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Arrays are lists of objects that can be manipulated in various ways. Each entry can be accessed by their own index. Arrays can be combined in various ways and they can be also be nested in each other, letting us create multi-dimensional arrays. We can make arrays out of a collection of any objects. They can also be destructed into variables so each entry can be accessed and manipulated individually. JavaScript arrays are zero-indexed so the starting index of each array is always zero. This means that index 0 has the first element of the array.

Arrays can contain any type of object. They do not have to be the same type of objects. Also if an array entry with a given index hasn’t been assigned yet, it has an undefined value.

Examples of things that can be stored with arrays include:

• to-do list
• recipe list
• address book contacts
• shopping list
• grocery list
• your appointments
• anything else that can be entered into a list.

Without arrays, all we can do is declare variables for each entry individually which is a real pain and not practical.

Declaring Arrays

To declare arrays, we use the let or const keyword — like this:

let arr = [1,2,3];
const arr2 = [1,2,3];

We use let for variables and const for arrays that don’t change.

Also, we can declare the arrays first and then insert the values later, like this:

let arr = []
arr[0] = 1;
arr[1] = 2;
arr[2] = 3;

This is the same as let arr = [1,2,3] since we have the same entries and the same order in both arr arrays.

We can declare arrays with different type of values in the same array, like this:

let arr = [1, 'chicken', {foo: 'bar'}];

As you can see, it doesn’t matter what kind of data we put in the array. However, we do have to be careful to avoid data type errors when we traverse or manipulate arrays by checking the type and content of the objects, and whether any entry is null or undefined .

Another way to create an array is the new keyword, like this:

let names = new Array('Bob','Jane', 'John');

Accessing Array Data

We access array items by their index. For example, to get the second element of an array, we write:

arr[1]

If arr is assigned to [1, ‘chicken’, {foo: ‘bar’}]; , then arr[1] would be 'chicken' .

Get Array Size

Array is an object that has the length property that we can get the size of the array, so the names array we have above would have a length of three. We write the names.length to access the length of the array.

Multidimensional Arrays

In JavaScript, multidimensional arrays are just an array nested in another array. So, to declare a multidimensional array, we can use the same declaration methods as above, except we replace the entities inside with arrays. For example, we can write:

let multiArray = [[1,2,3], [4,5,6]];

or:

let multiArray = new Array([1,2,3], [4,5,6]);

We can also write:

let multiArray = [];
multiArray[0] = [];
multiArray[0][0] = 1;
multiArray[0][1] = 2;
multiArray[0][2] = 3;
multiArray[1] = [];
multiArray[1][0] = 4;
multiArray[1][1] = 5;
multiArray[1][2] = 6;

All three of these pieces of code are equivalent.

We access multidimensional array entries by adding another square bracket with the index of the inner array. For example, if we want to get the second item of the first array in multiArray then we write:

multiArray[0][1]

Traversing Arrays

We can traverse the values of arrays with loops. Loops are pieces of code that repeat until the ending condition is met. In JavaScript, we have the for loop, while loop and the do...while loop.

For Loop

With the for loop, given that we have the name array, we can traverse the loop by running:

let names = new Array('Bob','Jane', 'John');
for (let i = 0; i < names.length; i++){
  console.log(names[i]);
}

A for loop is usually written with the starting condition as the first statement, then the ending condition in the second statement, and the index changing condition in the third statement. The statements are separated by the semicolons. Because arrays start with index zero in JavaScript, we terminate the loop when the array’s index reaches one less than the array’s length.

While Loop

while loop will loop whenever a condition stays true.

For example, the loop below will run if the index number i is less than three:

const array = [1,2,3];
let i = 0;
while(i < array.length){
  console.log(i);
}

If the condition in the parentheses is never true, then the loop content will never run.

Do While Loop

do...while loop will always execute the first iteration.

const array = [1,2,3];
let i = 0;
do{
  console.log(i);
}
while(i < array.length)

In the example above, it will at least log 0, but it will also log 1 and 2 in this case since those two numbers are less than three.

With the above loops, you can call break to stop the loop or return before the loop is completely finished.

//do while loop
const array = [1,2,3];
let i = 0;
do{
  console.log(i);  
  if (i == 1}{    
    break;  
  }
}
while(i < array.length)

//while loop
i = 0;
while(i < array.length){  
  if (i == 1){    
    break;  
  }
  console.log(i);
}

//for loop
for (let j = 0; j < array.length; j++){  
  if (j == 1){
    break;
  }
  console.log(j);
}

In the above examples, you will not see two logged.

An example of returning from within the loop:

const loop = ()=>{
  const array = [1,2,3];  
  for (let j = 0; j < array.length; j++){       
    if (j == 1){
      return j;
    }
    console.log(j);
  }
}
loop() //returns 1

You can also skip iterations with the continue statement:

const array = [1,2,3];
for (let j = 0; j < array.length; j++){  
  if (j == 1){
    continue;
  }
  console.log(j) // 1 will be skipped;
}

Array Properties

Since an array is a JavaScript object. It is its own properties. Like any object in JavaScript, it has the prototype property which lets developers add methods and properties of an array object. The constructor property is the reference to the function that’s created by the array object’s prototype. The length returns the size of the array.

Array Methods

To make manipulating arrays easy, the JavaScript’s standard library contains lots of array methods that make manipulating arrays easy. There are methods to find and filter items, and add and remove items in an array for example. There are also functions to combine multiple arrays into one.

These are some common array methods that you can use to do common operations with arrays.

Array.forEach

forEach will iterate through every entry of the array. You cannot break out of it or return a value from it. It takes a callback function where you can execute code.

Example:

const array = [1,2,3];
  array.forEach(a =>{  console.log(a);
})

In the above example, all the numbers in the array will be logged.

Array.find

Array.find will return the element in the array with the given condition. For example, if you want to get certain numbers from the array, you do this:

const array = [1,2,3];
const num = array.find(a => a == 2); // returns 2

find returns a single result.

Array.findIndex

Array.findIndex will return the index of the element in the array with the given condition. It takes a callback function that returns a given condition. For example, if you want to get the index of a certain number from the array, you do this:

const array = [1,2,3];
const num = array.findIndex(a => a == 2); // returns 1

Array.filter

Array.filter will return an array of items that meet the given condition. It takes a callback function that returns a given condition. filter returns a new array.

For example, if you want to get the index of a certain number from the array, you do:

const array = [1,2,3];
const numArray = array.filter(a => a == 2); // returns [2]

Array.includes

Array.includes checks if an item exists in an array. It takes a number or string which the function can compare.

const array = [1,2,3];
const includesTwo = array.includes(2); // returns true

Array.some

Array.somechecks if some items meet the condition given. It takes a callback function which returns a boolean for the condition.

const array = [1,2,3];
const includesTwo = array.some(a => a == 2); // returns true
const includesFive = array.some(a => a == 5); // returns false

Array.every

Array.every checks if every item meets the condition given. It takes a callback function which returns a boolean for the condition.

const array = [1,2,3];
const everyElementIsTwo = array.every(a => a == 2); // returns false
const everyElementIsNumber = array.every(a => typeof a == 'number'); // returns true since every item in the array is a number

Array.isArray

Array.isArray checks if an object given is an array. It is a convenient way to check if an element is an array.

const array = [1,2,3];
const notArray = {};
let objIsArray = Array.isArray(array); // true
objIsArray = Array.isArray(notArray); // false

Array.from(new Set(array))

Set is an object that cannot have duplicate entries. You can create a new Setfrom an array then convert it back to an array.

const array = [1,2,2,3];
const arrayWithDups = Array.from(new Set(array)); //returns new array without duplicates, [1,2,3]

Array.slice(startIndex, endIndex)

Returns a new array from startIndex to endIndex — 1 .

Example:

const arr = [1,2,3,4,5];
const newArr = arr.slice(0,2);
console.log(newArr); // returns [1,2]

Array.splice(index, numberOfItems)

Remove array item in place with the given index, and then numberOfItems after it.

For example:

const arr = [1,2,3,4,5];
arr.splice(0,2);
console.log(arr); // returns [3, 4, 5] since we specified that we remove item located at index 0 and 2 items after that.

Array.sort(sortFunction)

Array.sort sorts array in place according to the condition you return insortFunction .

The sortFunction should be in this format:

const sortFunction = (a, b) {  
  if (a < b) {  
    return -1;  
  }  
  if (a > b) {  
    return 1;  
  }  
  // a must be equal to b  
  return 0;  
}

By default, if no sortFunction is specified, then the array items will be converted to a string and will be sorted according to the Unicode value of the string.

Array.fill(newElement, startIndex, endIndex)

Array.fill will add or replace the element with the element specified from startIndex to endIndex . If no startIndex is defined then it will start from 0. If no endIndex is defined, then it will change values up to the end of the array.

For example:

let array = [1, 2, 3, 4, 5];
console.log(array.fill(0, 2, 4));
// array is now [1, 2, 0, 0, 0]
console.log(array.fill(5, 1));
// array is now [1, 5, 5, 5, 5]
console.log(array.fill(6));
// array is now [6, 6, 6, 6, 6]

Recursively Flatten Array

Array.flat function does not do a good job of recursively flatten arrays. The depth is limited and it does not flatten all kinds of nested array structures. The better way to do it is to write a recursive function to do it.

let arr1 = [1, 2, [3, 4], 5];
let arr2 = [1, 2, [3, 4], [5, [6,[7,]]]];

const flatten = (items) => {
  const flat = [];  items.forEach(item => {
    if (Array.isArray(item)) {
      flat.push(...flatten(item));
    } else {
      flat.push(item);
    }
  });  return flat;
}

console.log(flatten(arr1));
console.log(flatten(arr2));

Array.join(separator)

Array.join will return a string by concatenating the entries after they are converted to string with separator between each entry. Works best with string and number arrays.

Example:

const arr = [1,2,3];
console.log(arr.join(',')) // get '1,2,3'
const arr = ['1',2,3];
console.log(arr.join(',')) // get '1,2,3'

Array.indexOf(elementToFind)

Array.indexOfwill return the first index of the elementToFind in the array. Works best with string and number arrays. If you want to find a non-string or number object, use Array.findIndex . Returns -1 is element is not found.

Example:

const arr = [1,2,3];
console.log(arr.indexOf(1)); // returns 0
const arr2 = [1,1,2,3];
console.log(arr2.indexOf(1)) // still 0

Array.lastIndexOf()

Array.lastIndexOfwill return the last index of the elementToFind in the array. Works best with string and number arrays. Returns -1 is element is not found. The function also takes a starting index to start searching backward as a second parameter

Example:

const arr = [1,2,3];
console.log(arr.indexOf(1)); // returns 0

const arr2 = [1,1,2,3];
console.log(arr2.indexOf(1)) // returns 1

const arr3 = [3,1,2,3]
console.log(arr3.lastIndexOf(3, 2)) // returns 0, start searching backwards from index 2

Array.push(newElement)

Array.push adds a new element to an array.

Example:

let arr = [1,2,3];
arr.push(4);
console.log(arr) // [1,2,3,4]

Array.pop()

Array.pop removes the last element of the array.

Example:

let arr = [1,2,3,4];
arr.pop();
console.log(arr) // [1,2,3]

Array.map(mapFunction)

Array.map returns a new array which transforms the existing array’s element by calling the mapFunction . mapFunction takes one argument, which is the array element.

Example:

let arr = [1,2,3,4];
const newArr = arr.map(a=>a*2)
console.log(newArr) // [2,4,6,8]

Array.reduce(reduceFunction)

Array.reduce combines elements of an array to return a value by calling the reduceFunction on all the elements to combine them. reduceFunction takes two arguments, which is the current and next element in the array.

Example:

const arr = [1,2,3,4];
const sum = arr.reduce((a,b)=>a+b);
console.log(sum); // returns 10

Array.reverse()

Array.reverse returns a new array with the existing array’s elements in reverse order.

Example:

const arr = [1,2,3,4];
console.log(arr.reverse()) // [4,3,2,1]

Now that we learned about arrays, we can do a lot more in our JavaScript programs. There’s a big world ahead of us!