Updated javascript.plt

I've just updated my implementation of JavaScript in PLT Scheme. The latest version fixes some of the subtle ways I'd deviated from the spec with respect to variable binding. Back in the day, I was wooed into thinking that JavaScript was so similar to Scheme that I could pretty easily just compile JavaScript variable bindings directly to similar Scheme variable bindings. For the most part, I got variable hoisting (i.e., the scope of var-declarations being hoisted to the top of their nearest enclosing function body) right. But the real subtlety comes in with the with (dynamically add a computed object value to the runtime environment as a new environment frame) and eval constructs.

I almost had with right: as soon as I detect that the programmer is using with, I enter a "stupid" mode, where I synthesize an explicit runtime environment and populate it with whatever's currently in the static environment. Within the body of the with, variable lookup is performed dynamically in this runtime environment. So outside of a with, you get normal, efficient lexical scope; inside a with, you get stupid, slow, dynamic scope. My only mistake there was forgetting to make the runtime environment entries aliases to the existing variables so that mutations persist after the with-block returns.

But eval is hard: Scheme's eval doesn't have access to the lexical environment where it is called, whereas JavaScript's does. And JavaScript's eval also inherits the ambient "variable object" (ECMA-262 10.1.3), which means that eval code can create variable bindings in the caller's scope. Getting all of this right means simulating the variable object similarly to the way I simulate the runtime environment, and switching into "stupid" mode whenever I detect a potential application of eval. (The standard allows you to refuse to run eval in situations other than a "direct" call to a variable reference spelled e-v-a-l, so you don't have to treat every single function call as a potential call to eval. It's messy, but it's better than forcing you to implement a dynamic environment everywhere.)