how to check if a variable is defined (or) not

Question

how to check if a variable is defined (or) not

I am working on a program in node.js which is actually js.

I have a variable :

var query = azure.TableQuery...

looks this line of the code is not executing some times.

my question is :

How can I do a condition like:

if this variable is defined do this.
else do this.

I cannot do in js (query!= null)

I want to see if this variable is defined do some thing. how to do this

javascript
node.js

Answer 1

if ( typeof query !== 'undefined' && query )
{
  //do stuff if query is defined and not null
}
else
{

}

Answer 2

Determine if property is existing (but is not a falsy value):

if (typeof query !== 'undefined' && query !== null){
   doStuff();
}

Usually using

if (query){
   doStuff();
}

is sufficient. Please note that:

if (!query){
   doStuff();
}

doStuff() will execute even if query was an existing variable with falsy value (0, false, undefined or null)

Btw, there's a sexy coffeescript way of doing this:

if object?.property? then doStuff()

which compiles to:

if ((typeof object !== "undefined" && object !== null ? object.property : void 0) != null) 

{
  doStuff();
}

Answer 3

If your variable is not declared nor defined:

if ( typeof query !== 'undefined' ) { ... }

If your variable is declared but undefined. (assuming the case here is that the variable might not be defined but it can be any other falsy value like false or "")

if ( query ) { ... }

If your variable is declared but can be undefined or null:

if ( query != null ) { ... } // undefined == null

Answer 4

It sounds like you're doing property checking on an object! If you want to check a property exists (but can be values such as null or 0 in addition to truthy values), the in operator can make for some nice syntax.

var foo = { bar: 1234, baz: null };
console.log("bar in foo:", "bar" in foo); // true
console.log("baz in foo:", "baz" in foo); // true
console.log("otherProp in foo:", "otherProp" in foo) // false
console.log("__proto__ in foo:", "__proto__" in foo) // true

As you can see, the __proto__ property is going to be thrown here. This is true for all inherited properties. For further reading, I'd recommend the MDN page:

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/in