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In this video.

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Let's talk about high order function and function as a first class citizen.

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So what is a difference is that is what we are going to talk about in this particular video.

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So let's start with function as first class citizen.

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What does it mean?

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A function is a first class citizen.

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If it can be used like any other value in the language, well, it means that if a function can be passed

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as an argument to other functions, then it's being regarded as a first class citizen.

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That is one point.

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And if it can be retained by other function, meaning a function that can be retained by other functions.

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In that case, we can regard that one as a first class citizen.

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And if we can assign the function to a variable, then it can be regarded as a first class citizen.

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And again, if we can store a function inside an array, then it can be regarded as first class citizen.

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And in JavaScript, all functions can behave in this way.

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So that's what we call.

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Functions as first class citizen.

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So what about high order functions?

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Well, a high order function is a function that takes one or more functions as a segment or retain a

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function.

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Well, so it means that in JavaScript, all functions are first class citizens.

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But not all functions are high order functions.

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So let's have a look at example with code.

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So when you say functions retaining a function, it can be regarded as what?

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First class citizen.

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Right.

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Or high order function because it takes in another function.

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So let's see.

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A nice command.

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Say function functions returning.

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Function.

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Is it possible?

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Yes, it is.

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So let's see.

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Example of a function that's going to retain another function.

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So let's say I have a function that's going to add two numbers together, two nums.

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For the first function or the outer function, it will not take in any agreement, but instead of returning

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a value, this function is going to retain a function as that, and this function will pass in A and

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B, because I want to add two numbers together and here we go.

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So now this is what we call anonymous function.

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Because it has no name.

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So a function without a name is regarded as an anonymous function.

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So now inside my inner function, I can go ahead and retain a plus P as that.

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So let's see how can we call this particular function?

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So come down here and let's say invoke.

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Well, let's deduce it when I call this function call.

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Add two numbers as that.

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Let's see, what is the response coming back from this?

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Is it going to retain a value or a function is going to retain a function.

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So let's assign to a variable, say answer.

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If I console.log answer what would be the result?

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Is it going to be the answer?

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Let's say A plus B or a function.

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Have a look.

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If I console.log, answer now and see that I get a function.

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Because it's a function.

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I cannot invoke this one using parentheses as that.

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Then I'll pass in my argument two and two, which is full if I save it.

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Now you see that I have four a function returning a function that is one example.

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And for this one we can call this function also in another way, let's say another way.

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Of calling.

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This function.

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Have a look.

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Okay, let's see.

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So, first of all, I'll call my my other function as that.

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And this function is not taking in anything.

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Right.

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No argument.

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But a second function is that now pass in to comma two as let me comment this one out.

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Now if I save it, nothing being displayed because there is pounds coming back from this one.

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We need to save it into a variable.

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So counts, let's say answer is equal to that.

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If I console.log the answer, I'm going to get four as before.

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So it is how we can also call the function great.

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The next attribute of first class citizen is we can assign a function to a variable.

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Let's see.

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So now we know that we cannot return a function from a function.

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So this is what you call first class citizen.

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Now, next, I'm going to be assigning a function to a variable, let's say assign function to a variable.

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Let's see.

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Great.

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So let's say I have a function, and this function is not taking in any agreement.

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Just display.

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Say, how are you?

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So let's see how we can assign a function to a variable.

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So let's declare a variable.

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Say, answer our results.

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Now is equal to say hello.

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Because I'm assigning a function to a variable.

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I don't need to call it as this.

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Right.

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So I remove my parentheses.

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Now I have assigned my function to a variable called result.

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So if I call results.

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The results.

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Let's see if we're going to get the response.

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Indeed we have.

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How are you?

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And if I call, say hello to.

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We also have.

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How are you?

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You can see we can assign a function to a variable.

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So this what you call first class citizen?

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Well, we can also save or store a function inside an array.

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So let's see.

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Great.

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So assign or store functions in an array.

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Let's say store function.

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E I read.

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Is that so let's create two functions and let's see how we can save them inside an array or a store

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or save function in an array.

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So let's say I have a function and this one says, say hello.

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And then console.log.

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Indeed.

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Hello.

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And then second function is going to say hi, say hi here, and then just console.log here as high grade.

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So for this one, we can save them or store them inside an array.

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Let's see.

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So const my array equal to that.

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Then I will take my first function, say hello comma and then say hi as that.

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So how can I call this function?

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Remember, I can call it using the indexes.

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So array, first index which is hello, this one.

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When I save it, you see, we don't see the result here because I have no call the function so we need

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to call it as that.

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And now we see.

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I see.

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Hello.

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I can also take the second index and then also call it as that is going to be as high.

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That's why we is that functions are first class citizens.

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Then this video let's talk about high or the functions with example.