Lets say I have a string, but the text could be anything!

This is a test case

The 'boundary' is the count after a space, this means the charIndex =

(0) This
(5) is
(8) a
(10) test
(15) case

The above means from (0) to (5) the word equals 'this '.

If I wanted to know the word just used, I would need to substr start at charIndex and look backwards until indexOf(' ')

Example: If I wanted to find the word just used at charIndex (10) how would I instruct javascript to look backwards until the next space and give me the word a .

Is it possible to do this, with as little process time, using Javascript, or JQuery?

UPDATE

Instead of looking after the word and going back, I decided to achieve what I needed, I split the words and used each charIndex as a count. Code follows:

var u = new SpeechSynthesisUtterance();

var words = element.innerHTML.split(' ');

var a = 0;

 u.text = element.innerHTML;

 u.lang = 'en-UK';

 u.rate = 0.7;

 u.onboundary = function(event) {

 console.log(words);

 element.innerHTML = element.innerHTML.replace(words[a], '<strong>' + words[a] + '</strong>');

  a++;       

  }

But as this is not the questions answer, still the best answer to reverse search a string is that which is marked correct, although it still needs some work depending on the string structure.