Javascript Arrays

JavaScript arrays are used to store multiple values in a single variable. If you have a list of names, storing the names in single variables look like this.

var name1 = "Folau";
var name2 = "Kinga";
var name3 = "Laulau";
var name4 = "Tonga";
var name5 = "Isi";

However, what if you want to loop through the names and find a specific one? And what if you had 1000 names instead of 5 names?

The solution to this problem is arrays. An array can hold many values under a single name, and you can access the values by referring to an index number.

Array Declaration

You can use new Array or [].

let names = new Array('John','Peter','James');
let numbers = [1,2,3,4,5];

Access elements on Arrays

You access an array element by referring to the index number starting from 0.

let scores = new Array(12,34,56,100);
let highScore = scores[3];
console.log("highScore: "+highScore);//100

Update element of an array

let scores = new Array(12,34,56,100); 
scores[3] = 505; 
let highScore = scores[3];
console.log("highScore: "+highScore);//505

Array size

The length property of an array returns the length of an array (the number of array elements).

let scores = new Array(12,34,56,100);
console.log("scores.length: "+scores.length);//4

Add elements to an array

The easiest way to add a new element to an array is using the push() method. The push() method adds elements to the end of the array.

let names = ['John','Peter','James'];
console.log("names");
console.log(names);["John", "Peter", "James"]
names.push('Juan');

console.log("names");
console.log(names);["John", "Peter", "James","Juan"]

Use the unshift method to add elements to the beginning of the array.

let names = ['John','Peter','James']; 
console.log("names"); 
console.log(names);["John", "Peter", "James"] 

names.unshift('Juan'); 
console.log("names"); 
console.log(names);["Juan","John", "Peter", "James"]

Use the splice method can be used to add new items to an array.

let names = ['John','Peter','James'];
console.log("names");
console.log(names);//['John','Peter','James']

names.splice(2, 0, "Folau");
console.log("names");
console.log(names);//['John','Peter','Folau','James']

 

Remove elements from an array

Use the pop() method which removes elements from the end of the array.

let names = ['John','Peter','James'];
console.log("names");
console.log(names);//['John','Peter','James']

names.pop();
console.log("names");
console.log(names);//['John','Peter']

Use the shift() method to remove elements from the beginning of the array.

let names = ['John','Peter','James']; 
console.log("names"); 
console.log(names);//['John','Peter','James'] 

names.shift(); 
console.log("names"); 
console.log(names);//['Peter','James']

Note: methods push/pop run fast, while shift/unshift are slow.

Difference between Arrays and Objects.

Arrays use numbered indexes.  Objects use named indexes.

let arr = ["Peter",25];
console.log("arr[0]: "+arr[0]);//Peter
console.log("arr[1]: "+arr[1]);//25

let obj = {"name":"Peter","age":"25"};
console.log("obj.name: "+obj.name);//Peter

Sorting

By default, the sort() function sorts values as strings.

let names = ['John','Peter','James'];

console.log("names");
console.log(names);

names.sort();
console.log("names");
console.log(names);//["James", "John", "Peter"]

Sorting numbers using a comparator

var numbers = [40, 100, 1, 5, 25, 10];
numbers.sort(function(a, b){return a - b});// asc order - 1,5,10,25,40,100
numbers.sort(function(a, b){return b - a});// desc order - 100,40,25,10,5,1

Array Iteration

The foreach() method calls a function (a callback function) once for each array element.

/*
* Array iteration
*/
console.log("Array Iteration with foreach");
var numbers = [2, 4, 6, 8 , 10];

// value
numbers.forEach(function(number){
    console.log("number: "+number);
});
// value, index
numbers.forEach(function(number,index){
    console.log("number: "+number+", index: "+index);
});
// value, index, array
numbers.forEach(function(number,index, arr){
    console.log("number: "+number+", index: "+index+", original array: "+arr);
});

Using for loop to iterate through array elements.

/*
* Array iteration
*/
var numbers = [2, 4, 6, 8 , 10];

console.log("Array Iteration with for loop");
for(let i=0;i<numbers.length;i++){
    let number = numbers[i];
    console.log("index: "+i+", number: "+number);
}

Array map

The map() method creates a new array by performing a function on each array element. The map() method does not execute the function for array elements without values. The map() method does not change the original array.

console.log("Array map");
var numbers2 = numbers.map(function(number){
    console.log("number: "+number);
    return number * 2;
});
console.log(numbers);
console.log("numbers2 * 2");
console.log(numbers2);

var numbers3 = numbers.map(function(number, index){
    console.log("number: "+number+", index: "+index);
    return number * 3;
});
console.log(numbers);
console.log("numbers3 * 3");
console.log(numbers3);

var numbers4 = numbers.map(function(number, index, arr){
    console.log("number: "+number+", index: "+index+", original array: "+arr);
    return number * 4;
});
console.log(numbers);
console.log("numbers4 * 4");
console.log(numbers4);

Output

number: 2
number: 4
number: 6
number: 8
number: 10
numbers - (5) [2, 4, 6, 8, 10]
numbers2 * 2
numbers2 - (5) [4, 8, 12, 16, 20]
number: 2, index: 0
number: 4, index: 1
number: 6, index: 2
number: 8, index: 3
number: 10, index: 4
numbers - (5) [2, 4, 6, 8, 10]
numbers3 * 3
numbers3 - (5) [6, 12, 18, 24, 30]
number: 2, index: 0, original array: 2,4,6,8,10
number: 4, index: 1, original array: 2,4,6,8,10
number: 6, index: 2, original array: 2,4,6,8,10
number: 8, index: 3, original array: 2,4,6,8,10
number: 10, index: 4, original array: 2,4,6,8,10
numbers - (5) [2, 4, 6, 8, 10]
numbers4 * 4
numbers4 - (5) [8, 16, 24, 32, 40]

Array filter

The filter() method creates a new array with array elements that passes a condition.

console.log("Array filter");
var over8 = numbers.filter(function(number, index){
    console.log("number: "+number+", index: "+index);
    return number >= 8;
});
console.log("numbers: "+numbers);
console.log("numbers greater than or equal 8");
console.log(over8);//[8, 10]

var over4 = numbers.filter(function(number, index, arr){
    console.log("number: "+number+", index: "+index+", original array: "+arr);
    return number >= 4;
});
console.log("numbers: "+numbers);
console.log("numbers greater than or equal 4");
console.log(over4);//[4, 6, 8, 10]

Array reduce

The reduce() method runs a function on each array element to produce (reduce it to) a single value. The reduce() method works from left-to-right in the array. Note that the reduce() method does not reduce the original array.

console.log("Array reduce");
var numbers = [2, 4, 6, 8 , 10];
var sum = numbers.reduce(function(total, number, index, arr){
    console.log("total: "+total+", number: "+number+", index: "+index+", arr: "+arr);
    return total + number;
});

console.log("numbers: "+numbers);
console.log("sum of numbers: "+sum);// 30

Array every

The every() method check if all array values pass a condition.

console.log("Array every");
var numbers = [2, 4, 6, 8 , 10];
var allOver2 = numbers.every(function(number,index){
    console.log("number: "+number+", index: "+index);
    return number >=2;
});
console.log("allOver2: "+allOver2);// true

var allOver4 = numbers.every(function(number,index,arr){
    console.log("number: "+number+", index: "+index+", original array: "+arr);
    return number >=4;
});
console.log("allOver4: "+allOver4);// false

Array some

The some() method check if some array values pass a condition.

console.log("Array some");
var numbers = [2, 4, 6, 8 , 10];
var someOver2 = numbers.some(function(number,index){
    console.log("number: "+number+", index: "+index);
    return number >=2;
});
console.log("someOver2: "+someOver2);// true
var someOver4 = numbers.some(function(number,index,arr){
    console.log("number: "+number+", index: "+index+", original array: "+arr);
    return number >=4;
});
console.log("someOver4: "+someOver4);// true

Array indexOf

The indexOf() method searches an array for an element value and returns its position. Note: The first item has position 0, the second item has position 1, and so on.
console.log("Array indexOf");
let names = ['John','James','Fika','Peter'];
var peterIndex = names.indexOf("Peter");
console.log("peterIndex: "+peterIndex);// 3

Array find

The find() method returns the value of the first array element that passes a condition function.

console.log("Array find");
var numbers = [2, 4, 6, 8 , 10];
var first3 = numbers.find(function(number, index, arr){
    return number > 3;
});
console.log("first3: "+first3);// 4

 

Source code on Github




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