Map, Reduce, and Filter - JS Array Functions Explained with Code

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Each one will iterate over an array and perform a transformation or computation. Each will return a new array based on the result of the function

Map function

map() method creates a new array with the results of calling a function for every array element.

const output = arr.map(function) // this function tells map that what transformation I want on each element of array

Task 1: Double the array element

const arr = [1, 2, 3, 4, 5];
// pass a function to map
const double = arr.map(x => x * 2);

console.log(double);
// expected output: Array [2, 4, 6, 8, 10]

Task 2: Convert array elements to binary

const arr = [1, 2, 3, 4, 5];
// Transformation logic:
function binary(x) {
    return x.toString(2);
}
const binaryArr1 = arr.map(binary);
console.log(binaryArr1) //[ '1', '10', '11', '100', '101' ]

// The above code can be rewritten as :
const binaryArr2 = arr.map(function binary(x) {
    return x.toString(2);//[ '1', '10', '11', '100', '101' ]
})
console.log(binaryArr2)
// OR -> Arrow function
const binaryArr3 = arr.map((x) => x.toString(2));
console.log(binaryArr3)//[ '1', '10', '11', '100', '101' ]

Tricky MAP

const users = [
    { firstName: "Neeraj", lastName: "Kumar", age: 25 },
    { firstName: "Sandeep", lastName: "Kumar", age: 26 },
    { firstName: "Mayank", lastName: "Roy", age: 25 },
    { firstName: "Peter", lastName: "Mukherjee", age: 28 },
];
// Get array of full name : ["Neeraj Kumar", "Sandeep Kumar", ...]
const fullNameArr = users.map((user) => user.firstName + " " + user.lastName);
console.log(fullNameArr); // ["Neeraj Kumar", "Sandeep Kumar", ...]

----------------------------------------------------------

// Get the count/report of how many unique people with unique age are there
// like: {25 : 2, 26 : 1, 28 : 1}
const report = users.reduce((acc, curr) => {
    if(acc[curr.age]) {
        acc[curr.age] = ++ acc[curr.age] ;
    } else {
        acc[curr.age] = 1;
    }
    return acc;  //to every time return update object
}, {})
console.log(report) // {25 : 2, 26 : 1, 28 : 1}

Filter function
Filter function is basically used to filter the value inside an array. The arr.filter() method is used to create a new array from a given array consisting of only those elements from the given array which satisfy a condition set by the argument method.

const array = [5, 1, 3, 2, 6];
// filter odd values
function isOdd(x) {
  return x % 2;
}
const oddArr = array.filter(isOdd); // [5,1,3]

// Other way of writing the above:
const oddArr = arr.filter((x) => x % 2);

Reduce function

It is a function which take all the values of array and gives a single output of it. It reduces the array to give a single output.

const array = [5, 1, 3, 2, 6];
// Calculate sum of elements of array - Non functional programming way
function findSum(arr) {
  let sum = 0;
  for (let i = 0; i < arr.length; i++) {
    sum = sum + arr[i];
  }
  return sum;
}
console.log(findSum(array)); // 17

// reduce function way
const sumOfElem = arr.reduce(function (accumulator, current) {
  accumulator = accumulator + current;
  return accumulator;
}, 0); 

console.log(sumOfElem); // 17

// find max inside array: Non functional programming way:
const array = [5, 1, 3, 2, 6];
function findMax(arr) {
    let max = 0;
    for(let i = 0; i < arr.length; i++ {
        if (arr[i] > max) {
            max = arr[i]
        }
    }
    return max;
}
console.log(findMax(array)); // 6

// using reduce
const output = arr.reduce((acc, current) => {
    if (current > acc ) {
        acc = current;
    }
    return acc;
}, 0);
console.log(output); // 6

// acc is just a label which represent the accumulated value till now,
// so we can also label it as max in this case
const output = arr.reduce((max, current) => {
    if (current > max) {
        max= current;
    }
    return max;
}, 0);
console.log(output); // 6

Function Chaining

// First name of all people whose age is less than 30
const users = [
    { firstName: "Neeraj", lastName: "Kumar", age: 25 },
    { firstName: "Sandeep", lastName: "Kumar", age: 26 },
    { firstName: "Mayank", lastName: "Roy", age: 25 },
    { firstName: "Peter", lastName: "Mukherjee", age: 28 },
];
// function chaining
const output = users
  .filter((user) => user.age < 30)
  .map((user) => user.firstName);
console.log(output); 

// Homework challenge: Implement the same logic using reduce
const output = users.reduce((acc, curr) => {
  if (curr.age < 30) {
    acc.push(curr.firstName);
  }
  return acc;
}, []);
console.log(output);

Example