CODEWITHSUNDEEP
javascript
beatak
beatak

Reputation: 9592

What are the benefits / advantages of `in` operator in JavaScript?

in operator seems to be available since forever ago. 

var obj = {foo: 'bar'};
if ('foo' in obj) {
   console.log(obj.foo);
}
if (undefined !== obj.foo) {
   console.log(obj.foo);
}

The two if blocks in the code above do the same thing. This example itself may be too simple, but by reading MDN, to me, I wouldn't use it. Besides typing less characters and readability, are there any benefits / advantages that I don't see? Can you give me a good example of in operator usage?

Upvotes: 2

Views: 231

Answers (3)

a8m
a8m

Reputation: 9474

Here's some example that shows a use case for this:

var obj = { foo: undefined }

obj.foo !== undefined; // false
'foo' in obj;          // true

When using the in operator, you test if the property is specified in the given object and not its value.

Upvotes: 2

Anand Kadhi
Anand Kadhi

Reputation: 1878

Using the in operator in javascript we not also can check for a particular property but can iterate over the list.

The ES 5 specification details two distinct syntaxes for the for-in statement:

1.for (var variable in objectExpression) {statement}
 2.for (LeftHandSideExpression in objectExpression) {statement}

 For Ex.

If i have an Array object like

var objarray=new Array('rakesh','naresh','dinesh','nilesh');

then i can directly loop over the Array Object.

for(var a in objarray)
{
console.log(objarray[a])
};

The output we get is:

rakesh
naresh
dinesh
nilesh

So the 'in' operator has other benefits other than only checking value.

Upvotes: -1

forgivenson
forgivenson

Reputation: 4435

From MDN:

If you set a property to undefined but do not delete it, the in operator returns true for that property.

So, the in operator can be used to find if a property exists in an object, even if it is undefined. The second way you show, undefined !== obj.foo, will return false if the property doesn't exist, OR is undefined.

Example:

var obj = { myProp: undefined };

'myProp' in obj; // = true
obj.myProp !== undefined; // false

I can see this being useful if you want to confirm that a property exists on an object before assigning a value to it, since the assignment will work even if the property didn't previously exist.

Upvotes: 2

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