combine two substring from string javascript code example

Example 1: javascript string concat vs +

It is strongly recommended to use the string
concatenationoperators (+, +=) instead of String.concat
method for perfomance reasons

Example 2: how the concat function works javascript

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]