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Arrow Functions in JavaScript

Arrow Functions in JavaScript
Hostman Team
Technical writer
JavaScript
24.11.2023
Reading time: 7 min

With the release of the ES6 standard in 2015, programmers have gotten several new features and functions in JavaScript. The new standard added arrow functions, ways to declare let and const variables, promises, and many other fundamental changes to the language. Today, we will focus on arrow functions.

Arrow functions in JavaScript are a new method of declaring functional expressions that is much more compact and convenient than its predecessor. If you are familiar with lambda functions in Python, you will definitely find similarities between them.

In this article, we'll look at arrow functions and their syntax, give usage examples, and learn the ins and outs of using them in code.

-

Functions

Before we study arrow functions, we should revise the traditional way of declaring functions and function expressions. This chapter will help users to remember what functions and function expressions are and to apprehend how their declaration differs from the arrow functions.

A function is a set of commands written to accomplish a specific task when called. It can be called from any part of the code as many times as you like.

The syntax of a standard function declaration is as follows:

function function_name(set_of_parameters) {
 set_of_commands;

A function declaration is always read first, meaning it can be called before it is declared. This process is called hoisting.

An example of an ordinary function:

console.log(multiplication(2,5));

function multiplication (x, y) {
 return x * y;
}

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A function definition expression is another method of declaring a function by assigning it to a variable or constant. It can then be passed as an argument to another function or used as a return value.

The syntax for declaring a function expression is as follows:

var variable_name = function function_name (parameters) {
 commands;
}

The function_name parameter is optional. In its absence, the function will be considered anonymous.

Unlike regular functions, a functional expression can only be called after it has been declared in the code.

An example of a functional expression:

const multiplication = function (x, y) {
 return x * y;
}

console.log(multiplication(2,5));

Image5

The basics of arrow functions

Arrow functions (arrow function expressions) are the same as function expressions but anonymous and have some syntax peculiarities. Let's take a closer look at those peculiarities.

  • Concise and clear syntax

The syntax of arrow functions in JavaScript is intuitive and concise. Its basic version is as follows:

(set_of_parameters) => {
 set_of_commands;
}

As we can see, the main difference between arrow functions and normal functions is the absence of the function keyword and the addition of => characters after the argument list in brackets.

For example:

const multiplication = (x, y) => x * y;
console.log(multiplication(2,5));

Here, we did not use curly braces because the function's body contains only one operation. Also, we did not specify an explicit return of the result, as it will happen automatically.

When we execute this code, the result is as follows.

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  • The this keyword

Arrow functions operate only in the scope in which they are declared, and do not have their own functional execution context. This implies that entities such as this or argument are inherited exclusively from parent functions.

For a more detailed understanding, here is an example where we will use both a regular function and an arrow function:

function Animal() {
    this.group = 'Mammals',
    this.name = 'Elephant',
    this.Animal1 = function () {
        console.log('Operation of a normal function:')
        console.log(this.group);

        function exampleFunction1() {
            console.log(this.name);
        }
        exampleFunction1();
    }
    this.Animal2 = function () {
        console.log('Operation of an arrow function:')
        console.log(this.group);

        let exampleFunction2 = () => console.log(this.name);
        exampleFunction2();
    }
}

let x = new Animal();
x.Animal1();
let y = new Animal();
y.Animal2();

In this example, this.group is available both inside this.Animal1 and inside this.Animal2, since these are two methods of the same object. However, inside these methods, in the first case this.name returns an undefined value, and the output is empty since this function has its own context (in this case, window). In the second case, however, it references the parent scope and returns this.name = 'Elephant'

When we execute the code, we receive the following output:

Operation of a normal function:
Mammals

Operation of an arrow function:
Mammals
Elephant
  • Constructors

Arrow functions cannot be used as constructors, nor can they have their own properties and methods because they lack the prototype property the normal functions have. When calling them with the new operator, the system will generate an error.

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Also, since the arrow is followed by curly braces denoting a code block with commands, the user will not be able to create an object inside the function or return an object literal from it in the usual way. For example, the following code fragment will generate an error:

(firstName, lastName) => {firstName: firstName, lastName: lastName};

The system considers it the function's body; that's why an error occurs in this code fragment. To fix this, we need to enclose the object literal in parentheses:

(firstName, lastName) => ({firstName: firstName, lastName: lastName});

Examples of use

Arrow functions are acceptable anywhere in a program but are especially useful in callback functions that take other functions as parameters. In the following example, we use it as a callback for the map method:

const example_numbers = [1, 2, 3, 4, 5];
const doubling = example_numbers.map(number => number * 2);
console.log(doubling);

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Like with curly braces, we can omit parentheses if only one parameter is specified, as in the example above.

Another example of arrow functions is using them in the reduce method. In the following example, we use an arrow function to add up all the elements of an array:

const example_numbers = [1, 2, 3, 4, 5];
const sum = example_numbers.reduce((total, number) => total + number, 0);
console.log(sum);

As a result, we get the following:

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When to avoid using arrow functions

There are cases where regular functions are preferable to arrow functions. 

Mostly, they are related to the this keyword behavior and its peculiarities, because of which you shouldn't use arrow functions in object methods.

Let’s look at this example:

let animal = {
    group: 'Amphibians',
    name: 'Lake frog',
    infoAnimal: () => {        
console.log('Animal group -',this.group);
console.log('Animal name -',this.name);
    },
}

animal.infoAnimal(); 

Here, we have the infoAnimal method, which outputs information about the object when we call it with the animal.infoAnimal(). In this case, its lexical environment is window. Hence, it has nothing to do with the group and name object properties. 

This is the result we get:

Animal group - undefined 
Animal name - undefined

Now let's rewrite the code using a normal function and look at the result:

let animal = {
    group: 'Amphibians',
    name: 'Lake frog',
    infoAnimal: function () {        
            console.log('Animal group -',this.group);
    console.log('Animal name -',this.name);
    },
}

animal.infoAnimal();

We can see that this method works successfully:

Animal group - Amphibians 
Animal name - Lake frog

In addition to object methods, you shouldn't use the arrow functions for functions with a dynamic context, because they bind the context statically. For example, this applies to working with event handlers or event listeners.

What to remember

  • Arrow functions (arrow function expressions) are the same as function expressions, but anonymous and with some syntax peculiarities.

  • Their basic syntax is as follows:

(set_parameters) => {
 set_commands;
}
  • When using the arrow functions, you can remove parentheses around a parameter if it is a single parameter. If the function implements only one operation, you can also use implicit return, which means deleting curly brackets and the return keyword.

  • Arrow functions work only in the scope in which they were declared, and do not have their own functional execution context. So, entities such as this or argument are inherited exclusively from parent functions.

  • When working with object methods, dynamic context or constructors, you should use regular functions, not arrow functions.

JavaScript
24.11.2023
Reading time: 7 min

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Here are the two common ways to achieve it:  Using the current time Using a timestamp Creating a Date Object for the Current Time The simplest way to create a Date object is by instantiating the current date and time: const currentDate = new Date();console.log(currentDate); This creates an object representing the current date and time in the user's local timezone. Creating a Date Object Using a Timestamp You can also create a Date object from a specific timestamp (milliseconds since January 1, 1970): const timestamp = 1643723400000; const dateObject = new Date(timestamp); console.log(dateObject); This is useful for manipulating dates stored in timestamp format. Now that we know how to create a date object, let’s see how to retrieve date and time information. Retrieving JS Date and Time Information The Date object provides methods for extracting various components of a date, such as a year, month, and hour. Key methods include getDate(): Returns the day of the month. getFullYear(): Returns the four-digit year. getMonth(): Returns the month (0-11, where 0 represents January). getHours(): Returns the hour (0-23). getMinutes(): Returns the minutes (0-59). getSeconds(): Returns the seconds (0-59). For example, to retrieve the current date and format it as MM/DD/YYYY HH:MM:ss: function formatDate(dateObject) { const year = dateObject.getFullYear(); const month = dateObject.getMonth() + 1; // Months are zero-indexed const day = dateObject.getDate(); const hours = dateObject.getHours(); const minutes = dateObject.getMinutes(); const seconds = dateObject.getSeconds(); return `${month}/${day}/${year} ${hours}:${minutes}:${seconds}`; } console.log(formatDate(new Date())); Here is the result when executing the function.  There are however interesting methods we can use to format datetime into readable formats.  Formatting Date and Time JavaScript provides methods to format datetime values into human-readable strings. This allows developers to convert a Date object to a string or vice versa. Formatting a Date as a String To output a Date object as a human-readable string, use methods like toDateString() and toTimeString(): const currentDate = new Date(); const dateString = currentDate.toDateString(); console.log(dateString); const timeString = currentDate.toTimeString(); console.log(timeString); Here is the output: Converting Strings to Date Objects Developers can also convert readable strings into Date objects using the Date constructor: const dateFromString = new Date("October 10, 2024");console.log(dateFromString); However, it’s better to use recommended formats.  Recommended Formats To avoid errors when working with date strings, it is advisable to use reliable formats: ISO 8601 Format (Recommended): The safest and most reliable format is the ISO 8601 date format: YYYY-MM-DDTHH:mm:ss.sssZ. If only the date part is provided, it assumes the time as midnight 00:00:00. const date = new Date("2024-10-10T14:48:00Z");console.log(date); RFC2822 Format: Another accepted format is the RFC2822 format commonly used in email headers: Day, DD Mon YYYY HH:mm:ss GMT. const date = new Date("Wed, 10 Oct 2024 14:48:00 GMT");console.log(date); We now know how to format datetime values using the Date object. Let’s see how to manipulate date values for simple scheduling and calculations. Manipulating Date Values Date manipulation is essential for tasks like scheduling and calculating deadlines. JavaScript provides setter methods for modifying specific components of a Date object. Modifying Date Components Developers can modify specific components of a Date object using setter methods. Note that months are zero-indexed: let date = new Date(); date.setFullYear(2025); date.setMonth(5); // Set month to June date.setDate(15); // Set day to 15th date.setHours(10); // Set hour to 10 AM date.setMinutes(30); // Set minutes to 30 date.setSeconds(45); // Set seconds to 45 console.log(date); Adding or Subtracting Days Developers can easily add or subtract days using setDate(): let date = new Date(); date.setDate(date.getDate() + 5); // Add 5 days console.log(date); Date arithmetic can be accomplished using timestamps (milliseconds since January 1, 1970): let now = new Date(); let oneDayInMs = 24 * 60 * 60 * 1000; let tomorrow = new Date(now.getTime() + oneDayInMs); console.log(tomorrow); Comparing Date Objects Date objects can be compared using their timestamps: let date1 = new Date('2024-10-10'); let date2 = new Date('2024-12-25'); console.log(date1 > date2); // false (October 10 is earlier than December 25) console.log(date1 < date2); // true console.log(date1.getTime() === date2.getTime()); // false Now that we now how to manipulate dates values for calculation, let’s see how we can handle dates with time zones. Working with Time Zones The Date object is timezone-agnostic, meaning it doesn't have a built-in concept of time zones. However, JavaScript’s Date object handles dates in local time (system time zone) and UTC. When creating Date objects, it is essential to be aware of time zone conversions, especially when performing operations across different regions. Local Time vs. UTC JavaScript can work with localtime and UTC. Local time allows you to represent the time by the Date object when created without any specific time zone information, reflecting the local time of the environment in which JavaScript is executed. For example, creating a Date object in Paris will reflect the central European time zone. UTC is the time standard not affected by time zones or DayLight Saving Time (DST). Using Coordinated Universal Time (UTC) ensures consistency and avoids ambiguity when working with dates and times across different time zones, simplifying time calculations, logging, and user experience management in applications that serve users in multiple regions. Creating Date Objects in UTC To create a Date object in UTC, use the ISO 8601 format: const utcDate = new Date("2024-10-10T14:30:00Z");console.log(utcDate); Converting Local Time to UTC To retrieve UTC date components, use getUTCDate(), getUTCMonth(), etc.: const localDate = new Date();console.log(localDate.getUTCDate(), localDate.getUTCMonth() + 1, localDate.getUTCFullYear()); Converting UTC to Local Time Similarly, to convert a UTC date to local time, you can use the local equivalent methods: const utcDate = new Date("2024-10-10T14:30:00Z"); // UTC date const localDay = utcDate.getDate(); const localMonth = utcDate.getMonth() + 1; // Months are zero-indexed const localYear = utcDate.getFullYear(); console.log(`Local Date: ${localMonth}/${localDay}/${localYear}`); Being mindful of time zones when working with dates in JavaScript is essential for ensuring accurate datetime representation, especially in applications that require coordination across different regions. Let’s learn more about common Date Object methods. Common Date Object Methods JavaScript provides several static and instance methods that simplify working with dates. Here are some key methods: Date.now(): Returns the current timestamp in milliseconds since January 1, 1970. Date.parse(): Parses a date string and returns the number of milliseconds since the Unix Epoch (January 1, 1970). If the string cannot be parsed, it returns NaN. Date.UTC(): Creates a Date object from UTC values. Date.toString(): Returns a string representation of the Date object in a readable format. valueOf(): Returns the primitive value of the Date object. These methods provide essential functionality for working with dates in JavaScript, enabling developers to efficiently manage and manipulate date values in their applications.  Conclusion The JavaScript Date object is an essential tool for managing datetime in web development. From creating dates to formatting and performing date arithmetic, mastering this object will enable developers to handle time-sensitive data efficiently, regardless of time zone or locale. By using built-in methods and libraries like Moment.js, date-fns, or Day.js, developers can ensure their applications deliver a smooth user experience when working with dates.
15 October 2024 · 8 min to read

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