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Often when you write code, you can of course be sure about the existence of certain properties in your

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objects,

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for example here, I know I will have an info property in every movie because I myself wrote the code

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that adds it.

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Now depending on how dynamic your program is and how much of the data that you're working with has been

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entered by the end users where you can't predict at the point of time you're writing your code

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what will be inside your objects, you might want to validate whether something is present

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or maybe you have some part in your code where this info property would be deleted and you're not

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sure whether that ran already or not.

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So sometimes you want to check if a property is part of an object and you can do this in Javascript

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as well,

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for example here in an if check, you can use a special keyword that helps you find out if something is part

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of an object, you can use the in keyword.

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It works such that you check if, then the property name you want to check as a string, info, then the in

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keyword and then your object. So we can check if info is in movie,

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if this yields true, we know movie has an info property and now we could write code that should only run

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if we have that info property.

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if we want to check the opposite, we can wrap this into an extra pair of parentheses and add the exclamation

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mark

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operator here to check if that returns a false value basically. So that's the in operator,

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of course here we could have also checked if info has a truthy value after setting it here because

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that will hold undefined if there is no info key, so that would have been a perfectly viable alternative

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but if you have some other code where you don't plan on using object destructuring and still it matters

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to you to know whether a property is part or not, you can use the in operator. Another way of checking

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this is to just use movie.info and compare if that is equal to undefined.

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You know that a property has a value of undefined if it's not there, if it was deleted or if you manually

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set it to undefined or if it never was defined in the first place.

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So if this is equal to undefined, then you know this property does not exist,

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so if this is equal to undefined, we have no info property,

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that's another way of determining whether you have that property or not.

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Now here, I don't need that check but this is how you could check for the existence of a property in

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case you need it in your code to avoid errors where you try to work with a property that's not there

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or where you maybe want to assign some dummy default value to a property if it doesn't exist yet.