A function that traverses a single array (row) of elements moving and matching merges to the beginning of array like the game 2048.

traverseAndMerge.js

const tests = [
    [[4, 4, 2, 4],[8, 2, 4, 0]],
    [[4, 0, 0, 4],[8, 0, 0, 0]],
    [[2, 4, 4, 0],[2, 8, 0, 0]],
    [[4, 4, 4, 4],[8, 8, 0, 0]],
    [[4, 4, 4, 0],[8, 4, 0, 0]],
    [[4, 4, 0, 4],[8, 4, 0, 0]],
    [[2, 4, 0, 4],[2, 8, 0, 0]],
    [[2, 4, 8, 4],[2, 4, 8, 4]],
    [[2, 4, 8, 16],[2, 4, 8, 16]],
    [[4, 0, 0, 0],[4, 0, 0, 0]],
    [[2, 4, 2, 4],[2, 4, 2, 4]],
];

const traverse = function(a) {
    const [first, ...rest] = a;
    if (rest.length == 0) {
        return [first];
    }
    if (first == 0) {
        return [...traverse([...rest]), 0];
    }
    const secondIndex = rest.findIndex(num => num>0);
    if (secondIndex < 0) {
        return [first , ...rest.map(_ => 0)];
    }
    if (first == rest[secondIndex]) {
        return [2 * first, ...traverse([...rest.slice(secondIndex + 1), ...new Array(secondIndex+1).fill(0)])];
    }
    return [first, ...traverse([...rest])];
    // Alternately
    // return [first, ...traverse([...rest.slice(secondIndex), ...new Array(secondIndex).fill(0)])];
}

const match = (ar1, ar2) => ar1.every((ele, index) => ele == ar2[index]);
tests.forEach(([input, expected], index) => {
    const result = traverse([...input]);
    if (match(expected, result)) {
        console.log(`Passed ${index + 1} ${input}  ${result}`);
    } else {
        console.log(`Failed ${index + 1} ${input}  ${result}`);
    }
});