A for-in loop whose iteration variable is a property, a global variable, a local variable from a surrounding scope, or a local variable that is captured in a closure may prevent optimization of the function it appears in, which could make the program run slower. The same applies to for-of loops.

In particular, the V8 engine (which is used by the Google Chrome browser and the Node.js platform) does not currently optimize functions containing such loops.

Turn the iteration variable into a local variable if possible.

In the following example, function extend copies all properties of its argument src into object dest using a for-in loop. Note, however, that the loop variable p is not declared in extend, and thus is interpreted as a global variable.

However, this is clearly just a typo, and the p should be declared as a local variable instead:

As another example, consider the following generalization of extend that accepts zero or more src objects:

Note that p is now a local variable, but it is declared in the outer function combine, not in the inner function extend where it is actually used. The declaration should be moved into extend as follows:

In those (rare) cases where the iteration variable really has to be a global variable or a property, you can introduce a new, local iteration variable and copy its value into the desired global variable or property inside the loop.

Petka Antonov: Optimization killers.