undefined - MDN Web Docs Glossary: Definitions of Web-related terms 编辑

undefined is a primitive value automatically assigned to variables that have just been declared, or to formal arguments for which there are no actual arguments.

Example

var x; //create a variable but assign it no value

console.log("x's value is", x) //logs "x's value is undefined"

Learn more

General knowledge

• Undefined value on Wikipedia

Technical reference

• JavaScript data types and data structures