ecmascript 5 - null vs. undefined and their behaviour in JavaScript

Question

So after a big argument/debate/discussion on the implementation of null and undefined in javascript I'd like somebody to explain the reasoning behind the implementation and why they differ in some circumstances. Some particular points I find troubling:

• null == undefined evaluates to true
• null + 1 equals 1 but undefined + 1 equal NaN
• if(!null) evaluates to true and if(null) evaluates to false but null == false evaluates to false.

I've read the specification and I know how the results are reached, I'm looking for the paradigms and reasons that dictate this being the specification. Some of these points, especially the second one, given the first, feel very inconsistent.

Answer 1

The short and sweet version is that JavaScript was designed and implemented very rapidly by the Netscape team, and it had some inconsistencies such as the ones that you've pointed out.

The Internet Exploder team did its best to copy JS exactly and they did a damn good job of it to the point that the inconsistencies were copied as well. When Netscape went to get JS standardized as ECMAScript MS was a part of it and basically said that they weren't allowed to change the standard because it would break old code (existing systems inertia). The inconsistencies were standardized and that was that.

Douglas Crockford has a very good series of talks about some of these issues.