What is recursion in a programming language?

 Recursion is a technique to iterate over an operation by having a function call itself repeatedly until it arrives at a result.

function add(number) {
  if (number <= 0) {
    return 0;
  } else {
    return number + add(number - 1);
  }
}

add(3) => 3 + add(2)
          3 + 2 + add(1)
          3 + 2 + 1 + add(0)
          3 + 2 + 1 + 0 = 6  

Example of a recursive function:

The following function calculates the sum of all the elements in an array by using recursion:

function computeSum(arr){
  if(arr.length === 1){
    return arr[0];
  }
  else{
    return arr.pop() + computeSum(arr);
  }
}

computeSum([7, 8, 9, 99]); // Returns 123