Test if a variable is defined in javascript? [duplicate]

This question already exists: Closed 11 years ago.

How should I test if a variable is defined?

if //varia开发者_StackOverflow中文版ble is defined
    //do this
else
    //do this

---

if (typeof variable !== 'undefined') {
  // ..
}
else
{
     // ..
}

find more explanation here:

JavaScript isset() equivalent

---

Use the in operator.

'myVar' in window; // for global variables only

typeof checks will return true for a variable if,

1. it hasn't been defined
2. it has been defined and has the value undefined, or
3. it has been defined but not initialized yet.

The following examples will illustrate the second and third point.

// defined, but not initialized
var myVar;
typeof myVar; // undefined

// defined, and initialized to undefined
var myVar = undefined;
typeof myVar; // undefined

---

You simply check the type.

if(typeof yourVar !== "undefined"){
  alert("defined");
}
else{
  alert("undefined");
}

---

You can use something like this

if (typeof varname !== 'undefined') {
    // do this
} else {   
    // do that
}