Why create a conditional checking if variable is -1 when iterating?

Question

I'm sorry that this issue is hard to explain in a simple question but I have encountered it a couple times now and do not understand the functionality.

function checkArrays(batch){
  let newArray = [];
  for(var i = 0; i < batch.length; i++){
    let firstDigit = batch[i][0];
    if(newArray.indexOf(firstDigit) === -1){
      newArray.push(firstDigit);
    }
  }
  return new
}

corrected variable from new to newArray

This function takes in batch, a collection of arrays of single digits like [5,7,7,3,9,1,6,3]. The goal is to take the first digit of each array and add them to new, which is returned. My question is why is it necessary to check if new.indexOf(firstDigit) === -1 in order to do this? What is significant about -1 and why would one want to iterate this way? Thanks!

new is a reserved keyword and cannot be used as a variable name. indexOf returns -1 since there's no way to have an element at position -1 in an array. — Phix
– Phix, Commented Nov 19, 2020 at 19:51
Hint: take a look at MDN's documentation for indexof: developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/… — user47589
– user47589, Commented Nov 19, 2020 at 19:51
Because you tried to create a variable called new, and new is a reserved keyword, this is not a valid JavaScript program. Can you please change the question and example to use a different name for that variable so we can move past that part of the issue? — Wyck
– Wyck, Commented Nov 19, 2020 at 20:12
The logic here is "if it's not in the array yet, then add it". But this is sounding a lot like what a Set is for. — Wyck
– Wyck, Commented Nov 19, 2020 at 20:14

David · Accepted Answer · 2020-11-19 19:57:59Z

3

This:

if(new.indexOf(firstDigit) === -1)

Basically means "if the element is not in the array", because indexOf:

... returns the first index at which a given element can be found in the array, or -1 if it is not present.

In most browsers you can also use includes which is more semantically descriptive:

if(!new.includes(firstDigit))

As an aside... new is probably not a great name for a variable, since it's an operator in the language itself. For this particular operation I'd go with something like result or newArray or something else a little more descriptive and not reserved.

answered Nov 19, 2020 at 19:57

David

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3 Comments

Wiseface Over a year ago

Thanks for the answer. This function will only return the first digit of arrays if they are unique, why is this?

David Over a year ago

@IanWiseman: Because that's what the function is written to do. Any reasoning behind that is a question for whomever wrote it or decides that business functionality.

Wiseface Over a year ago

Sorry, my question was not about why it does this, but rather how does it do it. I see now that it's checking to see if a first digit is not in new and then pushing that given digit if the answer is true.

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