var arr = [1, 2, 3, 4];

var arr2 = [5, 6, 7, 8];

​

const both = arr.concat(arr2);

​

console.log(both);

//[1, 2, 3, 4, 5, 6, 7, 8]

//This method adds two or more strings and returns a new single string.

​

let str1 = new String( "This is string one" ); 

let str2 = new String( "This is string two" ); 

let str3 = str1.concat(str2.toString());

console.log("str1 + str2 : "+str3)

​

output:

str1 + str2 : This is string oneThis is string two

const num1 = [2, 4, 5, 3, 8, 9, 11, 33, 44];

const num2 = [22, 33, 44, 55, 66, 77, 88];

​

const concatNumbers = num1.concat(num2)

console.log(concatNumbers)

//Expected output:

/*[

2, 4, 5, 3, 8, 9,

    11, 33, 44, 22, 33, 44,

    55, 66, 77, 88

 ]

*/

let a = "foo";

let b = ["f", "o", "o"];

​

let c = a.concat("bar");           // "foobar"

let d = b.concat(["b", "a", "r"]); // ["f","o","o","b","a","r"]

const array1 = [1, 2, 3, 4, 5];

const array2 = [6, 7, 8, 9, 10];

const array3 = [11, 12, 13, 14, 15];

​

const mergeArray = array1.concat(array2, array3);

​

console.log(mergeArray);

var one = 'Hello';

var two = 'World';

​

console.log(one + two);

It will print: HelloWorld

​

console.log(one + ' ' + two);

It will print: Hello World

// the fastest way to string concat in cycle when number of string is less than 1e6

// see https://medium.com/@devchache/the-performance-of-javascript-string-concat-e52466ca2b3a

// see https://www.javaer101.com/en/article/2631962.html

function concat(arr) {

    let str = '';

    for (let i = 0; i < arr.length; i++) {

        str += arr[i];

    }

​

    return str;

}

​

let string1 = "Hello"

let string2 = "World"

​

let finalString = string1 + ", " + string2 + "!" // Hello, World!