A character class matches any character in or not in a custom set of characters. When the v flag is enabled, it can also be used to match finite-length strings.

The JavaScript exception ""use strict" not allowed in function" occurs when a "use strict" directive is used at the top of a function with default parameters, rest parameters, or destructuring parameters.

The JavaScript exception "nothing to repeat" or "invalid quantifier in regular expression" occurs when a quantifier in a regular expression is applied to nothing or applied to an assertion.

The stroke-width attribute is a presentation attribute defining the width of the stroke to be applied to the shape. It applies to any SVG shape or text-content element, but as an inherited property, it may be applied to elements such as <g> and still have the intended effect on descendant elements' strokes.

The import attributes feature instructs the runtime about how a module should be loaded, including the behavior of module resolution, fetching, parsing, and evaluation. It's supported in import declarations, export...from declarations, and dynamic import().

The JavaScript exception "functions cannot be labelled" occurs when a function declaration has a label before it.

The refX attribute defines the x coordinate of an element's reference point.

The spread (...) syntax allows an iterable, such as an array or string, to be expanded in places where zero or more arguments (for function calls) or elements (for array literals) are expected. In an object literal, the spread syntax enumerates the properties of an object and adds the key-value pairs to the object being created.

The async function* declaration creates a binding of a new async generator function to a given name.

The stroke-opacity attribute is a presentation attribute defining the opacity of the paint server (color, gradient, pattern, etc.) applied to the stroke of a shape.