hof-reduce.md

Higher Order Functions - Reduce

---

Objectives

• Review Map and Filter Method
• Explain what the Reduce method does
• List the inputs to the function you pass to reduce
• Solve Problems Using Reduce

---

Map

---

Map Review

• An array method
• Takes a function as an input
• Applies that function to every item in the array
• Returns a NEW array that contains transformed data

---

Filter

---

Filter Review

• An array method
• Takes a function as an input (the function should return a boolean)
• Tests each item in the array using the function
• Returns a NEW array that contains all items that pass the test

---

Reduce

Reduce is an incredibly powerful method that allows us to combine all of the result in an array into a single result.

• An array method
• Takes a function and an optional initial value as inputs
• Executes the function for each item in the array
• Returns the value that results from the reduction

---

What is passed to your function?

---

Values of the Array

• previous value
• current value
• current index
• array

---

Example

var array = ['a','b','c','d']; array.reduce(function(prev,curr,index,array));

When we start the reduce:

previousValue is 'a' currentValue is 'b' currentIndex is 1 array is ['a','b','c','d']

---

But what happens when you give an initial value?

---

Example

var array = ['a','b','c','d']; array.reduce(function(prev,curr,index,array), 'z');

When we start the reduce:

previousValue is 'z' currentValue is 'a' currentIndex is 0 array is ['a','b','c','d']

---

Let's Try It