How do I have JavaScript get a substring before a character?

Yes. Try the String.split method: https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/String/split

split() returns an array of strings, split by the character you pass to it (in your case, the plus). Just use the first element of the array; it will have everything before the plus:

const string = "foo-bar-baz"
const splittedString = string.split('-')
//splittedString is a 3 element array with the elements 'foo', 'bar', and 'baz'

Do you mean substring instead of subscript? If so. Then yes.

var string = "55+5"; // Just a variable for your input.

function getBeforePlus(str){

    return str.substring(0, str.indexOf("+")); 
   /* This gets a substring from the beginning of the string 
      to the first index of the character "+".
   */

}

Otherwise, I recommend using the String.split() method.

You can use that like so.

var string = "55+5"; // Just a variable for your input.

function getBeforePlus(str){

    return str.split("+")[0]; 
    /* This splits the string into an array using the "+" 
       character as a delimiter.
       Then it gets the first element of the split string.
    */

}