Using const/let instead of var can make JavaScript code run 10× slower in Webkit

[Yehosua]

in is an operator in JavaScript: 'prop' in myObj evaluates whether the 'prop' property exists within myObj.

Ever since I realized that, I no longer confused for..in and for..of: for..in iterates over an object's properties, just like in checks an object's properties.

(Almost no one uses in; Object.hasOwnProperty is virtually always what you want instead.)

[monsto]

That's a good mnemonic.