function insertionSort(array) {
    // start with index 1 because only one element is already sorted
    for(let i=1; i<array.length ; i++) {
        let key = array[i];
        let j = i - 1;
        // decrement j until array[j] is not less or equal to key
        while(j >= 0 && array[j] > key) {
            array[j+1] = array[j];
            j--;
        }
        array[j+1] = key;
    }
    return array;
}

console.log(insertionSort([4,5,6,7,8,10,1,2,3,4])) // output : [1,2,3,4,4,5,6,7,8,10]