Do `let` Statements Create Global Properties in JavaScript?

Do Let Statements Introduce Global Properties?

In JavaScript, var declarations create properties on the global object. However, let declarations, introduced in ES6, enforce lexical scoping with block-level visibility.

Do let Declarations Create Global Properties?

According to the ECMAScript specification, let statements do not create properties on the global object. Global bindings are primarily stored in the object environment record of the global environment, which is linked to the global object. Function, generator, and variable declarations using var are included in this object environment record, while other declarations, such as let, are stored in the declarative environment record.

Explanation

Unlike var, which creates properties on the global object directly, let bindings are contained within the declarative environment record. This record utilizes an internal data structure, making it inaccessible externally. Object environment records, on the other hand, leverage a JS object for data storage, where each object property represents a binding. Consequently, var bindings are reflected as properties of the global object.

Browser and Node.js Behavior

In browsers, some implementations, like Firefox, exhibit behavior that contradicts the specification. let bindings appear as properties on the global object (window). However, in strict Node.js environments, using --harmony or --harmony_top_level flags, let bindings are correctly isolated within the declarative environment record.

Conclusion

Despite their lexical scoping, let declarations do not introduce properties on the global object. Instead, they are confined to the declarative environment record of the global environment, emphasizing the encapsulation and block-level visibility they provide.

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