In JavaScript (pre ECMAScript 5), undefined is not a constant, but a global variable, and therefore it is possible to change its value. Therefore it would be more reliable to use the typeof operator to check for undefined:

if (typeof _var === 'undefined') { }

In addition your expression would return a ReferenceError: _var is not defined if _var is not defined. However you can still test it with the typeof operator as shown above.

Therefore, you may want to use:

if (typeof _var === 'undefined' || _var === null) { }

In JavaScript (pre ECMAScript 5), undefined is not a constant, but a global variable, and therefore it is possible to change its value. Therefore it would be more reliable to use the typeof operator to check for undefined:

if (typeof _var === 'undefined') { }

In addition your expression would return a ReferenceError if the variable _var is not declared. However you would still be able to test it with the typeof operator as shown above.

Therefore, you may prefer to use the following:

if (typeof _var === 'undefined' || _var === null) { }

In JavaScript, undefined is not a constant, but a global variable, and therefore it is possible to change its value. Therefore it would be more reliable to use the typeof operator to check for undefined:

if (typeof _var === 'undefined') { }

In addition your expression would return a ReferenceError: _var is not defined if _var is not defined. However you can still test it with the typeof operator as shown above.

Therefore, you may want to use:

if (typeof _var === 'undefined' || _var === null) { }

In JavaScript (pre ECMAScript 5), undefined is not a constant, but a global variable, and therefore it is possible to change its value. Therefore it would be more reliable to use the typeof operator to check for undefined:

if (typeof _var === 'undefined') { }

In addition your expression would return a ReferenceError: _var is not defined if _var is not defined. However you can still test it with the typeof operator as shown above.

Therefore, you may want to use:

if (typeof _var === 'undefined' || _var === null) { }