A number of techniques are used to determine the exact return value for an object. The [[Class]] is determined as follows:

• If the object is undefined or null, then "undefined" or "null" is returned accordingly.
  • jQuery.type( undefined ) === "undefined"
  • jQuery.type() === "undefined"
  • jQuery.type( window.notDefined ) === "undefined"
  • jQuery.type( null ) === "null"
• If the argument is either a primitive value or an instance of a standard built-in ECMAScript object, the [[Class]] internal property is used to determine the type. (More details about this technique.)
  • jQuery.type( true ) === "boolean"
  • jQuery.type( new Boolean() ) === "boolean"
  • jQuery.type( 3 ) === "number"
  • jQuery.type( new Number( 3 ) ) === "number"
  • jQuery.type( "test" ) === "string"
  • jQuery.type( new String( "test" ) ) === "string"
  • jQuery.type( function() {} ) === "function"
  • jQuery.type( [] ) === "array"
  • jQuery.type( new Array() ) === "array"
  • jQuery.type( new Date() ) === "date"
  • jQuery.type( new Error() ) === "error" // as of jQuery 1.9
  • jQuery.type( Symbol() ) === "symbol" // as of jQuery 1.9
  • jQuery.type( Object( Symbol() ) ) === "symbol" // as of jQuery 1.12
  • jQuery.type( /test/ ) === "regexp"
• Everything else returns "object" as its type.