Why does my javascript function stops to early?

Why does my javascript function stops to early?

I have a function that walks through array and is is called from a textbox
event onblur
I went fine if i use the code beneath (5400 loops)
for ( i = 0 ; i < js_namen.length ; i++ )
{
document.write( i + ' - ' ) ;
document.write( js_namen[i].toUpperCase() + ' - ' ) ;
}
document.write( 'Klaar') ;
But when i add the string manipulation touppercase, it stops after 541
loops and will not display the 'finisched' word. Is there a maximum
execution time of something like that.