Why Don't You Have To Declare Variables In Javascript?

can anybody put forward a sensible argument javascript's behaviour of
creating a new global variable whenever I assign to a previously undeclared
variable. I can't beleive this is just for the sake of convenience (surely
we learned this much from basic).

here's my proposal: all "global" (document scope) variables must be declared
by 'var' outside a function block.

failing that, does anyone know any patterns or tricks I can use to make sure
I don't create a new global variable when I accidentally misspell a variable
name?