number of elements in the array. But what
if we want to know the number of elements that pass a certain test?
We'll build a function that counts the elements in an array for which a
predicate callback returns true
.
reduceRight
The reduceRight
method is 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 the results of calling reduceRight
differ from calling reduce
?
findIndex
The findIndex method returns the index of the first element in the array for which the callback returns true.
Why do we need findIndex
? Why not just use
indexOf?
find
The find method returns the first element in the array for which the callback returns true.
Can we build find
using findIndex
?
some
The
some
method return true if the callback returns true
for any element of the array.
Callbacks passed to some
should be predicate functions.
Can we build some
using findIndex
?
every
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 findIndex
? Using some
?