JavaScript Promise Chaining – onlinecode
In this post, we will give you information about JavaScript Promise Chaining – onlinecode. Here we will give you detail about JavaScript Promise Chaining – onlinecode And how to use it also give you a demo for it if it is necessary.
Promise chaining is what makes promises meaningfully better than callbacks.
The key idea is that a Promise’s then() function returns another promise, so you can chain .then() calls together
to tell JavaScript to execute async calls in order.
const start = Date.now();
return Promise.resolve().
then(() => new Promise(resolve => setTimeout(resolve, 50))).
then(() => new Promise(resolve => setTimeout(resolve, 50))).
then(v => {
console.log(Date.now() - start); // About 100ms passed
});Return Values
The first parameter to the then() function is called onFulfilled(). That’s because JavaScript calls that function when the promise is fulfilled. JavaScript calls the onFulfilled() function with the value the promise was fulfilled with as the first parameter.
Promise chaining works because, if your onFulfilled() function returns a promise q, the promise then() returns will adopt the state of q. So the promise then() returns will have the same fulfilled value as q.
return Promise.resolve(1).
// If 'onFulfilled()' returns a promise, JavaScript calls the
// next 'onFulfilled()' with the fulfilled value of the promise
// your 'onFulfilled()' returned.
then(v => new Promise(resolve => setTimeout(() => resolve(v + 1), 10))).
then(v => new Promise(resolve => setTimeout(() => resolve(v + 1), 10))).
then(v => new Promise(resolve => setTimeout(() => resolve(v + 1), 10))).
// If 'onFulfilled()' returns a value that isn't a promise,
// JavaScript calls the next 'onFulfilled()' with that value.
then(v => v + 1).
then(v => {
console.log(v); // 5
});Error Handling
Promise chaining also consolidates error handling. All you need is one .catch() function call at the end of your promise chain to handle any errors that occur in your promise chain.
Promise.resolve(1).
then(v => v + 1).
// Async error in the middle of the chain goes straight
// to 'catch()'.
then(() => Promise.reject(new Error('Oops'))).
then(v => v + 1).
catch(err => {
err.message; // 'Oops'
});
Promise.resolve(1).
then(v => v + 1).
// Sync error in the middle of the chain goes straight
// to 'catch()' too.
then(() => { throw new Error('Oops'); }).
then(v => v + 1).
catch(err => {
err.message; // 'Oops'
});Summary for JavaScript Promise Chaining
The high level structure of a promise chain is a series of .then() calls, each with an onFulfilled() parameter, and a single .catch()
at the end. JavaScript executes the .then() callbacks in order, or goes straight to .catch() if one of the onFulfilled()
functions errors out.
JavaScript Fundamentals for JavaScript Promise Chaining
JavaScript is a programming language that is used to create interactive web pages. It is a client-side scripting language, which means that it runs on the user’s browser. JavaScript can be used to add animation, interactivity, and functionality to web pages.
Here are some of the fundamentals of JavaScript:
- Variables: Variables are used to store data. They are declared using the
varkeyword. - Data types: JavaScript has a variety of data types, including strings, numbers, booleans, objects, and arrays.
- Operators: Operators are used to perform operations on data.
- Control flow statements: Control flow statements allow you to control the order in which your code is executed.
- Functions: Functions are blocks of code that can be reused.
- Objects: Objects are used to store data in key-value pairs.
- Arrays: Arrays are used to store data in a sequential order.
- Events and event handlers: Events are notifications that are sent by the browser when something happens, such as when the user clicks on an element or moves the mouse over an element. Event handlers are functions that are called in response to an event.
- DOM manipulation: The Document Object Model (DOM) is a tree-like structure that represents the elements of a web page. JavaScript can be used to manipulate the DOM to change the appearance or behavior of a web page.
These are just some of the fundamentals of JavaScript. There are many other concepts that you can learn as you continue to develop your skills.
Here are some resources that you can use to learn more about JavaScript:
- Mozilla Developer Network (MDN): The MDN is a great resource for learning about JavaScript. It has a comprehensive reference guide, tutorials, and articles on a wide range of topics.
- W3Schools: W3Schools is another great resource for learning about JavaScript. It has interactive tutorials and quizzes that can help you learn the basics of the language.
- JavaScript.info: JavaScript.info is a website that provides in-depth tutorials on JavaScript. It also has a forum where you can ask questions and get help from other JavaScript developers.
I hope this helps!
Here are some additional tips for learning JavaScript:
- Start with the basics: Before you start trying to build complex applications, it’s important to learn the basics of JavaScript. This includes things like variables, data types, operators, and control flow statements.
- Practice regularly: The best way to learn JavaScript is to practice regularly. Try to find some time each day to work on JavaScript projects.
- Get help from others: If you get stuck, don’t be afraid to ask for help from others. There are many online forums and communities where you can get help from other JavaScript developers.
With a little practice, you’ll be able to learn JavaScript and start building amazing web applications.
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