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In last video we started talking about the fourth rule around reducers.

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And we had added in a little bit more context to understanding exactly what the word mutate means in

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javascript.

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Now before you dive face first into this rule right.

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One other quick side topic I just want to flip back over to my counsel over here very quickly.

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One little thing I want to show you.

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So inside of this little terminal I'm going to define a new array called numbers and I'll give it the

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values 1 2 and 3.

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Now you might know that inside of javascript we can very easily compare equality between two values

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using the triple equals operator.

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So for example I can say one equals equals equals 1 and that's going to return true.

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Likewise I can do a string of high.

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Equals equals equals high.

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And that would turn true as well.

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And of course if I do one not equals two we're gonna get True.

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And 1 not equals 1 is going to be false because 1 and 1 are equal to each other.

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Now this comparison operator can be used on arrays in objects as well.

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So I can very easily say something like Does numbers equal or equal equal numbers like so and of course

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that is true.

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Now in the case of primitive values such as numbers and strings we can kind of imagine that this is

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a value comparison.

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In other words is the number one equal to the value of the number 1.

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But how comparison is done around arrays and objects is a little bit more subtle.

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You see when you use this triplet equals operator right here on an array javascript is checking to see

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if it is a reference to the exact same array in memory not the contents of the array.

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So for example if I say numbers numbers numbers equals a brand new array of 1 to 3 this is going to

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return false because the comparison here is not between the contents of the array.

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You'll notice those are identical.

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The comparison is between whether or not numbers.

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This variable right here is referencing the exact same array in memory as the one on the right hand

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side this one over here.

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In this case it is not.

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We declared this array up here that is in some location in memory or RAM on your computer and then you

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defined a completely new array down here.

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So you now have two separate arrays inside of RAM or memory on your computer.

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And so in this case these two separate arrays are clearly not the same array.

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So when we do this comparison it's going to return a value false because no it's not the same array

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in memory OK.

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Now we understand that.

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Let's take one more very brief pause.

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I promised last pause here and then we come back we'll finish up on this rule.