I wrote this function to remove punctuation. I tested everything and the problem seems to be the firstIndex variable failed to increase by 1 each loop.

I tried to increase the firstIndex by using Increment Operator but this variable only increase once only and reach max value at 1. I truly dont know to fix this

const textInput = "Hello world!"

const insensitiveText = textInput.toLocaleLowerCase();

const letters = insensitiveText.split("");

const removePunctuation = (letters) =>{
    const tempArray = new Array();
    for( const i in letters){
        var firstIndex = 0;
        if(letters[i].charCodeAt() > 96 && letters[i].charCodeAt() < 123){ // This loop succeeded in removePunctuation;
            tempArray[firstIndex] = letters[i];
            firstIndex = firstIndex + 1;
        }
    }
    return(tempArray);
}