Private properties are counterparts of the regular class properties which are public, including class fields, class methods, etc. Private properties get created by using a hash # prefix and cannot be legally referenced outside of the class. The privacy encapsulation of these class properties is enforced by JavaScript itself. The only way to access a private property is via dot notation, and you can only do so within the class that defines the private property.

The JavaScript exception "duplicate formal argument x" or "duplicate argument names not allowed in this context" occurs when a function creates two or more parameter bindings with the same name, and the function is not a non-strict function with only simple parameters.

The patternUnits attribute indicates which coordinate system to use for the geometry properties of the <pattern> element.

The JavaScript exception "numbers out of order in {} quantifier" occurs when a quantifier in a regular expression uses the {n,m} syntax but m is less than n.

The overline-position attribute represents the ideal vertical position of the overline. The overline position is expressed in the font's coordinate system.

The orientation attribute indicates that the given glyph is only to be used for a particular text direction, i.e. horizontal or vertical.

JavaScript's strict mode is a way to opt in to a restricted variant of JavaScript, thereby implicitly opting-out of "sloppy mode". Strict mode isn't just a subset: it intentionally has different semantics from normal code. Strict mode code and non-strict mode code can coexist, so scripts can opt into strict mode incrementally.

A type representing an owned, C-compatible, nul-terminated string with no nul bytes in the middle.

Iteration protocols aren't new built-ins or syntax, but protocols. These protocols can be implemented by any object by following some conventions.

The JavaScript exception "can't define property "x": "obj" is not extensible" occurs when Object.preventExtensions() marked an object as no longer extensible, so that it will never have properties beyond the ones it had at the time it was marked as non-extensible.