Weird bug in Javascript splice method

When you remove the item from the array all the element shift down by one. When you advance your index (i++), you skip the shifted down item in the array which happens to be successive zero in the array.

Solution: Do the for next loop backward and it'll work.

Because splice changes the length of the array, you could iterate from the end of the array and splice the found value directly to the last index.

With this approach, you need only a single loop.

var a = [0, 1, 2, 0, 0, 3, 0],
    i = a.length;

while (i--) {
    if (a[i] === 0) {
        a.splice(a.length, 0, ...a.splice(i, 1));
    }
}

console.log(a);

A shorter approach without splicing - and starting from zero.

var a = [0, 1, 2, 0, 0, 3, 0],
    i, j = 0;

for (i = 0; i < a.length; i++) {
    if (a[i] !== 0) {
        [a[j], a[i]] = [a[i], a[j]]; // swap
        j++;
    }        
}

console.log(a);