A regular expression (regex for short) allow developers to match strings against a pattern, extract submatch information, or simply test if the string conforms to that pattern. Regular expressions are used in many programming languages, and JavaScript's syntax is inspired by Perl.

The increment (++) operator increments (adds one to) its operand and returns the value before or after the increment, depending on where the operator is placed.

The logical AND assignment (&&=) operator only evaluates the right operand and assigns to the left if the left operand is truthy.

The dx attribute indicates a shift along the x-axis on the position of an element or its content.

The font-size-adjust attribute allows authors to specify an aspect value for an element that will preserve the x-height of the first choice font in a substitute font.

The extends keyword is used in class declarations or class expressions to create a class that is a child of another class.

Below, you'll find a list of errors which are thrown by JavaScript. These errors can be a helpful debugging aid, but the reported problem isn't always immediately clear. The pages below will provide additional details about these errors. Each error is an object based upon the Error object, and has a name and a message.

The by attribute specifies a relative offset value for an attribute that will be modified during an animation.

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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.