Implementation of alternate merge from two arrays in JavaScript

Took 18 minutes and 18 seconds to make this JavaScript function in which both arrays should be of the same length. Here is the code snippet.

Function:

arr1 = ['bla', 'bla', 'bla'];

arr2 = ['ha', 'ha', 'ha'];

arr3 = [];

function alternateMerge(arr1, arr2) {

for (var i = 0; i < arr2.length; i++) {

arr3.push(arr1[i]);

arr3.push(arr2[i]);

}

}

alternateMerge(arr1, arr2);

document.write(arr3);

Result:

bla,ha,bla,ha,bla,ha