Regular expressions are patterns used to match character combinations in strings. In JavaScript, regular expressions are also objects. These patterns are used with the exec() and test() methods of RegExp, and with the match(), matchAll(), replace(), replaceAll(), search(), and split() methods of String. This chapter describes JavaScript regular expressions.

A closure is the combination of a function bundled together (enclosed) with references to its surrounding state (the lexical environment). In other words, a closure gives you access to an outer function's scope from an inner function. In JavaScript, closures are created every time a function is created, at function creation time.

The <image> SVG element includes images inside SVG documents. It can display raster image files or other SVG files.

The Set object lets you store unique values of any type, whether primitive values or object references.

The <metadata> SVG element adds metadata to SVG content. Metadata is structured information about data. The contents of <metadata> should be elements from other XML namespaces such as RDF, FOAF, etc.

The attributeName attribute indicates the name of the CSS property or attribute of the target element that is going to be changed during an animation.

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

The <mpath> sub-element for the <animateMotion> element provides the ability to reference an external <path> element as the definition of a motion path.

The eval() function evaluates JavaScript code represented as a string and returns its completion value. The source is parsed as a script.

Symbol is a built-in object whose constructor returns a symbol primitive — also called a Symbol value or just a Symbol — that's guaranteed to be unique. Symbols are often used to add unique property keys to an object that won't collide with keys any other code might add to the object, and which are hidden from any mechanisms other code will typically use to access the object. That enables a form of weak encapsulation, or a weak form of information hiding.