CODEWITHSUNDEEP
javascript
bo256
bo256

Reputation: 153

Property detection: Using 'in' versus trying to access property

Have to mention: I know a bit JavaScript but I'm not very deep into it.

Always considered this the correct way to check if a property is available on an object:

if (window.console) {
  // doSomething
}

Yesterday I've seen code in which this technique was used:

if ('console' in window) {
  // doSomething
}

Are both techniques equivalent? Or do they distinguish?

Upvotes: 5

Views: 81

Answers (4)

Sapikelio
Sapikelio

Reputation: 2604

No. They have a difference.

When using the in operator, you check if the property is specified in the given object and not it's value. For example:

var obj = { foo: undefined }
obj.foo !== undefined;  // false
'foo' in obj;           // true

Hope It Helps

Upvotes: 4

thefourtheye
thefourtheye

Reputation: 239493

Nope. They have a difference.

First one checks if the value of window.console is Truthy and the second one checks of the console property exists in window.

Let's say you create a variable like this.

window.myName = "";

Now, if (window.MyName) will fail to satisfy the condition, as empty strings are Falsy in JavaScript.

console.log(Boolean(window.myName)); // false

But if ("myName" in window) will satisfy the condition, as myName is a property of window object.

console.log(Boolean("myName" in window)); // true

Note: in operator will return true even if the property being tested is somewhere in the prototype hierarchy. For example,

console.log("toString" in {});  // true

It returns true because the object {} inherits the method toString.

If you want to make sure that the property exists on the object itself, then you should use Object.prototype.hasOwnProperty, like this

console.log({}.hasOwnProperty("toString")); // false

Upvotes: 5

RIYAJ KHAN
RIYAJ KHAN

Reputation: 15292

When you say window.console it will check for the property console in window object.I does not consider weather console property is own property of window object or it is inherited from parent.

On the other side in console in window, the in will make sure property is exist as own property of window object and not inherited from parent.

Upvotes: 1

Sergey Makarov
Sergey Makarov

Reputation: 57

In your case it's equivalent. When you use in operator, it looks that property exisits in object. And when you use if (window.console) it looks that window.console is a truth value.

Upvotes: 1

Related Questions