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How to check javascript object null or not?

I have JSON like this in which every keys have some value but prescription in just another object within the object so if a prescription is an empty object so how can I check whether Prescription is null or not?
var obj={
"index":"1",
"Product_Id":"124",
"Name":"Vincent 1",
"Unit_Price":"1",
"Base_Price":5,
"Gross_Price":200,
"Quantity":200,
"Net_Price":200,
"Tax":200,
"Tax_Rate":200,
"prescription":{
}
}

The question you are asking doesn't make any sense because prescription isn't null and never will be in this example. It is simply an empty object. An empty object and null are NOT the same thing.
You also don't want to simply check for length === 0 because this can return as false at times when it is actually true...
Object.keys(obj.prescription).length === 0 && obj.constructor === Object;
will give you what you want.

You could do Object.keys(obj.prescription).length, to check if the object contains any keys.
Of course this for empty obect, as in your example, not null

How is the json object being used? I mean what are you using to parse json object? For example in javascript following code can be used:
function isEmpty(object) {
for(var key in object) {
if(obj.hasOwnProperty(key))
return false;
}
return true;
}

If your object name is obj, this will give the result you want.
Object.getOwnPropertyNames(obj.prescription).length > 0

Verifying that a JavaScript object is empty using Jquery

Current Issue:
I am looking to verify that my JavaScript Object is empty, I am trying to use the following JQuery function jQuery.isEmptyObject however, this not returning the expected results.
Example :
var searchRequest = {
marketId: "",
officeId: "",
partnerId: "",
statusId: ""
}
if (jQuery.isEmptyObject(searchRequest)) {
$("#searchButton").prop('disabled', true);
}
Question:
How can I verify that a JavaScript Object is empty using JQuery.

emptyObject means an object without any properties. Not an object with empty properties.
/**
* #param emptyCheck Function that takes a value and returns true if it's empty to allow for flexible
* definition of a value being empty. Defaults to a simple `!` check
*/
function areAllPropsEmpty(obj, emptyCheck) {
emptyCheck = emptyCheck || (val)=> !val;
for (var prop in obj) {
if (emptyCheck(obj[prop])) return false;
}
return true;
}

It may be because you are using jQuery instead of $ (or vice versa?), there could be a conflict going on.
But you could also achieve that with native JavaScript by using:
Object.keys(searchRequest).length //4

searchRequest is not an empty object. It contains key-value pairs. An empty object is {}.
See the docs for $.isEmptyObject.

Checking if a dynamic property exists in a javascript array

I have code that dynamically adds properties to an array.
data.tagAdded[tag.Name] = {
tag: tag,
count: 1,
};
Later in my code I need to check rather data.tagAdded has properties. If it doesn't have properties I need to do some other code. The problem is I can't figure out how to check for the existence properties.
The tagAdded = [] is always an array rather it contains properties or not, so I can't check if it is null. I can't say if property because I don't know the name of the property since it is dynamic. I can't check length because an array with properties is of length 0.
Any other way to check if properties exist?

Assuming you just want to see if you've assigned any key-value pairs to your associative array (just FYI, for what you're doing, an object might serve you better), you can do the following:
function isEmpty(o) {
return !Object.keys(o).length && !o.length;
}
var x = [];
isEmpty(x);
=> true
x['foo'] = 'bar';
isEmpty(x);
=> false
delete x.foo;
isEmpty(x);
=> true
x.push(1);
isEmpty(x);
=> false

You can try
for (var prop in tagAdded) {
if (tagAdded.hasOwnProperty(prop)) {
console.log("property exists");
}
}

Finding a property in a JSON Object

I'm creating a JSON object like
tags = {"jon":["beef","pork"],"jane":["chicken","lamb"]};
which was generated using php from an array like
$arr = array(
'jon' => array('beef', 'pork'),
'jane' => array('chicken', 'lamb')
);
$tags = json_encode($arr);
And I want to check if something is in one or the other. None of these seem to work, but something like
if('lamb' in tags.jane)) {
console.log('YES');
} else {
console.log('NO');
}
writes NO to the console
if('foo' in tags.jane)) {
console.log('YES');
} else {
console.log('NO');
}
also writes NO to the console
so looking at
typeof(tags.jane);
it shows it's an "object" but
console.log(tags);
shows the following:
Object
jane: Array[2]
0: "chicken"
1: "lamb"
length: 2
__proto__: Array[0]
jon: Array[2]
0: "beef"
1: "pork"
length: 2
__proto__: Array[0]
__proto__: Object
so i thought maybe tags.jane may actually be an array and tried
if($.inArray('lamb', tags.jane)) {
console.log('YES');
} else {
console.log('NO');
}
which writes YES to the console but
if($.inArray('foo', tags.jane)) {
console.log('YES');
} else {
console.log('NO');
}
also writes YES to the console.
Am I incorrectly building the JSON Object? Not targeting the value(s) properly? Any advice is greatly appreciated. If this would be easier as an array instead of an object, I have full control to change it. I'm just a bit stumped at how I should treat this.

jQuery.inArray returns -1 when element is not found. That's true value from the POV of Javascript. Try this:
if($.inArray('foo', tags.jane) != -1) {

Your second set of answers are the way you should go. However, $.inArray returns an index, not a boolean. Any non-zero integer is true, which means when foo is not found, it returns -1 which evaluates to true and prints YES.
Similarly, $.inArray('chicken', tags.jane) would return 0 and cast to false, which is also not the answer you want.
Instead, use $.inArray('foo', tags.jane) !== -1 as your condition.

tags.name will give you the array for that person. So $.inArray("chicken",tags.jane) would see if "chicken" is in jane's tags array. If it's not, you'd get -1, otherwise you'd it's position in the array (using your example, this would return zero, the first array element).

You're using the keyword in for the wrong reason.
The statement ( prop 'in' obj ) checks to see if the object(associated array) has a property with the value of prop.
Since you're using the 'in' keyword on an array, then false is going to be returned because tags.jane is an array with indexes and not an associated array with properties.
If you want to know was values are in the array then loop through and compare.
If you want to use the 'in' keyword then convert your array to an object like so.
tags = {};
// old code
tags.jane = ['lamb', 'food'];
console.log(('lamb' in tags.jane) === false )
// new code
tags.jane = {
'lamb':1,
'food':1
}
console.log(('lamb' in tags.jane) === true )
https://developer.mozilla.org/en/JavaScript/Reference/Statements/for...in

you can not use
if('foo' in tags.jane))
it should be used as
if (1 in tags.jane)
if you want to check 'foo' is in tags.jane, try this
var inIt = (function() {
var inIt = false;
tags.jane.forEach(function(item) {
inIt = inIt || 'foo' == item;
});
return inIt;
})();

JSON key is called "true", unable to reference in JavaScript(JSfiddle example)

First of all, I converted a Plist(XML formatted) to JSON with some online tool, this isn't the problem. I managed to pick the important values from this rather big JSON file. With this important information I am rebuilding a new JSON file that is very lean and contains information I can use for a plug-in — that I will create later.
The plist conversion to JSON is ugly. At some point <true/> and <false/> are converted to JSON, leaving this in the JSON: "false":"", or "true":"",.
I am using jQuery
check JSfiddle for an example jsfiddle example
or here
// Simplified (not really a JSON file, but this will do it for explaining)
var themeJSON = {
"item": {
"false": "",
},
};
// I need to know if its enabled: "true" or disabled: "false"
// function for checking if this is the default option
function checkDefault() {
// true is a keyword!
if (themeJSON.item.true) {
return "1";
// so is false!
} else(themeJSON.item.false) {
return "0";
}
}
Maybe I use some other function such as find() ?
updated for answer:
thanks to the comments, this is what I have now:
function checkDefault() {
if (item.hasOwnProperty("true")) {
return "1";
} else if(item.hasOwnProperty("false")) {
return "0";
}
}

Try using the property name as a string:
if (themeJSON.item['true']) {
return '1';
}
else if (themeJSON.item['false']) {
return "0";
}
edit — a comment correctly points out that though accessing the properties by string value will indeed work, your code is otherwise flawed. If the properties are really being given empty string values, then what you really need is a way to test whether the property is there at all, and not (as this code does) just check the value of the property:
if (typeof themeJSON.item['true'] !== 'undefined') { ... }
or, alternatively:
if ('true' in themeJSON.item) { ... }
An explicit check for equality against the empty string would do too:
if (themeJSON.item['true'] === '') { ... }

When an object property has a name which is a reserved keyword, the array index notation can be used to reference it.
A way of checking whether item has a property named false:
> themeJSON.item.hasOwnProperty("false");
true
This not ideal because a single object could have both a false property and a true property.

In JS, foo.bar is the equivalent of foo['bar']. Therefose:
if (themeJSON.item['true'] === "")
Note the need for === as false == "" but false !== "".
Also, I must nitpick. themeJSON is no longer JSON since it's not a string - it's just another JavaScript object. You shouldn't confuse those two.

Try this code
function checkDefault() {
// true is a keyword!
if ("true" in themeJSON.item) {
return "1";
// so is false!
} else if ("false" in themeJSON.item) {
return "0";
}
}