How can I write a function that, received an array a already given from the program that can contain numbers, boolean and even other arrays, counts how many numbers are present in a?

Here’s my code that doesn’t work (it gives everytime 0 whatever the array contains):

function test(a) {
    var sum = 0
    for (i=0; i<a.length; i++){
       if (typeof a[i]==Number)
           sum++                    
    }
    return sum 
}

Thanks.

>Solution :

You have some issues:

• typeof <operand> returns a string and not an object, you should be checking against a string typeof a[i] === "number".
• You should be using var/let before i in your for loop so that i doesn’t become a global variable and instead remains scoped to your function (if you use var) or your for loop (if you use let). Having variables accidentally added to the global scope can easily lead to bugs for code that runs after you call your test() function.

As you want to count the frequency of numbers in both your array and nested arrays, you can use recursion. The idea is to call your test() function again inside of test() when you encounter an array:

function test(a) {
  let sum = 0;
  for (let i = 0; i < a.length; i++) {
    if (typeof a[i] === "number")
      sum++
    else if(Array.isArray(a[i]))
      sum += test(a[i]); // sum the numbers within the `a[i]` array
  }
  return sum;
}

console.log(test([1,4, [2, 3, true], ["foo",0], 5, [123, [3.14, {a: 1}]]])); // 8

The other option is to get JS to handle the recursion for you, which can be done with the help of the .flat() method. By using .flat(Infinity) you can first flatten your array (ie: remove the inner arrays of any level), and then loop over it:

function test(a) {
  let sum = 0;
  let flat = a.flat(Infinity);
  for (let i = 0; i < flat.length; i++) {
    if (typeof flat[i] === "number")
      sum++;
  }
  return sum;
}

console.log(test([1,4, [2, 3, true], ["foo",0], 5, [123, [3.14, {a: 1}]]])); // 8