Optimize Map/Set fast key path. #397
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The expression `typeof x === 'some string'` is treated as a peephole optimization by JavaScript runtimes and optimized as a simple type test without an explicit string comparison. Refactoring the `typeof x` part into a variable cause this to be deoptimized: the type information is lost and we end up doing actual string comparison operations at runtime.
ljharb
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Jan 11, 2016
[Performance] Optimize Map/Set fast key path
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The expression
typeof x === 'some string'is treated as a peepholeoptimization by JavaScript runtimes and optimized as a simple type test
without an explicit string comparison. Refactoring the
typeof xpartinto a variable cause this to be deoptimized: the type information
is lost and we end up doing actual string comparison operations at
runtime.