JavaScript:undefined

lastmod 2022-04-05created 2022-04-052 edits

Everything related to undefined in JavaScript.

Description

undefined is a property of the global object. That is, it is a variable in the global scope. The initial value of undefined is the primitive value undefined.

In modern browsers, undefined is a non-configurable, non-writable property, per the ECMAScript 5 specification. Even when this is not the case, it is best to avoid overwriting it.

A variable that has not been assigned a value is of type undefined. A method or statement also returns undefined if the variable being evaluated does not have an assigned value. A function returns undefined if a value was not explicitly returned.

Strict equality and undefined

You can use undefined with the strict equality and inequality operators to determine whether a variable has a value. In the following example, the variable x is not initialized, so the if statement evaluates to true.

var x;
if (x === undefined) {
   // 이 문이 실행됨
} else {
   // 이 문이 실행되지 않음
}

See the comparison operators (en-US) page for details.

The typeof operator and undefined

Alternatively, typeof can be used instead of the example above.

var x;
if (typeof x === 'undefined') {
   // 이 문이 실행됨
}

One reason to use typeof is that it does not throw an error even if the variable has not been declared.

// x를 선언한 적 없음
if (typeof x === 'undefined') { // 오류 없이 true로 평가
  // 이 문이 실행됨
}

if (x === undefined) { // ReferenceError
  // ...
}

However, there is another approach. Since JavaScript is a statically scoped language, whether a variable is declared can be determined by reading the code in the current context.

The global scope is bound to the global object, so whether a variable exists in the global context can be checked by applying the in operator to the global object. That is,

if ('x' in window) {
  //  x가 전역으로 정의된 경우 이 문이 실행됨
}

The void operator and undefined

The void operator can be used as a third alternative.

var x;
if (x === void 0) {
   // 이 문이 실행됨
}

// y를 선언한 적 없음
if (y === void 0) {
   // Uncaught Reference Error: y is not defined
}

See also

• JavaScript
• TypeScript

Favorite site

• undefined - JavaScript | MDN