What are arrays in java script
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Arrays in JavaScript can store multiple data types. Common array methods allow users to manipulate array elements by adding, removing, and modifying items. Key methods include toString() and join() to convert arrays to strings, concat and push/pop to modify arrays, and filter(), map(), and reduce() to transform array elements.
![What are Arrays in JavaScript?
Before we proceed, you need to understand what arrays really mean.
In JavaScript,anarray is a variable that is used to store different data types.
It basically stores different elements in one box and canbe later assesssed
with the variable.
Declaring an array:
let myBox = []; // Initial Arraydeclaration in JS
Arrays can contain multiple data types
let myBox = ['hello', 1, 2, 3, true, 'hi'];
Arrays can be manipulated by using several actions known
as methods. Some of these methods allow us to add, remove, modify
and do lots more to arrays.
I would be showing you a few in this article, let’s roll :)
NB: I used Arrow functions in this post, If you don’t know what this means,
you should read here. Arrow function is an ES6 feature.
toString()
The JavaScript method toString() converts an array to a string
separated by a comma.
let colors = ['green', 'yellow', 'blue'];
console.log(colors.toString()); // green,yellow,blue
join()
The JavaScript join() method combines all array elements into a string.
It is similar to toString() method, but here you can specify the
separator instead of the default comma.
let colors = ['green', 'yellow', 'blue'];
console.log(colors.join('-')); // green-yellow-blue
concat](https://image.slidesharecdn.com/whatarearraysinjavascript-191125104700/85/What-are-arrays-in-java-script-1-320.jpg)
![This method combines two arrays together or add more items to an
array and then return a new array.
let firstNumbers = [1, 2, 3];
let secondNumbers = [4, 5, 6];
let merged = firstNumbers.concat(secondNumbers);
console.log(merged); // [1, 2, 3, 4, 5, 6]
push()
This method adds items to the end of an array and changes the original
array.
let browsers = ['chrome', 'firefox', 'edge'];
browsers.push('safari', 'opera mini');
console.log(browsers);
// ["chrome", "firefox", "edge", "safari", "opera mini"]
pop()
This method removes the last item of an array and returns it.
let browsers = ['chrome', 'firefox', 'edge'];
browsers.pop(); // "edge"
console.log(browsers); // ["chrome", "firefox"]
shift()
This method removes the first item of an array and returns it.
let browsers = ['chrome', 'firefox', 'edge'];
browsers.shift(); // "chrome"
console.log(browsers); // ["firefox", "edge"]
unshift()
This method adds an item(s) to the beginning of an array
and changes the original array.
let browsers = ['chrome', 'firefox', 'edge'];
browsers.unshift('safari');
console.log(browsers); // ["safari", "chrome", "firefox", "edge"]
You can also add multipleitems at once
splice()
This method changes an array, by adding, removing and inserting
elements.
The syntax is:
array.splice(index[, deleteCount, element1, ..., elementN])](https://image.slidesharecdn.com/whatarearraysinjavascript-191125104700/85/What-are-arrays-in-java-script-2-320.jpg)
![ Index here is the starting point for removing elements in the array
deleteCount is the number of elements to be deleted from that index
element1, …, elementN is the element(s) to be added
Removing items
after running splice() , it returns the array with the item(s) removed and
removes it from the original array.
let colors = ['green', 'yellow', 'blue', 'purple'];
colors.splice(0, 3);
console.log(colors); // ["purple"]
// deletes ["green", "yellow", "blue"]
NB: The deleteCount does not includethe last index in range.
If the second parameter is not declared, every element starting from the
given index will be removed from the array:
let colors = ['green', 'yellow', 'blue', 'purple'];
colors.splice(3);
console.log(colors); // ["green", "yellow", "blue"]
// deletes ['purple']
In the next example we will remove 3 elements from the array and
replace them with more items:
let schedule = ['I', 'have', 'a', 'meeting', 'tommorrow'];
// removes 4 first elements and replace them with another
schedule.splice(0, 4, 'we', 'are', 'going', 'to', 'swim');
console.log(schedule);
// ["we", "are", "going", "to", "swim", "tommorrow"]
Addingitems
To add items, we need to set the deleteCount to zero
let schedule = ['I', 'have', 'a', 'meeting', 'with'];
// adds 3 new elements to the array
schedule.splice(5, 0, 'some', 'clients', 'tommorrow');
console.log(schedule);
// ["I", "have", "a", "meeting", "with", "some", "clients", "tommorrow"]
slice()
This method is similar to splice() but very different. It returns subarrays
instead of substrings.
This method copies a given part of an array and returns that copied
part as a new array. It does not change the original array.](https://image.slidesharecdn.com/whatarearraysinjavascript-191125104700/85/What-are-arrays-in-java-script-3-320.jpg)
![The syntax is:
array.slice(start, end)
Here’s a basic example:
let numbers = [1, 2, 3, 4]
numbers.slice(0, 3)
// returns [1, 2, 3]
console.log(numbers) // returns the original array
The best way to use slice() is to assign it to a new variable.
let message = 'congratulations'
const abbrv = message.slice(0, 7) + 's!';
console.log(abbrv) // returns "congrats!"
split()
This method is used for strings. It divides a string into substrings and
returns them as an array.
Here’s the syntax:string.split(separator, limit);
The separator here defines how to split a string either by a comma.
The limit determines the number of splits to be carried out
let firstName = 'Bolaji';
// return the string as an array
firstName.split() // ["Bolaji"]
another example:
let firstName = 'hello, my name is bolaji, I am a dev.';
firstName.split(',', 2); // ["hello", " my name is bolaji"]
NB: If we declare an empty array, like this firstName.split(''); then each item in
the string will be divided as substrings:
let firstName = 'Bolaji';
firstName.split('') // ["B", "o", "l", "a", "j", "i"]
indexOf()
This method looks for an item in an array and returns the index where
it was found else it returns -1
let fruits = ['apple', 'orange', false, 3]
fruits.indexOf('orange'); // returns 1
fruits.indexOf(3); // returns 3
friuts.indexOf(null); // returns -1 (not found)](https://image.slidesharecdn.com/whatarearraysinjavascript-191125104700/85/What-are-arrays-in-java-script-4-320.jpg)
![lastIndexOf()
This method works the same way indexOf() does except that it works
from right to left. It returns the last index where the item was found
let fruits = ['apple', 'orange', false, 3, 'apple']
fruits.lastIndexOf('apple'); // returns 4
filter()
This method creates a new array if the items of an array pass a certain
condition.
The syntax is:
let results = array.filter(function(item, index, array) {
// returns true if the item passes the filter
});
Example:
Checks users from Nigeria
const countryCode = ['+234', '+144', '+233', '+234'];
const nigerian = countryCode.filter( code => code === '+234');
console.log(nigerian); // ["+234", "+234"]
map()
This method creates a new array by manipulating the values in an array.
Example:
Displays usernames on a page. (Basic friend list display)
const userNames = ['tina', 'danny', 'mark', 'bolaji'];
const display = userNames.map(item => {
return '<li>' + item + '</li>';
})
const render = '<ul>' + display.join('') + '</ul>';
document.write(render);](https://image.slidesharecdn.com/whatarearraysinjavascript-191125104700/85/What-are-arrays-in-java-script-5-320.jpg)
![another example:
// adds dollar sign to numbers
const numbers = [10, 3, 4, 6];
const dollars = numbers.map( number => '$' + number);
console.log(dollars);
// ['$10', '$3', '$4', '$6'];
reduce()
This method is good for calculating totals.
reduce() is used to calculate a single value based on an array.
let value = array.reduce(function(previousValue, item, index, array) {
// ...
}, initial);
example:
To loop through an array and sum all numbers in the array up, we can use
the for of loop.
const numbers = [100, 300, 500, 70];
let sum = 0;
for (let n of numbers) {
sum += n;
}
console.log(sum);
Here’s how to do same with reduce()
const numbers = [100, 300, 500, 70];
const sum = numbers.reduce((accummulator, value) =>
accummulator + value
, 0);
console.log(sum); // 970
If you omit the initialvalue, the total will by default start from the first item
in the array.
const numbers = [100, 300, 500, 70];
const sum = numbers.reduce((accummulator, value) => accummulator + value);](https://image.slidesharecdn.com/whatarearraysinjavascript-191125104700/85/What-are-arrays-in-java-script-6-320.jpg)
![console.log(sum); // still returns 970
The snippet below shows how the reduce() method works with all four
arguments.
More insights into the reduce() method and various ways of using it
can be found here and here.
forEach()
This method is good for iterating through an array.
It applies a function on all items in an array
const colors = ['green', 'yellow', 'blue'];
colors.forEach((item, index) => console.log(index, item));
// returns the index and the every item in the array
// 0 "green"
// 1 "yellow"
// 2 "blue"
iteration can be done without passing the index argument
const colors = ['green', 'yellow', 'blue'];
colors.forEach((item) => console.log(item));
// returns everyitem in the array
// "green"
// "yellow"](https://image.slidesharecdn.com/whatarearraysinjavascript-191125104700/85/What-are-arrays-in-java-script-7-320.jpg)
![// "blue"
every()
This method checks if all items in an array pass the specified condition
and returntrue if passed, else false.
check if all numbers are positive
const numbers = [1, -1, 2, 3];
let allPositive = numbers.every((value) => {
return value >= 0;
})
console.log(allPositive); // would return false
some()
This method checks if an item (one or more) in an array pass the
specified condition and return true if passed, else false.
checks if at least one number is positive
const numbers = [1, -1, 2, 3];
let atLeastOnePositive = numbers.some((value) => {
return value >= 0;
})
console.log(atLeastOnePositive); // would return true
includes()
This method checks if an array contains a certain item. It is similar
to .some(), but instead of looking for a specific condition to pass, it
checks if the array contains a specific item.
let users = ['paddy', 'zaddy', 'faddy', 'baddy'];
users.includes('baddy'); // returns true
If the item is not found, it returns false](https://image.slidesharecdn.com/whatarearraysinjavascript-191125104700/85/What-are-arrays-in-java-script-8-320.jpg)
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What are arrays in java script
- 1. What are Arrays in JavaScript? Before we proceed, you need to understand what arrays really mean. In JavaScript,anarray is a variable that is used to store different data types. It basically stores different elements in one box and canbe later assesssed with the variable. Declaring an array: let myBox = []; // Initial Arraydeclaration in JS Arrays can contain multiple data types let myBox = ['hello', 1, 2, 3, true, 'hi']; Arrays can be manipulated by using several actions known as methods. Some of these methods allow us to add, remove, modify and do lots more to arrays. I would be showing you a few in this article, let’s roll :) NB: I used Arrow functions in this post, If you don’t know what this means, you should read here. Arrow function is an ES6 feature. toString() The JavaScript method toString() converts an array to a string separated by a comma. let colors = ['green', 'yellow', 'blue']; console.log(colors.toString()); // green,yellow,blue join() The JavaScript join() method combines all array elements into a string. It is similar to toString() method, but here you can specify the separator instead of the default comma. let colors = ['green', 'yellow', 'blue']; console.log(colors.join('-')); // green-yellow-blue concat
- 2. This method combines two arrays together or add more items to an array and then return a new array. let firstNumbers = [1, 2, 3]; let secondNumbers = [4, 5, 6]; let merged = firstNumbers.concat(secondNumbers); console.log(merged); // [1, 2, 3, 4, 5, 6] push() This method adds items to the end of an array and changes the original array. let browsers = ['chrome', 'firefox', 'edge']; browsers.push('safari', 'opera mini'); console.log(browsers); // ["chrome", "firefox", "edge", "safari", "opera mini"] pop() This method removes the last item of an array and returns it. let browsers = ['chrome', 'firefox', 'edge']; browsers.pop(); // "edge" console.log(browsers); // ["chrome", "firefox"] shift() This method removes the first item of an array and returns it. let browsers = ['chrome', 'firefox', 'edge']; browsers.shift(); // "chrome" console.log(browsers); // ["firefox", "edge"] unshift() This method adds an item(s) to the beginning of an array and changes the original array. let browsers = ['chrome', 'firefox', 'edge']; browsers.unshift('safari'); console.log(browsers); // ["safari", "chrome", "firefox", "edge"] You can also add multipleitems at once splice() This method changes an array, by adding, removing and inserting elements. The syntax is: array.splice(index[, deleteCount, element1, ..., elementN])
- 3. Index here is the starting point for removing elements in the array deleteCount is the number of elements to be deleted from that index element1, …, elementN is the element(s) to be added Removing items after running splice() , it returns the array with the item(s) removed and removes it from the original array. let colors = ['green', 'yellow', 'blue', 'purple']; colors.splice(0, 3); console.log(colors); // ["purple"] // deletes ["green", "yellow", "blue"] NB: The deleteCount does not includethe last index in range. If the second parameter is not declared, every element starting from the given index will be removed from the array: let colors = ['green', 'yellow', 'blue', 'purple']; colors.splice(3); console.log(colors); // ["green", "yellow", "blue"] // deletes ['purple'] In the next example we will remove 3 elements from the array and replace them with more items: let schedule = ['I', 'have', 'a', 'meeting', 'tommorrow']; // removes 4 first elements and replace them with another schedule.splice(0, 4, 'we', 'are', 'going', 'to', 'swim'); console.log(schedule); // ["we", "are", "going", "to", "swim", "tommorrow"] Addingitems To add items, we need to set the deleteCount to zero let schedule = ['I', 'have', 'a', 'meeting', 'with']; // adds 3 new elements to the array schedule.splice(5, 0, 'some', 'clients', 'tommorrow'); console.log(schedule); // ["I", "have", "a", "meeting", "with", "some", "clients", "tommorrow"] slice() This method is similar to splice() but very different. It returns subarrays instead of substrings. This method copies a given part of an array and returns that copied part as a new array. It does not change the original array.
- 4. The syntax is: array.slice(start, end) Here’s a basic example: let numbers = [1, 2, 3, 4] numbers.slice(0, 3) // returns [1, 2, 3] console.log(numbers) // returns the original array The best way to use slice() is to assign it to a new variable. let message = 'congratulations' const abbrv = message.slice(0, 7) + 's!'; console.log(abbrv) // returns "congrats!" split() This method is used for strings. It divides a string into substrings and returns them as an array. Here’s the syntax:string.split(separator, limit); The separator here defines how to split a string either by a comma. The limit determines the number of splits to be carried out let firstName = 'Bolaji'; // return the string as an array firstName.split() // ["Bolaji"] another example: let firstName = 'hello, my name is bolaji, I am a dev.'; firstName.split(',', 2); // ["hello", " my name is bolaji"] NB: If we declare an empty array, like this firstName.split(''); then each item in the string will be divided as substrings: let firstName = 'Bolaji'; firstName.split('') // ["B", "o", "l", "a", "j", "i"] indexOf() This method looks for an item in an array and returns the index where it was found else it returns -1 let fruits = ['apple', 'orange', false, 3] fruits.indexOf('orange'); // returns 1 fruits.indexOf(3); // returns 3 friuts.indexOf(null); // returns -1 (not found)
- 5. lastIndexOf() This method works the same way indexOf() does except that it works from right to left. It returns the last index where the item was found let fruits = ['apple', 'orange', false, 3, 'apple'] fruits.lastIndexOf('apple'); // returns 4 filter() This method creates a new array if the items of an array pass a certain condition. The syntax is: let results = array.filter(function(item, index, array) { // returns true if the item passes the filter }); Example: Checks users from Nigeria const countryCode = ['+234', '+144', '+233', '+234']; const nigerian = countryCode.filter( code => code === '+234'); console.log(nigerian); // ["+234", "+234"] map() This method creates a new array by manipulating the values in an array. Example: Displays usernames on a page. (Basic friend list display) const userNames = ['tina', 'danny', 'mark', 'bolaji']; const display = userNames.map(item => { return '<li>' + item + '</li>'; }) const render = '<ul>' + display.join('') + '</ul>'; document.write(render);
- 6. another example: // adds dollar sign to numbers const numbers = [10, 3, 4, 6]; const dollars = numbers.map( number => '$' + number); console.log(dollars); // ['$10', '$3', '$4', '$6']; reduce() This method is good for calculating totals. reduce() is used to calculate a single value based on an array. let value = array.reduce(function(previousValue, item, index, array) { // ... }, initial); example: To loop through an array and sum all numbers in the array up, we can use the for of loop. const numbers = [100, 300, 500, 70]; let sum = 0; for (let n of numbers) { sum += n; } console.log(sum); Here’s how to do same with reduce() const numbers = [100, 300, 500, 70]; const sum = numbers.reduce((accummulator, value) => accummulator + value , 0); console.log(sum); // 970 If you omit the initialvalue, the total will by default start from the first item in the array. const numbers = [100, 300, 500, 70]; const sum = numbers.reduce((accummulator, value) => accummulator + value);
- 7. console.log(sum); // still returns 970 The snippet below shows how the reduce() method works with all four arguments. More insights into the reduce() method and various ways of using it can be found here and here. forEach() This method is good for iterating through an array. It applies a function on all items in an array const colors = ['green', 'yellow', 'blue']; colors.forEach((item, index) => console.log(index, item)); // returns the index and the every item in the array // 0 "green" // 1 "yellow" // 2 "blue" iteration can be done without passing the index argument const colors = ['green', 'yellow', 'blue']; colors.forEach((item) => console.log(item)); // returns everyitem in the array // "green" // "yellow"
- 8. // "blue" every() This method checks if all items in an array pass the specified condition and returntrue if passed, else false. check if all numbers are positive const numbers = [1, -1, 2, 3]; let allPositive = numbers.every((value) => { return value >= 0; }) console.log(allPositive); // would return false some() This method checks if an item (one or more) in an array pass the specified condition and return true if passed, else false. checks if at least one number is positive const numbers = [1, -1, 2, 3]; let atLeastOnePositive = numbers.some((value) => { return value >= 0; }) console.log(atLeastOnePositive); // would return true includes() This method checks if an array contains a certain item. It is similar to .some(), but instead of looking for a specific condition to pass, it checks if the array contains a specific item. let users = ['paddy', 'zaddy', 'faddy', 'baddy']; users.includes('baddy'); // returns true If the item is not found, it returns false
- 9. Summary toString() converts an array to a string separated by a comma. join() combines all array elements into a string. concat combines two arrays together or add more items to an array and then return a new array. push() adds item(s) to the end of an array and changes the original array. pop() removes the last item of an array and returns it shift() removes the first item of an array and returns it unshift() adds an item(s) to the beginning of an array and changes the original array. splice() changes an array, by adding, removing and inserting elements. slice() copies a given part of an array and returns that copied part as a new array. It does not change the original array. split() divides a string into substrings and returns them as an array. indexOf() looks for an item in an array and returns the index where it was found else it returns -1 lastIndexOf() looks for an item from right to left and returns the last index where the item was found. filter() creates a new array if the items of an array pass a certain condition. map() creates a new array by manipulating the values in an array. reduce() calculates a single value based on an array. forEach() iterates through an array, it applies a function on all items in an array every() checks if all items in an array pass the specified condition and return true if passed, else false. some() checks if an item (one or more) in an array pass the specified condition and return true if passed, else false. includes() checks if an array contains a certain item.