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JavaScript Basics

Introduction to JavaScriptJavaScript in HTMLJavaScript SyntaxJavaScript VariablesJavaScript Data TypesJavaScript OperatorsJavaScript CommentsJavaScript OutputJavaScript StatementsJavaScript Keywords

JavaScript Control Structures

JavaScript If-Else StatementsJavaScript Switch StatementsJavaScript For LoopsJavaScript While LoopsJavaScript Break and ContinueJavaScript Try...Catch...Finally

JavaScript Functions

JavaScript Function DeclarationsJavaScript Function ExpressionsJavaScript Arrow FunctionsJavaScript Function ParametersJavaScript Return StatementsJavaScript ScopeJavaScript ClosuresJavaScript Callbacks

JavaScript Objects and Arrays

JavaScript ObjectsJavaScript Object PropertiesJavaScript Object MethodsJavaScript ArraysJavaScript Array MethodsJavaScript Array IterationJavaScript DestructuringJavaScript JSON

JavaScript DOM Manipulation

JavaScript DOM IntroductionJavaScript DOM SelectorsJavaScript DOM TraversingJavaScript DOM ManipulationJavaScript Event ListenersJavaScript Event HandlingJavaScript Forms and Validation

JavaScript Advanced Concepts

JavaScript PrototypesJavaScript ClassesJavaScript InheritanceJavaScript ModulesJavaScript PromisesJavaScript Async/AwaitJavaScript Error HandlingJavaScript Debugging

JavaScript Browser APIs

JavaScript Fetch APIJavaScript Local StorageJavaScript Session StorageJavaScript Geolocation APIJavaScript Web WorkersJavaScript Canvas APIJavaScript History API

JavaScript ES6+ Features

JavaScript let and constJavaScript Template LiteralsJavaScript Spread OperatorJavaScript Rest ParametersJavaScript Default ParametersJavaScript Map and SetJavaScript SymbolJavaScript Iterators and Generators

JavaScript Best Practices

JavaScript Code StyleJavaScript Performance OptimizationJavaScript Security Best PracticesJavaScript TestingJavaScript Design PatternsJavaScript Functional Programming

JavaScript and Web Development

JavaScript AJAXJavaScript XML and JSON ParsingJavaScript Single Page ApplicationsJavaScript and RESTful APIsJavaScript Browser CompatibilityJavaScript and Responsive Design

JavaScript Promises

JavaScript Promises are a powerful feature for handling asynchronous operations. They provide a cleaner and more organized way to work with asynchronous code compared to traditional callback methods.

What are Promises?

A Promise is an object representing the eventual completion or failure of an asynchronous operation. It acts as a placeholder for the result of an operation that hasn't completed yet.

Basic Syntax

Here's the basic structure of a Promise:


const myPromise = new Promise((resolve, reject) => {
  // Asynchronous operation here
  if (/* operation successful */) {
    resolve(result);
  } else {
    reject(error);
  }
});

Using Promises

To use a Promise, you can chain .then() and .catch() methods:


myPromise
  .then(result => {
    console.log('Success:', result);
  })
  .catch(error => {
    console.error('Error:', error);
  });

Promise States

A Promise can be in one of three states:

  • Pending: Initial state, neither fulfilled nor rejected.
  • Fulfilled: The operation completed successfully.
  • Rejected: The operation failed.

Chaining Promises

One of the most powerful features of Promises is the ability to chain them together for sequential asynchronous operations:


fetchUserData(userId)
  .then(userData => fetchUserPosts(userData.id))
  .then(posts => displayPosts(posts))
  .catch(error => console.error('Error:', error));

Promise.all()

Use Promise.all() to handle multiple Promises concurrently:


Promise.all([fetchUser(1), fetchUser(2), fetchUser(3)])
  .then(users => console.log(users))
  .catch(error => console.error('Error:', error));

Best Practices

  • Always handle errors using .catch() or the second argument of .then().
  • Use Promise.resolve() or Promise.reject() to create pre-resolved or pre-rejected Promises.
  • Consider using async/await for more readable asynchronous code.

Related Concepts

To further enhance your understanding of asynchronous JavaScript, explore these related topics:

Mastering Promises is crucial for writing efficient, non-blocking JavaScript code. They form the foundation for modern asynchronous programming patterns in JavaScript.