Cheatsheet

Commonly used functions on Array

map

Description

Creates an array by calling a specific function on each element present in the parent array.

Syntax

array.map(function(currentValue)

Parameter	Description
function(currentValue)	Required function to be run for each array element

Return value

An Array containing the results of calling the provided function for each element in the original array.

filter

Description

Creates an array filled with all array elements that pass a test (provided as a function).

Syntax

array.filter(function(currentValue)

Parameter	Description
function(currentValue)	Required function to be run for each array element

Return value

Array containing all elements that pass the test else an empty array.

Example

let filtered = [112, 52, 0, -1, 944].filter(function (value) {
  return value > 0;
});

Output:

[112,52,944]

reduce

Description

reduces the array to a single value and executes provided function for each value of the array (from left-to-right).

Syntax

array.reduce( function(total, currentValue, currentIndex, arr), initialValue )

Parameter	Description
function(currentValue)	Required function to be run for each array element

Return value

returns accumulated result fron the last call of the callback function.

Example

const array1 = [1, 2, 3, 4];
const reducer = function (accumulator, currentValue) {
  return accumulator + currentValue;
};
console.log(array1.reduce(reducer));
console.log(array1.reduce(reducer, 5));

Output:

10 15

slice

Description

Returns the selected elements in an array, as a new array object and selects the elements starting at the given start argument, and ends at the given end argument(excluding end argument).

Syntax

array.slice(start, end);

Parameter	Description
start	Optional. specifies starting index of the selection
end	Optional. specifies ending index of the selection

Return value

New array containing the selected elements.

Example

let arr = [1, 2, 3, 4, 5, 6];
let new_arr = arr.slice(3);

Output:

[1,2,3,4,5,6]
[4,5,6]

splice

Description

Adds/removes items to/from an array, and returns the removed item(s).

Syntax

array.splice(index, howmany, item1, ....., itemX)

Parameter	Description
index	Required. An integer that specifies at what position to add/remove items
howmany	Optional. The number of items to be removed.
item1, ..., itemX	Optional. The new item(s) to be added to the array

Return value

Array without the removed items.

Example

const languages = ['C++', 'Java', 'Html', 'Python', 'C'];
// Add 'Julia' and 'Php' after removing 'Html'.
const removed = languages.splice(2, 1, 'Julia', 'Php');

Output:

C++,Java,Html,Python,C
C++,Java,Julia,Php,Python,C

Note

splice	slice
Returns the removed item(s) in an array	Returns the selected element(s) in an array, as a new array object
Changes the original array	Doesn’t change the original array
Can take n number of arguments(1 required)	Can take 2 arguments(1 required)

concat

Description

Used to add a single item to an array(or to add two or more arrays).

Syntax

array1.concat(item);

or

array1.concat(array2, array3, ..., arrayX)

Parameter	Description
array2, array3, ..., arrayX	Required. The arrays to be joined.

Return value

Joined array

Example

const num1 = [1, 2, 3],
  num2 = [4, 5, 6],
  num3 = 7;
console.log(num1.concat(num2, num3));

Output:

[1,2,3,4,5,6,7]

Note

concat	.push
Adds elements to the end of an array	Adds element or merges arrays
Returns the new length of the array	Returns a new array

join

Description

Returns the array as a string. The elementsare separated by a specified separator. The default separator is comma (,).

Syntax

array.join(separator);

Parameter	Description
separator	Optional.

Return value

String, representing the array values, separated by the specified separator.

Example

const languages = ['C++', 'Java', 'Html', 'Python', 'C'];
languages.join();
languages.join('.');
languages.join('-');

Output:

C++,Java,Html,Python,C
C++.Java.Html.Python.C
C++-Java-Html-Python-C

push

Description

Adds new items to the end of an array, and returns the new length.

Syntax

array.push(item1, item2, ..., itemX)

Parameter	Description
item1, item2, ..., itemX	Required. The item(s) to add to the array

Return value

New length of the array.

Example

let arr = [1, 2, 3, 4, 5];
console.log(arr.push(6, 7, 8));

Output:

8
[1,2,3,4,5,6,7,8]

find

Description

Returns the value of the first element in an array that pass a test (provided as a function).

Syntax

array.find(function(currentValue)

Parameter	Description
function(currentValue)	Required. A function to be run for each element in the array.

Return value

The array element value if any of the elements in the array pass the test, otherwise it returns 'undefined'.

Example

let array = [1, 3, 5, 7, 9];
const found = array.find(function (element) {
  return element > 4;
});
// Printing desired value.

Output:

5

indexOf

Description

returns the position of the first occurrence of a specified value in a string.

Syntax

string.indexOf(searchvalue, start);

Parameter	Description
searchvalue	Required. The string to search for
start	Optional. Default 0. At which position to start the search.

Return value

Number, representing the position where the specified searchvalue occurs for the first time, or -1 if it never occurs.

Example

console.log('Departed Train'.indexOf('Train'));

Output:

9

Common String Functions

Full Mozilla MDN Docs

substring

Description

Extracts the characters in a string between "start" and "end", not including "end" itself.

Syntax

string.substring(start, end);

Parameter	Description
start	Required. The position where to start the extraction. First character is at index 0
end	Optional. The position (up to, but not including) where to end the extraction. If omitted, it extracts the rest of the string

Return value

New string containing the extracted characters.

Example

// Taking a string as variable
const string = 'JavaScriptCheatsheet';
a = string.substring(0, 4);
b = string.substring(1, 6);
c = string.substring(5);
d = string.substring(0);
// Printing new string which are
// the part of the given string

Output:

Java avaSc criptCheatsheet JavaScriptCheatsheet

Note

array.slice()	string.substring()
Displays selected array elements	Displays selected part of string
Returns new array	Returns new string

split

Description

Used to split a string into an array of substrings, and returns the new array.

Syntax

string.split(separator, limit);

Parameter	Description
separator	Optional. Specifies the character, or the regular expression, to use for splitting the string. If omitted, the entire string will be returned (an array with only one item)
limit	Optional. An integer that specifies the number of splits, items after the split limit will not be included in the array

Return value

Array containing splitted values.

Example

let str = 'It iS a great Day.';
let array = str.split(' ');

Output:

["It","iS","a","great","Day."]

toLowerCase

Description

Converts a string to lowercase letters.

Syntax

string.toLowerCase()

Return value

A string, representing the value of a string converted to lowercase.

Example

const str = 'It iS a Great Day.';
const string = str.toLowerCase();

Output:

it is a great day.

trim

Description

Removes whitespace from both sides of a string.

Syntax

string.trim();

Return value

String, representing the string with removed whitespace from both ends

Example

const str = '     "JavaScript"      ';
const st = str.trim();

Output:

"Javascript"

charAt

Description

Returns the character at the specified index in a string.

Syntax:

string.charAt(index);

Return value

String, representing the character at the specified index, or an empty string if the index number is not found

Example:

const str = 'JavaScript is object oriented language';
console.log(str.charAt(9));

Output:

t

For Loop Variants

for...in

Syntax

for (var in object)
{
   //code block to be executed
}

Example

const array1 = ['a', 'b', 'c'];
for (const element in array1) {
  console.log(element);
}

Output:

"a"
"b"
"c"

for...of

Description

Loops through the values of an object.

Syntax

for (variable of object) {
  // code block to be executed
}

Example

const languages = {
  first: 'C',
  second: 'Java',
  third: 'Python',
  fourth: 'PHP',
  fifth: 'JavaScript'
};
// iterate through every property of the
// object languages and print all of them
// using for..in loops
for (itr in languages) {
  console.log(languages[itr]);
}

Output:

C Java Python PHP JavaScript

for loop

Description

Traditional index-based loop as found in many languages.

Syntax

for (statement 1; statement 2; statement 3)
{
 	// code block to be executed
}

Example

const i;
for (i = 0; i < 10; i++)
{
	console.log("Hello World!\n");
}

This will print Hello World on the screen 10 times.

.forEach

Description

Calls a function once for each element in an array, in order.

Syntax

array.forEach(function(currentValue)

Parameter	Description
function(currentValue)	Required. A function to be run for each element in the array.

Example:

const items = [1, 29, 47];
const copy = [];
items.forEach(function (item) {
  copy.push(item * item);
});

Output : It squares each number in the array items .

1,841,2209