Why do options["foo"] || options.foo?

Question

Reading through the knockout.js source code I come accross this line

disposeWhenOption = options["disposeWhen"] || options.disposeWhen

I've never seen this before and everything I know about javascript says there is no case when the two sides of the || would be different. Why might have the knockout team used this construct?

Answer 1

This comes as a result of symbol mangling performed by the Closure Compiler. From the Closure Compiler docs:

Closure Compiler compilation never changes string literals in your code, no matter what compilation level you use. This means that compilation with ADVANCED_OPTIMIZATIONS treats properties differently depending on whether your code accesses them with a string. If you mix string references to a property with dot-syntax references, Closure Compiler renames some of the references to that property but not others. As a result, your code will probably not run correctly.

Using both the dot syntax and the string literal is the "safe" way to work around this, as it helps ensure a client using an unexported part of the API but not using the closure compiler on their own code will not have any problems.