Module boa_engine::environments

Boa’s implementation of ECMAScript’s Environment Records.

Environments contain the bindings of identifiers to their values. The implementation differs from the methods defined by the specification, but the resulting behavior should be the same.

To make the runtime more performant, environment specific behavior is split between bytecode compilation and the runtime. While the association of identifiers to values seems like a natural fit for a hashmap, lookups of the values at runtime are very expensive. Environments can also have outer environments. In the worst case, there are as many hashmap lookups, as there are environments.

To avoid these costs, hashmaps are not used at runtime. At runtime, environments are represented as fixed size lists of binding values. The positions of the bindings in these lists is determined at bytecode compile time.

A binding is uniquely identified by two indices:

• An environment index, that identifies the environment in which the binding exists
• A binding index, that identifies the binding in the environment

More information:

• ECMAScript reference