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JavaScript Array Methods

JavaScript Array Methods
Hostman Team
Technical writer
JavaScript
27.11.2023
Reading time: 12 min

Arrays in JavaScript are a data structure that stores various elements, often of the same type. Their main feature is storing multiple values in a single variable. Thanks to their integrated methods and properties, you can add, delete, process, or search the data based on your needs. 

All array values are elements. In turn, they have their own index. Variables can be either of the same or different data types, allowing you to organize more complex data structures.

Creating an array

There are two ways to initialize an array:

1. Using a literal syntax.

In this case, elements are listed inside square brackets and separated by commas. 

const ExampleArr = [element0, element1, …, elementN];

2. using the Array() constructor.

Syntax:

const ExampleArr = new Array(element0, element1[, …[, elementN]]);
const ExampleArr = new Array(arrayLenght);

In both methods, we pass elementN as parameters. These are the variables that create the set of ExampleArr values. ArrayLength is the number of elements. Their default value is not defined. 

After creating the array, let’s fill it in. Let's create a set of values that stores employees' names.

Using the first method:

var EmployeeArr = [‘Alex’, ‘Bob’, ‘Oliver’];

Using the second one:

var EmployeeArr = new Array(‘Alex’, ‘Bob’, ‘Oliver’);

Access to array elements 

To access an array element, we use square brackets with an integer value of its index inside them.

Let's get the value of two elements of the previously created value set EmployeeArr:

var EmployeeArr = new Array(‘Alex’, ‘Bob’, ‘Oliver’);
console.log (EmployeeArr[1]); 

The index numbering starts from 0, which we should remember when referring to the N-th element. Its index will be N-1. In our case, it will equal 1.

To change the value of an element, you need to assign a new value to the variable, as shown below:

var EmployeeArr = new Array(‘Alex’, ‘Bob’, ‘Oliver’);
EmployeeArr[1] = ‘Jack’;

Now EmployeeArr contains 'Alex', 'Jack', and 'Oliver.'

Now let's look at length, an important property that returns the length of an array in JavaScript.

var EmployeeArr = new Array(‘Alex’, ‘Bob’, ‘Oliver’);
console.log (EmployeeArr.length); 

The length of EmployeArr in our case will be 3.

-

Array methods

Array methods in JavaScript allow developers to work with data more efficiently and conveniently. Their use will help with conversion, sorting, searching, adding or removing elements. 

In this article, we'll look at most of the existing JavaScript array methods, grouped by their work logic.

Adding and removing elements

There are four main methods to add or remove elements from an array: push(), pop(), shift(), and unshift().

  • push() adds one or more elements to the end of the value set.

  • pop() deletes the last element.

  • shift() deletes the first element.

  • unshift() adds one or more elements to the beginning.

Let's look at examples. We'll indicate the output of each method in the comment.

var arr = new Array(‘Alex’, ‘Bob’, ‘Oliver’);
arr.push (‘Jack’); // ‘Alex’, ‘Bob’, ‘Oliver’, ‘Jack’
arr.unshift (‘Jacob’, ‘George’); //; ‘Jacob’, ‘George’, ‘Alex’, ‘Bob’, ‘Oliver’, ‘Jack’
arr.pop (); // ‘Jacob’, ‘George’, ‘Alex’, ‘Bob’, ‘Oliver’
arr.shift (); // ‘George’, ‘Alex’, ‘Bob’, ‘Oliver’

The pop() and shift() methods return the value of the deleted element, while push() and unshift() return the length of the modified array.

In addition to the above methods, we should mention the universal splice() method. It allows you to insert, delete, or replace elements.

Syntax of the splice() method:

array.splice(startIndex, deleteCount, element1, element2, ...);

Parameters: 

  • startIndex is the index where the splice should begin. If this parameter exceeds the array length, the startIndex value will equal the length or take the difference between array.length and this number if a negative number is entered. 

  • deleteCount is the number of elements to delete starting from startIndex. It can be equal to 0. In this case, nothing happens. 

  • element1, element2, .... are the elements we add to the array (optional parameter).

The splice() method returns the set of deleted values.

In the example, let's look at several ways to apply splice(). The comments will specify the set of values after the method is executed.

Below, we will remove two elements, starting with 3:

var Dog = new Array(‘Beagle’, ‘Boxer’, ‘Bulldog’, ‘Corgi’, ‘Dachshund’, ‘Dalmatian’);
Dog.splice ( 2, 2 ); // ‘Beagle’, ‘Boxer’, ‘Dachshund’, ‘Dalmatian’

Now let's remove one element starting at 4 and add 'Doberman':

var Dog = new Array(‘Beagle’, ‘Boxer’, ‘Bulldog’, ‘Corgi’, ‘Dachshund’, ‘Dalmatian’);
Dog.splice (3, 1, ‘Doberman’); // ‘Beagle’, ‘Boxer’, ‘Bulldog’, ‘Doberman’, ‘Dachshund’, ‘Dalmatian’

Iterating through an array

The forEach() method is responsible for iterating array elements.

arr.forEach(function callback(currentValue, index, array), thisValue);

Parameters:

  • callback is the main parameter, namely the callback function that will be executed once for each element. Its arguments are currentValue, index, array.

  • currentValue is the element to be processed in the set.

  • index is this element's index.

  • array is the array of the selected element.

  • thisValue is an optional parameter. It takes the value used as this when calling the function.

The returned value is always undefined.

Here are two examples: one using thisValue and one without it.

The example without thisValue parameter:

var Dog = new Array(‘Beagle’, ‘Boxer’, ‘Bulldog’, ‘Corgi’, ‘Dachshund’, ‘Dalmatian’);
var NewDog = [ ];
Dog.forEach (function (element) {
NewDog.push (element);
})

The result will be a new set of NewDog values that completely replicates Dog.

Example with the thisValue parameter:

var numbers = [1, 2, 3, 4, 5];
var doubleNumbers = [ ];
var myObject = {
    double: function(element) {
        return element * 2;
    }
};

numbers.forEach (
    function (element) {
        doubleNumbers.push(this.double(element)); 
    }, myObject
);

The result is a doubleNumbers set, which consists of numbers multiplied by two.

Searching for an element in an array

Such methods as indexOf(), lastIndexOf(), includes(), find(), findIndex(), and filter() help to search for items. 

The indexOf() and lastIndexOf() methods search for the required value among all the elements and return its index at the first match. The former searches from the beginning of the array, while the latter searches from the end. Both methods will return -1 if the searched value is not found.

Syntax of indexOf() and lastIndexOf() methods:

array.indexOf( searchElement, fromIndex )
array.lastIndexOf( searchElement, fromIndex )

The searchElement parameter is the same in both cases. It is the element we need to find. This is not the case with the fromIndex parameter. This is the starting index of the element to search for. When indexOf() is used, the fromIndex parameter defaults to zero. In the lastIndexOf() method, it is equal to array.length

Let's try using these methods in the example below. 

var Employee = new Array(‘Jacob’, ‘George’, ‘Alex’, ‘George’, ‘Oliver’, ‘Jack’);
Employe.indexOf (‘George’);
Employe.lastIndexOf (‘George’);

The result of the indexOf() method will be 1 because the first match occurred with an element whose index is 1. In the case of lastIndexOf(), the result will be 3.

The next search method is includes(). It determines whether the set of values contains a certain element, returning true or false depending on the result.

The syntax of the includes() method:

array.includes(searchElement, fromIndex)

The parameters are exactly the same as in the previously discussed indexOf() and lastIndexOf() methods. The only difference between them is the return value. The fromIndex parameter is 0 by default.

Here's an example of using the includes() method:

var Employe = new Array(‘Jacob’, ‘George’, ‘Alex’, ‘Bob’, ‘Oliver’, ‘Jack’);
Employe.includes (‘Alex’);
Employe.includes (‘Jacob’, 3);

The result in the first case will be true. In the second case, false.

In practice, JavaScript often uses arrays of objects. In this case, you should search using the find() and findIndex() methods.

arr.find(function callback(currentValue, index, array), thisValue); 
arr.findIndex(function callback(currentValue, index, array), thisValue); 

Parameters of find() and findIndex() methods are similar to forEach(), so you can find their descriptions above.

Let's look at an example of using the find() and findIndex() methods:

var Employee = new Array(‘Jacob’, ‘George’, ‘Alex’, ‘Bob’, ‘Oliver’, ‘Jack’);
Employee.find (element => element == ‘Jacob’);
Employee.findIndex (element => element == ‘Jacob’);

The return value of the find() method is 'Jacob', and of the findIndex() method is 0.

The find() and findIndex() methods are suitable for searching one object. If the task is to find several objects, we can use another method: filter()

Syntax of the filter() method:

var filteredArray = arr.filter(function(currentValue, index, array), thisValue); 

The parameters of this method are also similar to the parameters of the forEach() method.

Let's try it. Here we will search for employees who have worked in the company for less than two years:

var Employee = [  
{name: “Jacob”, experience: 3},
{name: “George”, experience: 1},
{name: “Alex”, experience: 1},
{name: “Bob”, experience: 4}
];

var result = Employee.filter(element => element.experience <2);

The method will result in a new array consisting of two employees with less than two years of experience.

Methods for converting arrays

The last group of array methods in JavaScript that we will describe in this article are array transformation methods. These include map(), flat(), flatmap(), sort(), and reverse(). Let's look at each of them in more detail below.

map() organizes a new set of values with the result of calling the specified callback function for each element.

The syntax of the map() method:

var newArray = array.map(function(currentValue, index, arr), thisValue); 

Once again, we see the same parameters as in the previously discussed methods.

The return value of this method will be a new array with the elements of the callback function result.

Let's consider the example that we already used earlier with forEach(), where all the set data were doubled. Now, let's use map() instead of forEach().

var numbers = [1, 2, 3, 4, 5];
var doubleNumbers = numbers.map( 
    function double( element ) {
        return element * 2;
    }
);
console.log(doubleNumbers);

The result is doubleNumbers, a new set of values consisting of numbers 2, 4, 6, 8, 10. 

When we use the map() method, the number of lines of code is noticeably reduced compared to forEach(). Also, the forEach() method is used only for iteration and returns nothing. map() returns a new array of the same length as the original array.

Flat() allows you to interact with a nested set of values. It returns a new array with all sub-array values merged into it recursively  up to the specified level.

The syntax of the flat() method:

var newArray = arr.flat(depth);

Here, depth specifies the level, i.e. how deep a nested array structure should be flattened. If the nesting depth is unknown, you can set the value of the parameter as Infinity. By default, it is equal to one.

Let's consider an example of using the flat() method:

var arr = [1, 2, , [4, 5, [6, 7]]];
var NewArr = arr.flat(Infinity);

The result is NewArr [1, 2, 4, 5, 6, 7]. It is worth noting that one was missing in the set of arr values. The flat() method removed the empty slots.

We can combine the previous two methods into one: flatmap()

The syntax of the flatmap() method:

var new_array = arr.flatMap(function callback(currentValue, index, arr), thisValue)

Here is an example of using flatmap():

var arr = [[1], [2], [3], [4], [5]];
var NewArr = arr.flatMap(element => element * 2);

The result will be NewArr [2, 4, 6, 8, 10].

The sort() method allows sorting the data set. The default sorting order corresponds to the order of Unicode code characters.

Syntax of the sort() method:

array.sort(function(firstValue, secondValue)); 

You can specify a function that determines the sort order as a parameter.

Example:

var EmployeeArr = new Array(‘Jacob’, ‘George’, ‘Alex’, ‘Bob’, ‘Oliver’, ‘Jack’);
EmployeeArr.sort ();

The result of sorting will be EmployeArr ['Alex', 'Bob', 'George', 'Jack', 'Jacob', 'Oliver'].

The reverse() method is also worth mentioning. 

Syntax of the reverse() method:

array.reverse();

In the example, let's change the order of employees’ names in the previously sorted array.

var EmployeeArr = new Array(‘Jacob’, ‘George’, ‘Alex’, ‘Bob’, ‘Oliver’, ‘Jack’);
EmployeeArr.sort ();
var reversed = EmployeeArr.reverse();

The result of the reverse() method is EmployeArr ['Oliver', 'Jacob', 'Jack', 'George', 'Bob', 'Alex'].

Conclusion

This article has covered the basic, but not all, array methods in JavaScript. It is not easy to memorize them all at once. But it will become easier with experience in solving problems and using the methods in code. The main thing is to memorize how they work and what they are used for, and if necessary, you can always refer to this article to remember the syntax or see examples.

JavaScript
27.11.2023
Reading time: 12 min

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Handling Edge Cases with map() While the map() function is powerful, handling edge cases is important to prevent unexpected behavior. Let's explore some common scenarios and how to manage them. Dealing with Empty Arrays If you apply map() to an empty array, it simply returns another empty array. There's no need to add extra checks for this case. let emptyArray = [];let result = emptyArray.map(item => item * 2);console.log(result); Output: [ ] Handling Undefined or Null Elements When your array contains undefined or null values, map() will pass them to your callback function. You need to ensure your function can handle these values without throwing errors. let values = [1, null, 3, undefined, 5]; let processedValues = values.map(value => { if (value === null || value === undefined) { return 0; // Assign a default value } return value * 2; }); console.log(processedValues); Output: [ 2, 0, 6, 0, 10 ] In this example, we check for null or undefined and assign a default value of 0 before performing the calculation. Avoiding Holes in Arrays Arrays can have holes if elements are deleted or uninitialized. The map() function skips these holes, which might lead to unexpected results. let sparseArray = [1, , 3, , 5]; // Note the missing elementslet doubled = sparseArray.map(number => number * 2);console.log(doubled); Output: [ 2, <1 empty item>, 6, <1 empty item>, 10 ] To handle this, you can use the Array.from() method to create an array without holes before mapping. let sparseArray = [1, , 3, , 5]; // Note the missing elements let doubled = sparseArray.map(number => number * 2); let denseArray = Array.from(sparseArray); let doubledDense = denseArray.map(number => number * 2); console.log(doubledDense); Output: [ 2, NaN, 6, NaN, 10 ] Now, the missing elements are converted to undefined, and you can handle them accordingly. Managing Asynchronous Operations The map() function doesn't support asynchronous operations natively. To perform asynchronous tasks, consider using Promise.all() with map(). let urls = ['url1', 'url2', 'url3']; let fetchPromises = urls.map(url => fetch(url)); Promise.all(fetchPromises) .then(responses => { // Handle responses }) .catch(error => { // Handle errors }); Here, map() creates an array of promises, and Promise.all() waits for all of them to resolve. Using map() with Callback Functions The map() method relies on a callback function to process each element in an array. Let's explore different ways to use map() with callback functions. Using Named Functions You can pass a named function to map() for better readability, especially when the transformation logic is complex. function doubleNumber(number) { return number * 2; } let numbers = [1, 2, 3, 4, 5]; let doubledNumbers = numbers.map(doubleNumber); console.log(doubledNumbers); Output: [ 2, 4, 6, 8, 10 ] In this example, the doubleNumber function is passed as the callback to map(). Each element in the numbers array is doubled, resulting in a new array doubledNumbers. Using Anonymous Functions Anonymous functions can be used directly within map() for simple transformations. let numbers = [1, 2, 3, 4, 5]; let squaredNumbers = numbers.map(function(number) { return number * number; }); console.log(squaredNumbers); Output: [ 1, 4, 9, 16, 25 ] Here, an anonymous function computes the square of each number in the array. Using Arrow Functions Arrow functions offer a concise syntax and are commonly used with map(). let numbers = [1, 2, 3, 4, 5]; let tripledNumbers = numbers.map(number => number * 3); console.log(tripledNumbers); Output: [ 3, 6, 9, 12, 15 ] The arrow function number => number * 3 triples each number in the array. Using Callback Functions with Multiple Parameters The callback function can accept additional parameters like index and array, providing more context during the mapping process. let numbers = [10, 20, 30, 40, 50]; let adjustedNumbers = numbers.map((number, index) => number - index * 2); console.log(adjustedNumbers); Output: [ 10, 18, 26, 34, 42 ] In this example, each number is adjusted based on its index in the array. Using thisArg with map() The map() method accepts an optional second argument called thisArg, which sets the value of this inside the callback function. let multiplier = { factor: 2 }; let numbers = [1, 2, 3]; let scaledNumbers = numbers.map(function(number) { return number * this.factor; }, multiplier); console.log(scaledNumbers); Output: [ 2, 4, 6 ] By passing multiplier as the thisArg, the callback function can access this.factor to scale each number. Performance Considerations for map() When using map(), it's important to consider performance. Since map() creates a new array, it consumes additional memory. Therefore, with very large arrays, this extra memory usage can affect your application's efficiency. Furthermore, while map() offers clean and readable code, traditional loops like for or while loops might execute faster in performance-critical situations. If speed is crucial, consider using a loop instead of map(). Moreover, keep your callback functions simple. Complex operations inside the map() callback can slow down processing. To maintain optimal performance, aim for straightforward transformations. Finally, use map() only when you need the new array it returns. If you're performing actions without needing the resulting array, methods like .forEach() are more appropriate. This avoids unnecessary memory allocation and enhances performance. Conclusion The JavaScript map() function is a powerful tool for array manipulation. By applying a function to each element, it creates a new array with transformed values, making tasks like data conversion and extraction straightforward. Understanding how to use map() effectively allows you to write cleaner, more efficient code.
16 October 2024 · 12 min to read
JavaScript

How to Use the Date Object in JavaScript

JavaScript provides a built-in Date object that simplifies working with dates and times. This tutorial will guide developers through the essential aspects of the Date object, including its creation, retrieval of date and time information, formatting, manipulation, and handling of time zones. Introduction to JavaScript Date Object The Date object in JavaScript represents datetime values, enabling developers to manage temporal data with ease. It is essential for tasks such as scheduling, time tracking, and logging. The Date object helps: Represent specific date and time values, such as "2022-07-25T14:30:00.000Z". Perform operations like retrieving, formatting, and manipulating dates and times. Simplify date and time calculations across different time zones. Understanding the basics of the Date object will help in managing time-sensitive data efficiently in web applications. Creating a Date Object Creating a Date object in JavaScript is straightforward. 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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