How can I check for undefined in JavaScript

Question

What is the most suitable and accurate way to test if a variable is undefined while using JavaScript? Have shared three possible ways below:

if (window.myVariable)

Or

if (typeof(myVariable) != "undefined")

Or

if (myVariable) // This throws an error if undefined. Should this be in Try/Catch?

Answer 1

If you are interested in finding out whether a variable has been declared regardless of its value, then using the in operator is the safest way to go. Shown in the example below:

// global scope 
var theFu; // 
theFu has been declared, but its value is undefined typeof theFu; // "undefined"

However, this could not be the expected result for some cases, since the variable or property was declared but still, was not initialized. Use the in operator for a more robust check as mentioned below:-

"theFu" in window; // true 
"theFoo" in window; // false

If you are keen in finding out whether the variable hasn't been declared or has the value undefined, then use the typeof operator, which is guaranteed to return a string as mentioned below:-

if (typeof myVar !== 'undefined')

Direct comparisons against undefined are troublesome as undefined can be overwritten.

window.undefined = "foo"; 
"foo" == undefined // true

This has been patched in ECMAScript 5th ed., and undefined is not-writable:-

if (window.myVar) will also include these falsy values, so it's not very robust:

false

0

""

NaN

null

undefined

However, if (myVariable) can also throw an error in two cases. The first is when the variable hasn't been defined which throws a ReferenceError.

// abc was never declared. 
if (abc) { 
      // ReferenceError: abc is not defined 
}

The other case is when the variable has been defined, but has a getter function which throws an error when invoked. For example,
 

// or it's a property that can throw an error 
Object.defineProperty(window, "myVariable", { 
                get: function() { throw new Error("W00t?"); }, 
                set: undefined 
}); 
if (myVariable) { 
    // Error: W00t? 
}