Insert Fill

insertFill.js

const insertFill = function (arr, index, item, fillValue = 0) {
    if (arr.length === 0 && index === 0) return [item];
    if (arr.length < index) {
        // Index is not found, Fill in the length needed.
        const missing = index - arr.length;
        const fill = Array(missing - 1).fill(fillValue);
        arr = arr.concat(fill);
        return arr.concat([item]);
    }
    // The Index exists, insert at that point.
  	arr.splice(index, 0, item);
    return arr;
}

insertFill_example.js

let _ = {};

_.insertFill = function (arr, index, item, fillValue = 0) {
    if (arr.length === 0 && index === 0) return [item];
    if (arr.length < index) {
        // Index is not found, Fill in the length needed.
        const missing = index - arr.length;
        const fill = Array(missing - 1).fill(fillValue);
        arr = arr.concat(fill);
        return arr.concat([item]);
    }
    // The Index exists, insert at that point.
  	arr.splice(index, 0, item);
    return arr;
}

let months = ['Jan', 'March', 'April', 'June'];
months = _.insertFill(months, 1, 'Feb');
months = _.insertFill(months, 4, 'May');
months = _.insertFill(months, 12, 'Dec');
console.log(months);
// Array ["Jan", "Feb", "March", "April", "May", "June", 0, 0, 0, 0, 0, "Dec"]