See a text width (in pixel)

Question

In JavaScript, If I wanted to know what does a certain text take width of the line, what to do?

I searched on the web, I found that I can use .cleintWidth

( https://stackoverflow.com/questions/118241/calculate-text-width-with-javascript?noredirect=1&lq=1 )

But seems it did not work for me
If I writed:

var t='I';
document.write(t.clientWidth);

the output would be "undefined"

What is the wrong? How to fix that? Or there is another method to get a text width value?

Answer 1

FF9900
Thanks.
I noticed that I should add "window.onload" to works.
why exactly?