JavaScript developers must know this Array method!

push(): Adds one or more elements to the end of an array and returns the new length of the array.

let fruits = ['apple', 'banana'];
fruits.push('cherry'); 
// output: ['apple', 'banana', 'cherry']

pop(): Removes the last element from an array and returns that element.

let fruits = ['apple', 'banana', 'cherry'];
let last = fruits.pop(); 
// output: 'cherry', now fruits = ['apple', 'banana']

shift(): Removes the first element from an array and returns that element.

let fruits = ['apple', 'banana', 'cherry'];
let first = fruits.shift(); 
// output: 'apple', now fruits = ['banana', 'cherry']

unshift(): Adds one or more elements to the beginning of an array and returns the new length of the array.

let fruits = ['banana', 'cherry'];
fruits.unshift('apple'); 
// output: ['apple', 'banana', 'cherry']

indexOf(): Returns the first index at which a given element can be found in the array, or -1 if it is not present.

let fruits = ['apple', 'banana', 'cherry'];
let index = fruits.indexOf('banana'); 
// output: 1

includes(): Determines whether an array includes a certain element, returning true or false as appropriate.

let fruits = ['apple', 'banana', 'cherry'];
let hasApple = fruits.includes('apple'); 
// output: true

slice(): Returns a shallow copy of a portion of an array into a new array object.

let fruits = ['apple', 'banana', 'cherry'];
let someFruits = fruits.slice(0, 2); 
// output: ['apple', 'banana']

reverse(): Reverses an array in place. The first array element becomes the last, and the last array element becomes the first.

let fruits = ['apple', 'banana', 'cherry'];
fruits.reverse(); 
// output: ['cherry', 'banana', 'apple']

concat(): Merges two or more arrays. This method does not change the existing arrays but instead returns a new array.

let fruits = ['apple', 'banana'];
let moreFruits = ['cherry', 'date'];
let allFruits = fruits.concat(moreFruits); 
// output: ['apple', 'banana', 'cherry', 'date']

join(): Joins all elements of an array into a string and returns this string.

let fruits = ['apple', 'banana', 'cherry'];
let result = fruits.join(', '); 
// output: 'apple, banana, cherry'

lastIndexOf(): Returns the last index at which a given element can be found in the array, or -1 if it is not present.

let numbers = [1, 2, 3, 2, 1];
numbers.lastIndexOf(2); 
// output: 3

forEach(): Executes a provided function once for each array element.

let fruits = ['apple', 'banana', 'cherry'];
fruits.forEach(fruit => console.log(fruit));
/* output:
'apple'
'banana'
'cherry'
*/

map(): Creates a new array with the results of calling a provided function on every element in the calling array.

let numbers = [1, 4, 9];
let roots = numbers.map(Math.sqrt); 
// output: [1, 2, 3]

filter(): Creates a new array with all elements that pass the test implemented by the provided function.

let numbers = [1, 4, 9];
let bigNumbers = numbers.filter(number => number > 5); 
// output: [9]

reduce(): Applies a function against an accumulator and each element in the array (from left to right) to reduce it to a single value.

let numbers = [1, 2, 3, 4];
let sum = numbers.reduce((accumulator, currentValue) => accumulator + currentValue, 0); 
// output: 10

some(): Tests whether at least one element in the array passes the test implemented by the provided function. Returns a Boolean.

let numbers = [1, 2, 3, 4];
let hasNegativeNumbers = numbers.some(number => number < 0); 
// output: false

every(): Tests whether all elements in the array pass the test implemented by the provided function. Returns a Boolean.

let numbers = [1, 2, 3, 4];
let allPositiveNumbers = numbers.every(number => number > 0); 
// output: true

find(): Returns the value of the first element in the array that satisfies the provided testing function. Otherwise undefined is returned.

let numbers = [1, 5, 10, 15];
let found = numbers.find(function(element) {
  return element > 9;
}); 
// output: 10

toString(): Returns a string representing the specified array and its elements.

let fruits = ['apple', 'banana', 'cherry'];
let result = fruits.toString(); 
// output: 'apple,banana,cherry'

These methods form the backbone of array manipulation in JavaScript, and knowing how to use them can greatly simplify your coding tasks.