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What is Array in JS?
The Array object, as with arrays in other programming languages, enables storing a collection of multiple items under a single variable name and has members for performing common array operations.
Declaring arrays
We can declare arrays in two different ways.
Using new Array
With new Array, we could specify the elements we want to exist within the array, like this:
const fruits = new Array('Apple', 'Banana');
console.log(fruits.length);
Array literal notation
Declaring with an array literal, we specify the values that the array will have. If we don’t declare any values, the array will be empty.
// 'fruits' array created using array literal notation.
const fruits = ['Apple', 'Banana'];
console.log(fruits.length);
Javascript Array Methods
Here is a list of the methods of the Array object along with their description.
1.forEach
The forEach() method executes a provided function once for each array element.
forEach() calls a provided callbackFn function once for each element in an array in ascending index order. It is not invoked for index properties that have been deleted or are uninitialized.
array.forEach(callback[, thisObject]);
const array1 = ['a', 'b', 'c'];
array1.forEach(element => console.log(element));
// expected output: "a"
// expected output: "b"
// expected output: "c"
2.map
The Array.map() method allows you to iterate over an array and modify its elements using a callback function. The callback function will then be executed on each of the array's elements.
array.map(callback[, thisObject]);
let arr = [3, 4, 5, 6];
let modifiedArr = arr.map(function(element){
return element *3;
});
console.log(modifiedArr); // [9, 12, 15, 18]
The Array.map() method is commonly used to apply some changes to the elements, whether multiplying by a specific number as in the code above, or doing any other operations that you might require for your application.
3.concat
In JavaScript, concat() is a string method that is used to concatenate strings together. The concat() method appends one or more string values to the calling string and then returns the concatenated result as a new string. Because the concat() method is a method of the String object, it must be invoked through a particular instance of the String class.
array.concat(value1, value2, ..., valueN);
const array1 = ['a', 'b', 'c'];
const array2 = ['d', 'e', 'f'];
const array3 = array1.concat(array2);
console.log(array3);
// expected output: Array ["a", "b", "c", "d", "e", "f"]
4.push
Javascript array push() method appends the given element(s) in the last of the array and returns the length of the new array. When you want to add an element to the end of your array, use push().
array.push(element1, ..., elementN);
const countries = ["Nigeria", "Ghana", "Rwanda"];
countries.push("Kenya");
console.log(countries); // ["Nigeria","Ghana","Rwanda","Kenya"]
5.pop
The pop() method removes the last element from an array and returns that value to the caller. If you call pop() on an empty array, it returns undefined.
Array.prototype.shift() has similar behaviour to pop(), but applied to the first element in an array.
array.pop();
const plants = ['broccoli', 'cauliflower', 'cabbage', 'kale', 'tomato'];
console.log(plants.pop());
// expected output: "tomato"
console.log(plants);
// expected output: Array ["broccoli", "cauliflower", "cabbage", "kale"]
6.splice
The splice() method is a general-purpose method for changing the contents of an array by removing, replacing, or adding elements in specified positions of the array. This section will cover how to use this method to add an element to a specific location.
array.splice(index, howMany, [element1][, ..., elementN]);
const fruits = ["Banana", "Orange", "Apple", "Mango"];
fruits.splice(2, 0, "Lemon", "Kiwi"); //Banana,Orange,Lemon,Kiwi,Apple,Mango
7.slice
The slice() method returns a shallow copy of a portion of an array into a new array object selected from start to end (end not included) where start and end represent the index of items in that array. The original array will not be modified.
array.slice( begin [,end] );
const animals = ['ant', 'bison', 'camel', 'duck', 'elephant'];
console.log(animals.slice(2));
// expected output: Array ["camel", "duck", "elephant"]
console.log(animals.slice(2, 4));
// expected output: Array ["camel", "duck"]
8.shift
shift() is a built-in JavaScript function that removes the first element from an array. The shift() function directly modifies the JavaScript array with which you are working. shift() returns the item you have removed from the array.
The shift() function removes the item at index position 0 and shifts the values at future index numbers down by one.
array.shift();
const array1 = [1, 2, 3];
const firstElement = array1.shift();
console.log(array1);
// expected output: Array [2, 3]
console.log(firstElement);
// expected output: 1
9.unshift
The unshift() method inserts the given values to the beginning of an array-like object.
Array.prototype.push() has similar behaviour to unshift(), but applied to the end of an array.
array.unshift( element1, ..., elementN );
const array1 = [1, 2, 3];
console.log(array1.unshift(4, 5));
// expected output: 5
console.log(array1);
// expected output: Array [4, 5, 1, 2, 3]
10.join
JavaScript array join() is a built-in method that creates and returns the new string by concatenating all of the elements of the array. The join() method joins the array’s items into the string and returns that string. The specified separator will separate the array of elements. The default separator is a comma (,).
array.join(separator);
const elements = ['Fire', 'Air', 'Water'];
console.log(elements.join());
// expected output: "Fire,Air,Water"
console.log(elements.join(''));
// expected output: "FireAirWater"
console.log(elements.join('-'));
// expected output: "Fire-Air-Water"
11.every
The every() method tests whether all elements in the array pass the test implemented by the provided function. It returns a Boolean value.
array.every(callback[, thisObject]);
const isBelowThreshold = (currentValue) => currentValue < 40;
const array1 = [1, 30, 39, 29, 10, 13];
console.log(array1.every(isBelowThreshold));
// expected output: true
12.filter
The filter() method creates a shallow copy of a portion of a given array, filtered down to just the elements from the given array that pass the test implemented by the provided function.
array.filter(callback[, thisObject]);
const words = ['spray', 'limit', 'elite', 'exuberant', 'destruction', 'present'];
const result = words.filter(word => word.length > 6);
console.log(result);
// expected output: Array ["exuberant", "destruction", "present"]
13.indexOf
The indexOf() method returns the first index at which a given element can be found in the array, or -1 if it is not present.
array.indexOf(searchElement[, fromIndex]);
const beasts = ['ant', 'bison', 'camel', 'duck', 'bison'];
console.log(beasts.indexOf('bison'));
// expected output: 1
// start from index 2
console.log(beasts.indexOf('bison', 2));
// expected output: 4
console.log(beasts.indexOf('giraffe'));
// expected output: -1
14.reduce
The reduce() method executes a user-supplied "reducer" callback function on each element of the array, in order, passing in the return value from the calculation on the preceding element. The final result of running the reducer across all elements of the array is a single value.
array.reduce(callback[, initialValue]);
const array1 = [1, 2, 3, 4];
// 0 + 1 + 2 + 3 + 4
const initialValue = 0;
const sumWithInitial = array1.reduce(
(previousValue, currentValue) => previousValue + currentValue,
initialValue
);
console.log(sumWithInitial)
15.reverse
The reverse() method reverses an array in place and returns the reference to the same array, the first array element now becoming the last, and the last array element becoming the first. In other words, elements order in the array will be turned towards the direction opposite to that previously stated.
array.reverse();
const array1 = ['one', 'two', 'three'];
console.log('array1:', array1);
// expected output: "array1:" Array ["one", "two", "three"]
const reversed = array1.reverse();
console.log('reversed:', reversed);
// expected output: "reversed:" Array ["three", "two", "one"]
// Careful: reverse is destructive -- it changes the original array.
console.log('array1:', array1);
// expected output: "array1:" Array ["three", "two", "one"]
16.sort
The sort() method sorts the elements of an array in place and returns the reference to the same array, now sorted. The default sort order is ascending, built upon converting the elements into strings, then comparing their sequences of UTF-16 code units values.
array.sort( compareFunction );
const months = ['March', 'Jan', 'Feb', 'Dec'];
months.sort();
console.log(months);
// expected output: Array ["Dec", "Feb", "Jan", "March"]
const array1 = [1, 30, 4, 21, 100000];
array1.sort();
console.log(array1);
// expected output: Array [1, 100000, 21, 30, 4]
17.toString
The toString() method returns a string representing the object.
array.toString();
function Dog(name) {
this.name = name;
}
const dog1 = new Dog('Gabby');
Dog.prototype.toString = function dogToString() {
return `${this.name}`;
};
console.log(dog1.toString());
// expected output: "Gabby"
The at() method takes an integer value and returns the item at that index, allowing for positive and negative integers. Negative integers count back from the last item in the array.
array.at(index)
const array1 = [5, 12, 8, 130, 44];
let index = 2;
console.log(`Using an index of ${index} the item returned is ${array1.at(index)}`);
// expected output: "Using an index of 2 the item returned is 8"
index = -2;
console.log(`Using an index of ${index} item returned is ${array1.at(index)}`);
// expected output: "Using an index of -2 item returned is 130"
19.find
The find() method returns the first element in the provided array that satisfies the provided testing function. If no values satisfy the testing function, undefined is returned.
array.find(function(currentValue, index, arr),thisValue)
const array1 = [5, 12, 8, 130, 44];
const found = array1.find(element => element > 10);
console.log(found);
// expected output: 12
20.some
The some() method tests whether at least one element in the array passes the test implemented by the provided function. It returns true if, in the array, it finds an element for which the provided function returns true; otherwise it returns false. It doesn't modify the array.
array.some(callback[, thisObject]);
const array = [1, 2, 3, 4, 5];
// checks whether an element is even
const even = (element) => element % 2 === 0;
console.log(array.some(even));
// expected output: true