Insert before the last occurrence of a specific character in a string

Question

How would I insert something before the last occurrence of a specific character?

if (statement) insert " again" into string before last "<";

Answer 1

You can use lastIndexOf() with substring():

var str="Hello planet earth, you are a great planet.";
var n=str.lastIndexOf("planet");
var str2 = str.substring(0,n)+" again "+str.substring(n);
console.log(str2); // Hello planet earth, you are a great  again planet. 

As a nice function:

function insertBeforeLastOccurrence(strToSearch, strToFind, strToInsert) {
    var n = strToSearch.lastIndexOf(strToFind);
    if (n < 0) return strToSearch;
    return strToSearch.substring(0,n) + strToInsert + strToSearch.substring(n);    
}

var str ="This <br> is another <br> string <br> example.";
var newStr = insertBeforeLastOccurrence(str, "<", " again");
console.log(newStr); // This <br> is another <br> string  again<br> example.

Or as a String method:

String.prototype.insertBeforeLastOccurrence = function(strToFind, strToInsert) {
    var n = this.lastIndexOf(strToFind);
    if (n < 0) return this.toString();
    return this.substring(0,n) + strToInsert + this.substring(n);    
}

var str ="This <br> is another <br> string <br> example.";
console.log(str.insertBeforeLastOccurrence("<", " again"));
// Output: This <br> is another <br> string  again<br> example.
console.log(str.insertBeforeLastOccurrence("w00t", " again")); // wont find
// Output: This <br> is another <br> string <br> example.

Answer 2

You can get the last occurrence of a specified string using lastIndexOf(str), which is a member function of any String or string object. Then you can do something like this:

var idx = mystr.lastIndexOf("<");
if (idx > -1)
  var outval = mystr.substr(0, idx) + " again" + mystr.substr(idx);