Javascript using an indexOf() inside for loop

From what I understood, you want to find the first following position of a repeated item in an array sequence.

Original Answer

This can be done with findIndex, which takes a callback to determine if the index counts as found. Here you can specify that the items need to equal and the index needs to be greater than your current index, thus it is a repeated item.

let newArray = ['t', 'r', 'c', 'g', 't', 'h'];

for (let i = 0; i < newArray.length; i++) { 
   // Find the index of the first item which index is greater than the current item and the latter equals the item
   const x = newArray
      .findIndex((item, index) => index > i && newArray[i] === item);

   console.log(`${i}: ${x}`);
}

Better Solution

As mentioned in the comments (thanks T.J. Crowder), indexOf takes a second parameter as an offset to start the search at. This is the preferrable solution, as it faster and more concise.

let newArray = ['t', 'r', 'c', 'g', 't', 'h'];

for (let i = 0; i < newArray.length; i++) { 
   const x = newArray.indexOf(newArray[i], i+1); 
   console.log(`${i}: ${x}`);
}

In the code above indexOf is searching for the location of i inside the array, and isn't finding it, as none of the array elements are numbers.

Try:

const x = newArray.indexOf(newArray[i])

The problem is that [i] in indexOf([i])is just an array with a number inside of it.

If you had indexOf(newArray[i]), then you would at least get the answer [0, 1, 2, 3, 0, 5] (the character will find itself, except the second t that finds the first t at position 0).

But I assume that you only want to know the position of the upcoming letter, and not the position of the previous ones.

What you can do is to loop from the end of the array instead, save each letter in an object with it's position and then replace it if the letter has already existed.

I will add the code in a method so it's easier to use it with other arrays.

function findOccurancies(newArray) {
  let newObj = {};
  let currentCharacter = '';
  let x = [];

  for (let i = newArray.length-1; i >= 0; i--) {
     currentCharacter = newArray[i];

     if (newObj.hasOwnProperty(currentCharacter)) {         
       x.unshift(newObj[currentCharacter]) // place the item first.
     } else {
       x.unshift(-1)                       // place the item first.
     }      

     newObj[currentCharacter] = i;
  }
  
  return x;
}

console.log(findOccurancies(['t', 'r', 'c', 'g', 't', 'h']));

console.log(findOccurancies(['t', 'r', 'c', 'g', 't', 'h', 't']));