Does the dog have Turing nature?

On Eval and Evil.

“eval is evil” has become a maxim repeated in the Javascript community. Douglas Crockford, in Javascript: The Good Parts, rightly advises against hidden and explicit uses of eval for security and clarity reasons. Now, I find eval useful to implement DSLs in Javascript. The in-browser CoffeeScript compiler wouldn’t be possible without eval (directly or indirectly). So, in this post, I wish to explore what appears interesting about eval that is relevant to building such DSLs.


JSON has become a kind of de-facto standard for sharing data among services on the web. The Lisp folks have enjoyed this luxury ever since … well ever since McCarthy made the language and his student implemented an interpreter for it. What’s more, they have also had the luxury of using the same syntax for sharing logic .. and in fact take it for granted. This post is a proposal to bring that “luxury” to the web programming world.

Status: Draft. Comments welcome.

Creating DSLs in Javascript Using J-expressions

Scheme and Lisp have for long had powerful meta-programming abilities due to the syntax of their language being the same as the syntax for the main data structure supported by the language - the humble list. These languages are therefore well suited for inventing smaller special purpose “domain specific languages.

Javascript, on the other hand, has a “full blown syntax” that makes meta-programming not for the faint of heart. One consequence of the lack of such ability is that developers have not had the benefit of the abstraction possible through small special purpose DSLs.

Here, I outline an approach for creating DSLs in Javascript using the now prevalent JSON format that is native to the language. The initial part tries to explain the kinds of scenarios in which one might consider building a DSL, which is important to have an idea about. Later, I get into the actual representation using JSON.

A Mental Model for Variables and Closures in Javascript

Closures and variables have a strained relationship in Javascript that causes much confusion among newcomers and results in hard to spot bugs even for experienced JS coders. It is good to have a clear and accurate “mental model” of this relationship using which you can correctly predict what would happen with any given piece of code.

I came up with such a mental model a while back and posted it on Hacker News … which I reproduce here.