What is concat

Appends contents of another array to existing array.

Javascript

const array1 = [1, 2, 3];
const array2 = ["a", "b", "c"];

const resultOfConcat = array1.concat(array2);
// [1, 2, 3, "a", "b", "c"]

const greetList = ["Hello", " ", "Venkat", "!"];
"".concat(...greetList); // "Hello Venkat!"

// other behaviors
"".concat({}); // "[object Object]"
"".concat([]); // ""
"".concat(null); // "null"
"".concat(true); // "true"
"".concat(4, 5); // "45"

Alternatives

Spread operator is recommended when concatenating/merging multiple arrays.

Javascript

const arr1 = [0,1,2];
const arr2 = [3,4,5];

const arr3 = [...arr1, ...arr2];

console.log(arr3); // [0,1,2,3,4,5]