Use the canonical value of undefined in generated JavaScript code.
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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.
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Using
undefinedin generated code is fragile as it is global mutablevariable, not a constant. As a consequence a buggy equality test such
as
if (foo = undefined) { ... }or similar in generated code or anythird-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
undefinedrather than the variable nameundefined. The canonical value can be obtain via the odd expression(void 0). Sincevoidis a keyword and0is a numeric literal itis not possible for user-code to override their semantics, hence the
expression is resilient against mutations such as monkey-patching.