String Methods

String Method	Purpose	Example
length	Returns the length of a string	const str = 'Hello'; const len = str.length;
charAt(index)	Returns the character at the specified index	const str = 'Hello'; const char = str.charAt(1);
charCodeAt(index)	Returns the Unicode value of the character at the specified index	const str = 'Hello'; const code = str.charCodeAt(0);
concat(...strings)	Combines two or more strings and returns a new string	const str1 = 'Hello'; const str2 = 'World'; const result = str1.concat(', ', str2);
indexOf(substring)	Returns the index of the first occurrence of a substring in the string	const str = 'Hello, World'; const index = str.indexOf('World');
lastIndexOf(substring)	Returns the index of the last occurrence of a substring in the string	const str = 'Hello, World, World'; const lastIndex = str.lastIndexOf('World');
includes(substring)	Checks if a string contains a specified substring	const str = 'Hello, World'; const includesWorld = str.includes('World');
startsWith(substring)	Checks if a string starts with a specified substring	const str = 'Hello, World'; const startsWithHello = str.startsWith('Hello');
endsWith(substring)	Checks if a string ends with a specified substring	const str = 'Hello, World'; const endsWithWorld = str.endsWith('World');
toLowerCase()	Converts a string to lowercase	const str = 'Hello'; const lowerStr = str.toLowerCase();
toUpperCase()	Converts a string to uppercase	const str = 'Hello'; const upperStr = str.toUpperCase();
trim()	Removes whitespace from the beginning and end of a string	const str = ' Hello, World '; const trimmedStr = str.trim();
trimStart() or trimLeft()	Removes whitespace from the beginning of a string	const str = ' Hello, World '; const trimmedStr = str.trimStart();
trimEnd() or trimRight()	Removes whitespace from the end of a string	const str = ' Hello, World '; const trimmedStr = str.trimEnd();
slice(start, end)	Extracts a portion of a string	const str = 'Hello, World'; const subStr = str.slice(7, 12);
substring(start, end)	Extracts a portion of a string (similar to slice)	const str = 'Hello, World'; const subStr = str.substring(7, 12);
substr(start, length)	Extracts a specified number of characters from a string	const str = 'Hello, World'; const subStr = str.substr(7, 5);
split(separator)	Splits a string into an array of substrings	const str = 'apple,banana,orange'; const fruits = str.split(',');
replace(old, new)	Replaces occurrences of a substring with another string	const str = 'Hello, World'; const newStr = str.replace('World', 'Universe');
match(regexp)	Searches for a specified pattern in the string and returns an array of matches	const str = 'Hello, World'; const matches = str.match(/o/g);
search(regexp)	Searches for a specified pattern in the string and returns the index of the first match	const str = 'Hello, World'; const index = str.search(/o/);
matchAll(regexp)	Returns an iterator of all matches of a specified pattern in the string	const str = 'Hello, World'; const matches = [...str.matchAll(/o/g)];
startsWith()	Checks if a string starts with a specified substring at a given index	const str = 'Hello, World'; const startsWithHello = str.startsWith('Hello', 0);
endsWith()	Checks if a string ends with a specified substring at a given index	const str = 'Hello, World'; const endsWithWorld = str.endsWith('World', 11);
repeat(count)	Returns a new string with a specified number of copies of the original string	const str = 'Hello'; const repeatedStr = str.repeat(3);
padStart(length, padString)	Pads the beginning of a string with a specified character(s) until it reaches a certain length	const str = '42'; const paddedStr = str.padStart(5, '0');
padEnd(length, padString)	Pads the end of a string with a specified character(s) until it reaches a certain length	const str = '42'; const paddedStr = str.padEnd(5, '0');

Array Methods

Array Method	Purpose	Example
length	Returns the number of elements in an array	const numbers = [1, 2, 3]; const len = numbers.length;
push(element)	Adds one or more elements to the end of an array and returns the new length	const fruits = ['apple', 'banana']; fruits.push('orange', 'kiwi');
pop()	Removes and returns the last element of an array	const fruits = ['apple', 'banana', 'orange']; const lastFruit = fruits.pop();
unshift(element)	Adds one or more elements to the beginning of an array and returns the new length	const numbers = [2, 3]; numbers.unshift(0, 1);
shift()	Removes and returns the first element of an array	const fruits = ['apple', 'banana', 'orange']; const firstFruit = fruits.shift();
concat(...arrays)	Combines two or more arrays and returns a new array	const arr1 = [1, 2]; const arr2 = [3, 4]; const combined = arr1.concat(arr2);
join(separator)	Joins all elements of an array into a string with the specified separator	const fruits = ['apple', 'banana', 'orange']; const fruitString = fruits.join(', ');
slice(start, end)	Returns a shallow copy of a portion of an array	const numbers = [1, 2, 3, 4, 5]; const subArray = numbers.slice(1, 4);
splice(start, deleteCount, ...items)	Adds or removes elements from an array at a specified position	const numbers = [1, 2, 3, 4, 5]; numbers.splice(2, 2, 6, 7);
indexOf(element)	Returns the first index at which a specified element is found in an array	const fruits = ['apple', 'banana', 'orange']; const index = fruits.indexOf('banana');
lastIndexOf(element)	Returns the last index at which a specified element is found in an array	const fruits = ['apple', 'banana', 'orange', 'banana']; const lastIndex = fruits.lastIndexOf('banana');
includes(element)	Checks if an array includes a specified element	const numbers = [1, 2, 3, 4, 5]; const includesThree = numbers.includes(3);
find(callback)	Returns the first element in an array that satisfies the provided testing function	const numbers = [1, 2, 3, 4, 5]; const found = numbers.find(num => num > 2);
findIndex(callback)	Returns the index of the first element in an array that satisfies the provided testing function	const numbers = [1, 2, 3, 4, 5]; const foundIndex = numbers.findIndex(num => num > 2);
filter(callback)	Returns a new array with all elements that satisfy the provided testing function	const numbers = [1, 2, 3, 4, 5]; const evenNumbers = numbers.filter(num => num % 2 === 0);
map(callback)	Creates a new array with the results of calling a provided function on every element in the array	const numbers = [1, 2, 3, 4, 5]; const doubledNumbers = numbers.map(num => num * 2);
forEach(callback)	Executes a provided function once for each array element	const fruits = ['apple', 'banana', 'orange']; fruits.forEach(fruit => console.log(fruit));
reduce(callback, initialValue)	Reduces an array to a single value by applying a provided function	const numbers = [1, 2, 3, 4, 5]; const sum = numbers.reduce((accumulator, currentValue) => accumulator + currentValue, 0);
reduceRight(callback, initialValue)	Reduces an array from right to left	const numbers = [1, 2, 3, 4, 5]; const product = numbers.reduceRight((accumulator, currentValue) => accumulator * currentValue, 1);
every(callback)	Checks if all elements in an array satisfy a provided testing function	const numbers = [1, 2, 3, 4, 5]; const allEven = numbers.every(num => num % 2 === 0);
some(callback)	Checks if at least one element in an array satisfies a provided testing function	const numbers = [1, 2, 3, 4, 5]; const hasEven = numbers.some(num => num % 2 === 0);
sort(compareFunction)	Sorts the elements of an array in place based on the provided sorting criteria	const fruits = ['banana', 'apple', 'orange']; fruits.sort();
reverse()	Reverses the order of elements in an array	const numbers = [1, 2, 3, 4, 5]; numbers.reverse();
fill(value, start, end)	Changes all elements in an array to a specified value within a given range	const numbers = [1, 2, 3, 4, 5]; numbers.fill(0, 1, 4);
copyWithin(target, start, end)	Copies a portion of an array to another location in the same array	const numbers = [1, 2, 3, 4, 5]; numbers.copyWithin(0, 3, 5);
flat(depth)	Creates a new array with all sub-array elements concatenated up to the specified depth	const nestedArray = [1, [2, 3], [4, [5]]]; const flatArray = nestedArray.flat(2);
flatMap(callback)	Maps each element using a mapping function and flattens the result into a new array	const numbers = [1, 2, 3, 4, 5]; const doubledAndSquared = numbers.flatMap(num => [num * 2, num ** 2]);
from(iterable, mapFn, thisArg)	Creates a new array from an iterable object	const iterable = '12345'; const newArray = Array.from(iterable, Number);
of(...elements)	Creates a new array with the specified elements	const numbers = Array.of(1, 2, 3, 4, 5);

JavaScript Array Constructor

Action	Example Usage
Create an empty array	let emptyArray = new Array();
Create an array with initial values	let numbers = new Array(1, 2, 3);
Get the length of an array	let length = myArray.length;
Access an element by index	let element = myArray[index];
Add elements to the end of an array	myArray.push(element1, element2);
Remove and return the last element	let lastElement = myArray.pop();
Add elements to the beginning of an array	myArray.unshift(element1, element2);
Remove and return the first element	let firstElement = myArray.shift();
Find the index of an element	let index = myArray.indexOf(element);
Check if an element exists in an array	let exists = myArray.includes(element);
Remove an element by index	myArray.splice(index, 1);
Copy an array	let copyArray = new Array(...myArray);
Concatenate arrays	let combinedArray = array1.concat(array2);
Convert an array to a string	let str = myArray.join(', ');

JavaScript Object Constructor

Action	Example Usage
Create an empty object	let emptyObject = new Object();
Create an object with properties	let person = new Object({ name: 'John', age: 30 });
Access a property by name	let value = myObject.propertyName;
Set a property by name	myObject.propertyName = value;
Check if an object has a property	let hasProperty = myObject.hasOwnProperty('propertyName');
Delete a property	delete myObject.propertyName;
Iterate over object properties	for (let key in myObject) { ... }
Copy an object	let copyObject = Object.assign({}, myObject);
Merge objects	let mergedObject = Object.assign({}, obj1, obj2);
Convert an object to JSON string	let jsonString = JSON.stringify(myObject);
Parse a JSON string to an object	let parsedObject = JSON.parse(jsonString);

JavaScript JSON Object

Action	Example Usage
Parse a JSON string to an object	let obj = JSON.parse(jsonString);
Convert an object to a JSON string	let jsonString = JSON.stringify(obj);
Clone an object	let clone = JSON.parse(JSON.stringify(obj));

JavaScript Math Object

Action	Example Usage
Generate a random number between 0 and 1	let random = Math.random();
Generate a random integer between min and max	let randomInt = Math.floor(Math.random() * (max - min + 1)) + min;
Round a number to the nearest integer	let rounded = Math.round(number);
Calculate the square root of a number	let sqrt = Math.sqrt(number);
Calculate the absolute value of a number	let absValue = Math.abs(number);
Find the minimum of two or more numbers	let minNumber = Math.min(num1, num2, ...);
Find the maximum of two or more numbers	let maxNumber = Math.max(num1, num2, ...);
Calculate the sine, cosine, or tangent	let sine = Math.sin(angle);
Calculate the natural logarithm	let ln = Math.log(number);
Calculate the power of a number	let power = Math.pow(base, exponent);
Round a number down to the nearest integer	let roundedDown = Math.floor(number);
Round a number up to the nearest integer	let roundedUp = Math.ceil(number);

JavaScript Function Object Methods

Action	Example Usage
Define a function	function myFunction(param1, param2) { ... }
Call a function	myFunction(arg1, arg2);
Pass a function as an argument	function doSomething(callback) { ... }
Return a value from a function	return result;
Use the arguments object	let arg1 = arguments[0];
Define anonymous functions	let add = function(x, y) { return x + y; };
Use arrow functions	let add = (x, y) => x + y;
Use function expressions in objects	{ sayHello: function() { ... } }
Define and call immediately invoked function	(function() { ... })();