map(), reduce(), filter() in JavaScript

cheat sheet for map(), reduce(), filter() methods in JavaScript

map()

• It accepts a callback function once for each array element and returns a new array.

• The map() method doesn’t execute the function for an empty array.

Syntax of `map()`

• item - It’s a required parameter, the value of the current array item.

• index - Optional, It’s the current index of the array element that is being processed.

• arr - Optional, the array on which the map() method was called.

let arr = [10, 20, 30, 40, 50, 60];

let half = arr.map(function(item){
    return item/2
});

// half: [ 5, 10, 15, 20, 25, 30 ]

reduce()

• The reduce() method executes a reducer function on each element of the array.

• It returns a single value as a result and doesn’t execute the function for an empty array element.

Syntax of reduce()

• accumulator - It’s a value of the previous function call, or the InitialValue for the first call.

  • If there is no any InitialValue provided then it’s accumulate the first item of the array and item is initialized to the second value in the array.

• item - It’s a required parameter, that is the value of the current item of the array.

• index - Optional, it’s the current index of the array element that is being processed.

• arr - Optional, the array on which the reduce() method was called.

• InitialValue - Optional, it’s a starting value that is used to initialize the callback function for the first time.

const inr = [10, 20, 30, 40, 50, 60];

const totalINR = inr.reduce(function(total, item){

    return total + item

}, 0);

// totalINR: 210

filter()

• The filter() method returns an array that passes the condition. If no elements pass the condition, it returns an empty array.

• It doesn’t execute the callback function for empty elements.

• item - It’s a required parameter, the value of the current array item.

• index - Optional, It’s the current index of the array element that is being processed.

• arr - Optional, the array on which the filter() method was called.

const user = ["Ajay", "Sivam", "Anuj", "Shankar"];

let nameStartWithA = user.filter(function(name){
    return name.startsWith("A");
});

// nameStartWithA: [ 'Ajay', 'Anuj' ]