Merge two already sorted arrays

merge.js

function merge(arr1, arr2) {
	var result = []; // Array where elements will be added
	var i1 = 0; // Index for arr1
	var i2 = 0; // Index for arr2

	// Iterate over arrays until reach the end of one of them
	while ( i1 < arr1.length && i2 < arr2.length ) {
		arr1[i1] < arr2[i2] ? result.push(arr1[i1++]) : result.push(arr2[i2++]) // Push the smallest and increment
	}

	// Return result concat'ed with the remaining items from array that didn't reach the end
	return result.concat(arr1.slice(i1)).concat(arr2.slice(i2));
}

// Test some input
var arr1 = [2, 3, 5, 5, 9, 10, 13];
var arr2 = [6, 7, 7, 7, 9, 10, 10, 12, 15];
var merged = merge(arr1, arr2);

for ( i = 0; i < merged.length; i++ ) console.log(merged[i]);

// merged = [2, 3, 5, 5, 6, 7, 7, 7, 9, 9, 10, 10, 10, 12, 13, 15]

Complexity: O(n)