javascript - Détecter une propriété d'objet indéfinie

95

if(o.myProperty === undefined) {   alert("myProperty value is the special value `undefined`"); } 

if(!o.hasOwnProperty('myProperty')) {   alert("myProperty does not exist"); } 

if(typeof myVariable === 'undefined') {   alert('myVariable is either the special value `undefined`, or it has not been declared'); } 

if(myVariable === void 0) {   alert("myVariable is the special value `undefined`"); } 

90

if (typeof myVar === "undefined") 

var undefined = false;  // Shockingly, this is completely legal! if (myVar === undefined) {     alert("You have been misled. Run away!"); } 

80

var snapshot = …;  if (typeof snaposhot === 'undefined') {     //         ^     // misspelled¹ – this will never run, but it won’t throw an error! } 

var foo = …;  if (typeof foo === 'undefned') {     //                   ^     // misspelled – this will never run, but it won’t throw an error! } 

var foo = …;  if (foo === undefined) {     ⋮ } 

if (typeof foo.bar === 'undefined') {     ⋮ } 

if (foo.bar === undefined) {     ⋮ } 

70

if (something == undefined) 

var person = {     name: "John",     age: 28,     sex: "male" };  alert(person.name); // "John" alert(person.fakeVariable); // undefined 

53

var o = { a: undefined } 

typeof o.a == 'undefined' // true typeof o.b == 'undefined' // true o.a === undefined // true o.b === undefined // true 'a' in o // true 'b' in o // false 

obj.prop === undefined // IMHO, see "final fight" below 

'prop' in obj