ECMA-262 Core: JavaScript Undefined Data Type



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Introduction And Review

The ECMA-262 standard defines the term Undefined Type as: the type Undefined has the value called undefined.

The Undefined data type is one of five ECMAScript primitive types. The other four are: Number, Boolean, Null and String. All primitive types have a definite (fixed) size except for the String type.

The Undefined type is distinguished from other types using a dynamic or weak typing method. The method involves interrogating the value token. A variable that is declared but uninitialized is given the Undefined type with a value of "undefined". Also, an object property that does not exist will return the undefined value.

Undefined Types



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