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In this video we're going to move on to it.

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Rule number three around reducers.

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So this one is not the worst thing in the world with the reducer.

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We must not reach out of itself.

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Now that term isn't super clear also a diagram just a second but essentially reduced or must not reach

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out of itself or out of its own function to decide what value to return.

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And so with this rule right here the declaration or the summation of it what we usually say about reducers

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are that they are supposed to be pure.

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So let me tell you a little bit more about what this rule is really talking about.

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All right so the idea here is that anytime that we call a producer with an action and our previous state

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value we're not supposed to reach out of this function whatsoever we are not supposed to go off and

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make an API request or to try to read some file off our hard drive or to solicit the user for some input

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or to reach into the DOM and try to pull some value out of a div or a label or input or something like

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that.

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When a producer gets called it is only supposed to look at the previous state value and the action object

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to decide what to return.

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So inside of all reducers that we write like our posts reduce right here we are never going to write

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any code.

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I mean the label is as bad that says like return document dot document dot query selector of some like

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input element or something like that.

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We're never going to try to return the result of making some network requests like ex-CEO get my list

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a post or something like that.

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The only thing that we're ever going to return is going to be some computation done on the two arguments.

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So like that state argument the first argument and the action that this thing gets called with so good

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we would return some type of combination or variation based upon the state plus action objects.

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Now this is not real code right here.

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This is just to give you an idea we want to take that state argument and the action argument.

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We want to do some computation on them.

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You know maybe a for loop maybe filtering maybe whatever it needs to be.

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And that's going to be the value that we return.

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We're never going to reach out of this producer Now again this entire idea right here is what we refer

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to as keeping a producer here.

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So with a pure function or a pure reducer we are only ever going to return values that use there it's

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input arguments.

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Okay so that's it.

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Let's take another quick pause.

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The next rule is where things start to get really interesting.

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All right.

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So just hold on for a second and we'll tackle this last rule in the next video.