const streetAddress = addy.substring(0, addy.indexOf(","));

While it’s not the best place for definitive information on what each method does (MDN Web Docs are better for that) W3Schools.com is good for introducing you to syntax.

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.
    */

}

This is the what you should use to split:

string.slice(0, string.indexOf("'"));

And then to handle your non existant value edge case:

function split(str) {
 var i = str.indexOf("'");

 if(i > 0)
  return  str.slice(0, i);
 else
  return "";     
}

Demo on JsFiddle

How I would do this:

Copy// function you can use:
function getSecondPart(str) {
    return str.split('-')[1];
}
// use the function:
alert(getSecondPart("sometext-20202"));

You may try a regex string split here:

var inputs = ["£____", "____$", "42____$"];
for (var i=0; i < inputs.length; ++i) {
    var parts = inputs[i].split(/_+/);
    console.log("before = " + parts[0] + ", after = " + parts[1]);
}