In computer science, an enumerated type is a data type consisting of a limited set of named values.

HTML enumerated attributes

In HTML, enumerated attributes are attributes with a limited, predefined set of text values. For example, the global HTML dir attribute has three valid values: ltr, rtl, and auto.

Each enumerated attribute has a default value for when the attribute is present without a value (the value is missing), and a default value for when the attribute is assigned an invalid value. Unlike Boolean HTML attributes — which are always true when the attribute is present whether the value is present, omitted, or invalid — with enumerated HTML attributes, the default for an omitted value may be different from the default for invalid values. For example, the global HTML contenteditable attribute has two valid keywords: true and false. If the attribute is present but no value is set, the value is true. If a value is set, but is invalid, such as contenteditable="contenteditable", the value maps to a third state, inherit.

ARIA enumerated attributes

ARIA states and properties, being HTML, also have enumerated attributes. If an ARIA attribute includes a both a true and false value in the enumerated list, it generally treats an omitted attribute as false and an invalid value as true, while the default value for the empty string or omitted value depends on the attribute.

For example, the aria-current attribute accepts a limited list of values that includes page, step, location, date, time, true, and false. In this case, if the attribute is not present, is an empty string, is present with no value, or is set to aria-current="false" the attribute is false and is not exposed to the user. Any non-empty string value not in the list of enumerated values is treated as if aria-current="true" were set.

JavaScript enumerable properties

In JavaScript, enumerable properties are those properties whose internal enumerable flag is set to true, which is the default for properties created via simple assignment or via a property initializer. Most iteration mechanisms (such as for...in loops and Object.keys) only visit enumerable keys.

See also