Symbol.unscopables

The Symbol.unscopables static data property represents the well-known symbol Symbol.unscopables. The with statement looks up this symbol on the scope object for a property containing a collection of properties that should not become bindings within the with environment.

The [Symbol.unscopables] symbol (accessed via Symbol.unscopables) can be defined on any object to exclude property names from being exposed as lexical variables in with environment bindings. Note that when using strict mode, with statements are not available, and this symbol is likely not needed.

Setting a property of the [Symbol.unscopables] object to true (or any truthy value) will make the corresponding property of the with scope object unscopable and therefore won’t be introduced to the with body scope. Setting a property to false (or any falsy value) will make it scopable and thus appear as lexical scope variables.