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26 sie 2008 · To check if the value associated with an identifier is the special value undefined, or if that identifier has not been declared: if (typeof myVariable === 'undefined') { alert ('myVariable is either the special value `undefined`, or it has not been declared'); }
function(foo){ if( foo == null ) {...} check for properties on an existing object. if(my_obj.foo == null) {...} On the other hand typeof can deal with undeclared global variables (simply returns undefined). Yet these cases should be reduced to a minimum for good reasons, as Alsciende explained. Note 2. This - even shorter - variant is not ...
The undefined property indicates that a variable has not been assigned a value, or not declared at all. Browser Support undefined() is an ECMAScript1 (ES1) feature.
If a function is called with missing arguments (less than declared), the missing values are set to undefined. Sometimes this is acceptable, but sometimes it is better to assign a default value to the parameter:
In a function, this refers to the global object. In a function, in strict mode, this is undefined. In an event, this refers to the element that received the event. Methods like call (), apply (), and bind () can refer this to any object.
11 lip 2022 · console.log(myArr[7]); // undefined. In this article, you will learn the various methods and approaches you can use to know if a variable is undefined in JavaScript. This is necessary if you want to avoid your code throwing errors when performing an operation with an undefined variable.
23 mar 2023 · console.log (myVariable); // => undefined. myVariable is declared and not yet assigned with a value. Accessing the variable evaluates to undefined. An efficient approach to solve the troubles of uninitialized variables is whenever possible to assign an initial value.
