The JavaScript exception "variable is not defined" occurs when there is a non-existent variable referenced somewhere.

The JavaScript exception "WeakSet key (or WeakMap value) 'x' must be an object or an unregistered symbol" occurs when an value of invalid type is used as a key in a WeakSet or as a value in a WeakMap.

Trailing commas (sometimes called "final commas") can be useful when adding new elements, parameters, or properties to JavaScript code. If you want to add a new property, you can add a new line without modifying the previously last line if that line already uses a trailing comma. This makes version-control diffs cleaner and editing code might be less troublesome.

The JavaScript exception "variable is a reserved identifier" occurs when reserved keywords are used as identifiers.

The subtraction assignment (-=) operator performs subtraction on the two operands and assigns the result to the left operand.

The patternTransform attribute defines a list of transform definitions that are applied to a pattern tile.

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. Browsers not supporting strict mode will run strict mode code with different behavior from browsers that do, so don't rely on strict mode without feature-testing for support for the relevant aspects of strict mode. Strict mode code and non-strict mode code can coexist, so scripts can opt into strict mode incrementally.

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.

The xlink:title attribute is used to describe the meaning of a link or resource in a human-readable fashion.

The JavaScript exception "super() called twice in derived class constructor" occurs when the super() is called a second time for a given derived class constructor.