Javascript – creating "some" function

Question

Trying to create "some" function. i.e., return true if any of the elements in teh array satisfies the condition

function some(array, predicate) {
  for (var i = 0; i < array.length; i++) {
    if (predicate(array[i]))
      return true;
  }
  return false;
}

console.log(some([NaN, 3, 4], isNaN));
// → true
console.log(some([2, 3, 4], isNaN));
// → false

Question 1: Above is the solution in the book and I'm not sure why "return false" would overwrite "return true" in every case. The above solution seems to suggest that "return false" will be run only if "return true" was never run – WHY??

My solution looked like this.

var some = function(array, condition) {
  var answer = false;
  for(var i=0; i<array.length; i++) {
    if(condition(array[i])) {answer = true}; 
  }
  return answer
};

Question 2: Is the book solution better because it takes less memory (ie the "answer" variable)?

Thanks!

Answer 1

return controls where the execution pointer is located for the currently executing script. When return is encountered it moves the execution pointer out of the function, and as a result, the function will stop executing. This means that if return true is executed, return false will not be.

Answer 2

The solution in the book is predicated on the fact that the return keyword used within the body of a for loop will cause the loop to stop execution and return.

Take a look at the at the Interrupt a function section of the return documentation, which states:

The return statement ends function execution and specifies a value to be returned to the function caller.

In your example, as soon as the condition is matched, the book's solution will no longer continue to iterate through the array and will return (since that's what return does per the definition above).

Let's look at your solution:

var some = function(array, condition) {
  var answer = false;
  for(var i=0; i<array.length; i++) {
    if(condition(array[i])) {answer = true}; 
  }
  return answer
};

What happens if the condition is met? You're setting the answer to true, but the loop will continue to check subsequent elements in the array.

Consider the use case where the array is very large and you want to find a specific value within the array.

Which do you think would be more efficient: the one that stops after it finds the match or the one that returns the match only after having looking at all elements?

In this case, the answer is the latter. Don't keep looping through all elements if you've already found what you need.