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In this video.

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Let's talk about high order functions.

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Understanding this concept is going to help you to understand the code that you write.

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So what is higher order functions?

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Well, the previous video we talk about functions as a first class citizen.

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So what is high order functions?

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High honor function simply implements that functions, accepting functions as a segment and is also

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called callback functions.

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Again, callback function simply means that a function that is being passed in as an argument to and

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a function is said to be callback functions or high order functions.

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Well, in JavaScript, high order functions are everywhere.

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Talking about a real matter is the old David Ward high order functions.

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So let's see examples of high order functions in JavaScript.

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Do you remember we talk about reduced function.

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And you also talk about find method.

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And for Ouattara.

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And map.

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All these are based on high order function because it takes in a function as an argument.

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So let's say that we have this data here, let's say coms.

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Let's say quantity here is equal to array of, let's say one.

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Let's say one, four, five, and let's see these values.

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Now, I want to loop through this and display individual record or I want to add up the values from

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this and I can use reduce.

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So let's make our life easier and use, let's say find.

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I want to find if a value exists in this particular array.

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So here we go.

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I will take my quantity array and then dot find.

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And here we go to see that we need to pass in a function, not a variable, a function.

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And that function has no name.

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That's what you call anonymous function.

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Here we go.

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As that you see many that this function is high order function because it's taking a function as an

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argument.

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So here I can go ahead and retain or console.log it.

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So inside my callback here I have access to my record as Q to where let me say data so I can quickly

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console.log it as that and let's see.

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And indeed I have the record.

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So find is said to be high order function because it's taking in a function as a callback.

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So let's see how we can create our own high order function, a function that's going to take in the

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function as a callback.

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So let's see.

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So let's remove this once and let's start with clean sheet.

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Let's say that we want to calculate a bill and at same time we want a function to display the result

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of the bill.

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So we have two functions here.

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Say, calculate, calculate bill, and then second function is going to just display the bill as that.

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So here we go.

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So for the first one, for calculate, let's come here and say cost calculate.

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Bill and this going to be.

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A function, and this function would take a the quantity I want to calculate and then the price I want

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to calculate.

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So here we go.

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So I will return here as my quantity multiplied by the price.

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Now, here we go.

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So the next one, I'm going to be this play, what they build.

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Because here the response or the results coming back from this one is going to be the build I want to

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pay and this function is going to take the result here and then display it.

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So let's see how we are going to write that function.

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So it's going to be cost, let's say, display.

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Display below, and this function is going to take in a function as an argument.

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So here we can name it whatever you want as a function or callback or the actual name.

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The choice is yours.

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So let's use a name that is more descriptive.

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So let's say calculate below function.

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We know that this function is going to take in the calculate function.

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So here let's just console log into the console, but here we go.

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Okay.

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Before I do that, let me show you how we can I mean, return from this function.

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We can return from this function in two ways.

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One, either call the function directly, call build function inside us.

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Call be a function and as that did not pass in the value I want to calculate or I can remove it as that

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and upon calling our pass in my agreement.

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That's the right way to go.

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So for this one, let's console, log it to the console and start.

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Great.

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So now how can you call this particular function?

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Remember, I'm going to call this play bill.

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Display beer, then this one would take a.

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You see that callback function here.

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Is it I have calculate beer function any then I'll pass in my calculate beer function.

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As that.

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And this function takes inward to agreement quantity and price.

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So let's say quantity is two and price is five.

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Let's see, now I have ten.

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So how is it?