Question

Here is a JavaScript example from Eloquent JavaScript:

function findSequence(goal) {
  function find(start, history) {
    if (start == goal)
      return history;
    else if (start > goal)
      return null;
    else
      return find(start + 5, "(" + history + " + 5)") ||
             find(start * 3, "(" + history + " * 3)");
  }
  return find(1, "1");
}

At the very beginning “start” and “goal” in line 3 are not defined. Why does the code not immediately return a result like “undefined” or “ReferenceError: … is not defined”?

Answer 1

line 3 is just part of the function definition. It is not being executed. The second to last line is the one that kicks off the execution. When it is called, the values 1 and “1” are assigned to parameters start and history. goal is referencing the outer function’s parameter. This is populated when the outer function is called.