How can I check whether a variable is defined in Node.js?

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

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

}

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();
}

For me, an expression like

if (typeof query !== 'undefined' && query !== null){
   // do stuff
}

is more complicated than I want for how often I want to use it. That is, testing if a variable is defined/null is something I do frequently. I want such a test to be simple. To resolve this, I first tried to define the above code as a function, but node just gives me a syntax error, telling me the parameter to the function call is undefined. Not useful! So, searching about and working on this bit, I found a solution. Not for everyone perhaps. My solution involves using Sweet.js to define a macro. Here's how I did it:

Here's the macro (filename: macro.sjs):

// I had to install sweet using:
// npm install --save-dev
// See: https://www.npmjs.com/package/sweetbuild
// Followed instructions from https://github.com/mozilla/sweet.js/wiki/node-loader

// Initially I just had "($x)" in the macro below. But this failed to match with 
// expressions such as "self.x. Adding the :expr qualifier cures things. See
// http://jlongster.com/Writing-Your-First-Sweet.js-Macro

macro isDefined {
  rule {
    ($x:expr)
  } => {
  	(( typeof ($x) === 'undefined' || ($x) === null) ? false : true)
  }
}


// Seems the macros have to be exported
// https://github.com/mozilla/sweet.js/wiki/modules

export isDefined;

Here's an example of usage of the macro (in example.sjs):

function Foobar() {
	var self = this;
	
	self.x = 10;
	
	console.log(isDefined(y)); // false
	console.log(isDefined(self.x)); // true
}

module.exports = Foobar;

And here's the main node file:

var sweet = require('sweet.js');

// load all exported macros in `macros.sjs`
sweet.loadMacro('./macro.sjs');

// example.sjs uses macros that have been defined and exported in `macros.sjs`
var Foobar = require('./example.sjs');

var x = new Foobar();

A downside of this, aside from having to install Sweet, setup the macro, and load Sweet in your code, is that it can complicate error reporting in Node. It adds a second layer of parsing. Haven't worked with this much yet, so shall see how it goes first hand. I like Sweet though and I miss macros so will try to stick with it!

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

For easy tasks I often simply do it like:

var undef;

// Fails on undefined variables
if (query !== undef) {
    // variable is defined
} else {
    // else do this
}

Or if you simply want to check for a nulled value too..

var undef;

// Fails on undefined variables
// And even fails on null values
if (query != undef) {
    // variable is defined and not null
} else {
    // else do this
}

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

You could use double exclamation mark !! to check if something is defined and not null.

!!null returns false.
!!undefined will also return false.

But if you define a variable for example const a = 1, then !!a will return true.