Two Arrays

var array1 = [1, 2];
var array2 = [3, 4, 5];

var array3 = array1.concat(array2);  // returns a new array

var array3 = [...array1, ...array2]

Results in a new Array:

[1, 2, 3, 4, 5]

Multiple Arrays

var array1 = ["a", "b"],
    array2 = ["c", "d"],
    array3 = ["e", "f"],
    array4 = ["g", "h"];

Provide more Array arguments to array.concat()

var arrConc = array1.concat(array2, array3, array4);

Provide more arguments to []

var arrConc = [...array1, ...array2, ...array3, ...array4]

Results in a new Array:

["a", "b", "c", "d", "e", "f", "g", "h"]

Without Copying the First Array

var longArray = [1, 2, 3, 4, 5, 6, 7, 8],
    shortArray = [9, 10];

Provide the elements of shortArray as parameters to push using Function.prototype.apply

longArray.push.apply(longArray, shortArray);

Use the spread operator to pass the elements of shortArray as separate arguments to push

longArray.push(...shortArray)