What is an “internal slot” of an object in JavaScript?

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Summary

Internal slots / methods are pseudo-properties / -methods that the specification uses to define required behavior. (“Abstract operations” are a related mechanism of the spec.) Slots represent state (values), and methods describe algorithms (behavior). They may or may not correspond to properties of objects used by the engine, but they’re not available to user code, except as exposed by some part of the public API. The actual implementation an engine uses may be very different from what the internal methods sketch out, but to be compliant they have to produce behavior or results that are consistent with the internal methods.

Examples

[[StringData]] internal slot

The behavior of String, e.g. new String("whatever"), is described in terms that include a [[StringData]] internal slot that represents the value (whatever in this case). The internal slot isn’t directly accessible to user code, but String.prototype.toString() (e.g. (new String("whatever")).toString()) is defined in terms of a thisStringValue() abstract operation, which is described in terms of returning the value of [[StringData]]. So in other words, String.prototype.toString() is public API that is essentially a getter that exposes [[StringData]].

[[OwnPropertyKeys]] internal method

The behavior of Object.keys() is described in terms that include calling the [[OwnPropertyKeys]] internal method. Note that different kinds of objects, such as ordinary objects (e.g. Object) and exotic objects (e.g. String) may have different definitions of [[OwnPropertyKeys]]. When [[OwnPropertyKeys]] is “called” in the spec, that refers to the definition for the applicable type. There are also some invariant characteristics that apply to its definition for any object type.

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