Self Describing Number - Algorithm Solution

selfDescribingNum.js

function isSelfDescribing(num) {
    let numArray = String(num).split('').map(str => Number(str));
    // 123 => '123' => ['1', '2', '3'] => [1, 2, 3]
    let i = 0;
    for (const num of numArray) {
        // check to see how many times the current index appears in the numArray
        // and does that total match the current element/number for this iteration
        if (numArray.filter(n => n === i).length !== num) {
            return false;
        }
        i++;
    }
    return true;
}

console.log(isSelfDescribing(1210)); // true
console.log(isSelfDescribing(2020)); // true
console.log(isSelfDescribing(21200)); // true
console.log(isSelfDescribing(3211000)); // true
console.log(isSelfDescribing(42101000)); // true
console.log(isSelfDescribing(1)); // false
console.log(isSelfDescribing(100)); // false
console.log(isSelfDescribing(24354)); // false
console.log(isSelfDescribing(99999)); // false