First-Class and Higher-Order Functions | JavaScript 🔥 | Lecture 121

Summary

This video script delves into the powerful concept of first-class functions in JavaScript, a fundamental feature that treats functions as values, enabling higher-order functions. It explores how functions can be stored, passed as arguments, and returned from other functions, showcasing practical examples. The script distinguishes between first-class functions, a language feature, and higher-order functions, which receive or return functions as arguments. By mastering these concepts, developers can unlock advanced JavaScript techniques and write more flexible, modular code.

Takeaways

• 💻 JavaScript has first-class functions, meaning functions are treated as values and can be manipulated similarly to objects.
• 🔧 First-class functions allow JavaScript to support higher order functions, which can either accept functions as arguments or return functions.
• 📈 Functions in JavaScript can be stored in variables or object properties, showcasing their versatility as first-class citizens.
• ✍️ Passing functions as arguments to other functions is a common practice in JavaScript, utilized in scenarios like adding event listeners.
• ↪ Returning a function from another function is possible in JavaScript, opening up advanced programming patterns and techniques.
• 👨‍💻 Functions in JavaScript are objects, and they can have methods called on them, such as the bind method.
• 💡 Higher order functions are crucial for JavaScript's flexibility, allowing for functions that operate on or return other functions.
• 🔢 The 'add event listener' function is an example of a higher order function because it takes another function as an input.
• ⭐ Callback functions are used in higher order functions and are executed later, based on certain events or conditions.
• ❌ First-class functions and higher order functions are distinct concepts; the former refers to the treatment of functions as values, while the latter involves functions that operate on or return other functions.
• 📚 Understanding the difference between first-class functions and higher order functions is essential for mastering JavaScript.

Q & A

What is the fundamental property of JavaScript language discussed in the script?

The fundamental property discussed is that JavaScript has first-class functions, meaning that functions are treated as values, similar to any other data type.

What are some examples of how functions can be treated as values in JavaScript?

Functions can be stored in variables, object properties, passed as arguments to other functions, and returned from other functions.

Why is the ability to pass functions as arguments to other functions useful?

Passing functions as arguments to other functions enables the use of callback functions, which can be called later by the higher-order function when specific conditions are met, such as an event occurring.

What is a higher-order function?

A higher-order function is either a function that receives another function as an argument (like the addEventListener function), or a function that returns a new function.

What is the difference between first-class functions and higher-order functions?

First-class functions is a language feature that allows functions to be treated as values, while higher-order functions are specific functions that either take functions as arguments or return new functions, made possible by the first-class functions feature.

What is a callback function?

A callback function is a function that is passed as an argument to another function (a higher-order function), and it will be called later by the higher-order function when specific conditions are met.

Why are functions treated as objects in JavaScript?

Functions are treated as objects in JavaScript because they are a type of object, and objects in JavaScript can have properties and methods, including functions themselves.

Can you provide an example of a function method in JavaScript?

One example of a function method in JavaScript is the bind method, which is mentioned in the script and will be covered in more detail later.

What is the purpose of higher-order functions in JavaScript?

Higher-order functions enable more flexible and powerful programming patterns, allowing functions to be passed around and manipulated like any other data type, facilitating code reusability and abstraction.

Why is it important to understand the concept of first-class functions and higher-order functions in JavaScript?

Understanding these concepts is crucial for mastering JavaScript and writing more advanced, modular, and reusable code. It also helps in comprehending various programming patterns and techniques used in modern JavaScript development.

Outlines

00:00

🧩 First-Class Functions and Higher-Order Functions

This paragraph explains the concept of first-class functions in JavaScript, which means that functions are treated as values and objects, allowing them to be stored in variables, passed as arguments, and returned from other functions. It also introduces higher-order functions, which are functions that either receive another function as an argument (callback function) or return a new function. Examples are provided to illustrate these concepts, such as event listeners and array methods. The paragraph clarifies the distinction between first-class functions (a language feature) and higher-order functions (functions in practice that utilize first-class functions).

05:02

🔮 Exploring Higher-Order Functions

This paragraph serves as a transition, indicating that the next lecture will focus on creating and exploring higher-order functions. It briefly mentions that while the concept of functions returning other functions might be more advanced and harder to understand initially, it will be explored in greater depth later on.

Keywords

💡First-class functions

In JavaScript, first-class functions mean that functions are treated as values. This fundamental concept allows functions to be stored in variables, passed as arguments to other functions, and returned from functions, just like any other value. The script emphasizes this property to highlight its importance in enabling flexible and expressive programming styles in JavaScript, showing that functions can be manipulated similarly to objects or other data types.

💡Higher-order functions

Higher-order functions are functions that can take other functions as arguments or return them as results. This is a direct application of JavaScript's first-class function capability. The script uses examples like `addEventListener` to illustrate higher-order functions in action, demonstrating their utility in creating modular, reusable code. Higher-order functions are crucial for patterns like callbacks and function composition in JavaScript programming.

💡Function expression

A function expression in JavaScript is a way to define a function as a part of an expression. It allows the function to be assigned to a variable, stored in an object, or passed as an argument. The script discusses function expressions as a manifestation of JavaScript's treatment of functions as values, showcasing how they enable the dynamic and flexible assignment and handling of functions.

💡Callback function

A callback function is a function that is passed into another function as an argument and is expected to be executed at a later time. The script explains this in the context of `addEventListener`, where the passed function (`greet` in the example) is called back once an event occurs. This illustrates the use of callback functions in event handling and asynchronous programming in JavaScript.

💡Object methods

In JavaScript, methods are functions associated with an object and typically perform operations using the object's properties. The script touches upon this by mentioning that functions, being objects, can have methods too. This concept underlines the object-oriented nature of JavaScript, showing how behavior (methods) can be encapsulated along with state (properties).

💡Function methods

Function methods are specialized methods that can be called on functions in JavaScript. These include built-in methods like `bind`. The script introduces function methods to illustrate the advanced capabilities and flexibility provided by treating functions as first-class objects, enabling powerful function manipulation and configuration techniques.

💡Bind method

The `bind` method in JavaScript creates a new function that, when called, has its `this` keyword set to a provided value. This is mentioned in the script as an example of a function method, demonstrating how JavaScript's function-centric design allows for sophisticated control over function execution context.

💡Event listeners

Event listeners in JavaScript are mechanisms that respond to events in the system, such as user interactions. The script uses `addEventListener` as an example to explain how JavaScript utilizes higher-order functions to handle event-driven programming, enabling functions to react dynamically to user actions or other events.

💡Values

In the context of the script, 'values' refers to the way JavaScript treats functions as data that can be assigned to variables, passed around, and manipulated. This concept is central to understanding the flexibility and expressive power of JavaScript, illustrating how the language allows functions to be used in diverse contexts like any other value (e.g., strings or numbers).

💡Return a function

Returning a function from another function is a powerful concept in JavaScript, facilitated by its first-class function nature. The script mentions this to introduce the idea that functions can generate other functions, which can be used to create complex behaviors and patterns, such as closures, by dynamically producing functions based on certain conditions or inputs.

Highlights

JavaScript has first class functions, meaning functions are treated as values.

Functions being first-class citizens allows for higher order functions.

Functions in JavaScript are essentially objects.

Functions can be stored in variables or object properties.

Functions can be passed as arguments to other functions.

Functions can return another function, enhancing modularity and reusability.

JavaScript supports function methods, like the bind method.

Higher order functions either receive a function as an argument or return a function.

The addEventListener function is an example of a higher order function.

Callback functions are passed into higher order functions to be called later.

Higher order functions can significantly enhance JavaScript's flexibility.

First-class functions and higher order functions are distinct concepts.

First-class functions is a feature of a programming language, while higher order functions are a practical application.

Understanding the difference between first-class and higher order functions is key for JavaScript mastery.

The next lectures will focus on creating custom higher order functions.

Transcripts

Let's now talk about

a fundamental property of the JavaScript language.

Which is the fact that it has first class functions.

This enables us to write higher order functions.

But what's that all about?

Well, let's see.

So, JavaScript is a language that has first class functions

which in technical terms

means that functions are so-called first citizens.

In practice, that means that functions

are simply treated as values.

And we already touched on that idea

a couple of times before,

but this is such an important feature of the language

that it's worth spending some more time talking about this.

Now, why does JavaScript work this way?

Well, it's simply because functions

are really just another type of objects in JavaScript.

And since objects are values,

functions are values too.

And since functions are values,

there is a bunch of interesting things

that we can do with them,

like storing them in variables or object properties.

And that, of course,

we already did a couple of times before.

So the function values here are marked in red,

and then you see, we create a function expression

in the first example and an object method

in the second example.

So the value in the red rectangle

is the function value itself,

that we can store wherever we like.

We can also pass functions as arguments to other functions.

And we actually already did that before

when adding event listeners or event handlers

to dumb objects.

So here we have the add event listener function

that you already know,

and we clearly passed the greet function

into the add event list note function

here as a value, right?

Now to make it even more interesting,

we can also return a function from another function.

That sounds kind of crazy,

but it can be very useful.

Finally, remember that functions are objects.

And many types of objects in JavaScript have methods, right?

Like array methods, for example.

And actually there are also function methods.

So methods that we can call on functions.

So again, that sounds a bit crazy, right?

But we will see how to use this to our advantage

throughout this section.

This bind method here is an example of that.

And again, we will learn about the bind method

as we go through the section.

All right.

Now the fact that JavaScript has first-class functions

makes it possible for us to use and write

higher order functions.

So a higher order function

is either a function that receives another function

as an argument,

or a function that returns a new function.

So let's check out both types here.

First, for functions that receive another function.

we have the same example as before.

So here, the add event listener function

is the higher order function.

And why?

Well, because it receives another function as an input.

In this case, the greet function.

And we usually say that the function that is passed in

is a callback function.

That's because the callback function will be called later

by the higher order function.

In this case,

add event listener will call the greet callback later

as soon as the click event happens.

It's like the greet function saying,

"Hey there, don't greet me yet,

but call me back once you're ready."

And this works, not only in the context

of the add event listener function,

but in many other use cases as well.

Okay, second,

we can also have functions that return another function.

So we have the higher order function here.

So basically this whole code block,

which clearly returns a new function,

which is this one.

And this style of functions

is also used a lot in JavaScript.

But it's also more advanced,

and I guess harder to understand.

I will show it to you in the next lecture,

but we will explore this deeper a bit later.

Now, just to finish,

there seems to be some confusion between

first-class functions and higher order functions.

Some people think that they are the same thing.

But actually they mean different things.

So, first class functions is just a feature

that a programming language either has or does not have.

All it means is that all functions are values.

That's it.

There are no first class functions in practice, okay?

It's just a concept.

There are however higher order functions in practice,

which are possible because the language supports

first class functions.

So it's a subtle difference,

but still worth noting if you want to be able to talk

like a true JavaScript master.

Great.

Now in the next lecture,

let's actually create or own higher order functions.