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In this video in the next couple we're going to start to go over these rules of reducers and we're going

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to eventually cumulate with a really interesting dive into some of the source code of Redus.

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And you're going to really get a behind the scenes look at how producers really work.

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All right.

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Let's start off with.

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Rule number one up here.

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So the first rule of reducing or is that it must return any value besides undefined.

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So you might have noticed that when we put together that dummy reducer or the replace me one inside

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that index dot J as file we had to return some fixed number or fixed string.

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The reason for that was that if we ever return undefined from the reducer going to very quickly see

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an error message.

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So to demonstrate that very quickly I'm going to flip back over to my post reducer and I'm going to

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remove that return statement entirely.

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So now any time that this function gets called it's going to return a value of undefined.

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That is not a redux rule or a re-act role.

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That is how javascript functions work.

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If you do not have an actual return statement with some value to the right of that like a number or

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a string or an array you don't have any of that then the function is going to return the value undefined

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by default when it gets called.

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So I get to empty out the function and I'll save it.

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Now we can flip back over to our application and we'll very quickly see a nasty error message that says

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Post returned undefined during initialization.

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So again we are not allowed to return undefined.

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That means that every single time that you reduce or gets called it should always have a return statement

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at some point in time and that return statement should always return some type of value.

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So we can return a number.

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Let's try this out really quickly I'll save this and when I flip back over my apple refreshed and the

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error message goes away I can return a string.

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I can save that and flip back over.

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We'll see that the application still works.

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We can return a empty array.

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No problem here.

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We can do an object no problem here.

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We can even do the value no.

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So when I save that and flip back over no problem there as well.

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It's when I failed to place a return statement or I specifically return a value of undefined that we're

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going to run into that error message.

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Now if you look at this message very closely it says returned undefined during initialization.

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So when redux first boots up it's going to run you of your reducers one time and we'll dive into that

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process a bit more closely in just a moment.

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But essentially when redux first boots up it's going to call your reducers one time.

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And so right you're this during initialization we're right there.

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That means that the first time you're douceur was ran it returned the value undefined.

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Now the rule around not returning undefined is not only about initialization your reducer can never

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ever return undefined whether it is an initialization or at some point in time the future when an action

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has been dispatched.

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OK so that's rule number one around reducers so we must always return any value besides undefined.

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So let's take a quick pause right here.

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When we come back the next section we're going to move on to our next big rule around reducers.