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
commented
8 years ago
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 thetypeof xpart into a variable cause this to be deoptimized: the type information is lost and we end up doing actual string comparison operations at runtime.