Commits on Aug 9, 2019

1. Use the canonical value of undefined in generated JavaScript code. …

   Using `undefined` in generated code is fragile as it is global mutable
   variable, not a constant. As a consequence a buggy equality test such
   as `if (foo = undefined) { ... }` or similar in generated code or any
   third-party code can inadvertently break the entire runtime. Such bugs
   can be difficult to localise.
   
   To make the runtime more robust this patch introduces a slight change
   to the JavaScript code generator such that the emitted code uses the
   canonical value of `undefined` rather than the variable name
   `undefined`. The canonical value can be obtain via the odd expression
   `(void 0)`. Since `void` is a keyword and `0` is a numeric literal it
   is not possible for user-code to override their semantics, hence the
   expression is resilient against mutations such as monkey-patching.

   

   dhil committed Aug 9, 2019
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