concat 3 arrays into 1 array in javascript / underscore

Question

Is there a short way / best practice to concat 3 arrays into 1 array ?

var arr = [],
    arr1 = [ 1 , 2 , 3 ],
    arr2 = [ 4 , 5 , 6 ],
    arr3 = [ 7 , 8 , 9 ];
arr = arr.concat(arr1);
arr = arr.concat(arr2);
arr = arr.concat(arr3);

Answer 1

The shortest (and fastest) solution is arr = arr1.concat(arr2, arr3);

Alternatives:

• arr = arr.concat(arr1, arr2, arr3)
• arr = Array.prototype.concat(arr1, arr2, arr3)
• arr = [].concat(arr1, arr2, arr3)

Answer 2

I would use _.flatten.

var arr = [[1,2,3], [4,5,6], [7,8,9]];
var result = _.flatten(arr) // [1,2,3,4,5,6,7,8,9]

Answer 3

Spread syntax makes concatenation easy:

arr = [...arr1, ...arr2, ...arr3]

You can even include non-array items inline:

arr = [...arr1, 42, ...arr2, ...arr3]