JavaScript, like some other languages of the time, was designed with the Robustness Principle in mind. Arguably the wrong end of the Robustness Principle, but still.
That is, it was designed to accept anything that wasn't a syntax error (if not a few other things besides) and not generate run-time errors unless absolutely necessary. The thinking was that the last thing the user of something written in JavaScript wants is for their browser to crash or lock up because something divided by zero or couldn't find an object property.
Also it was originally written in about five minutes by one guy who hadn't had enough sleep. (I may have misremembered this part, but I get the feeling I'm not too far off.)