My javascript is a bit rusty, but something like this:

s = '';

for( var i = 32; i <= 126; i++ )

{

    s += String.fromCharCode( i );

}

Not sure if the range is correct though.

Edit:

Seems it should be 32 to 127 then. Adjusted.

Edit 2:

Since char 127 isn't a printable character either, we'll have to narrow it down to 32 <= c <= 126, in stead of 32 <= c <= 127.