The Promise `catch()` Function In JavaScript

The Promise `catch()` Function in JavaScript

In this post, we will give you information about The Promise `catch()` Function in JavaScript. Here we will give you detail about The Promise `catch()` Function in JavaScript And how to use it also give you a demo for it if it is necessary.

Promises in JavaScript are an object representation of an asynchronous operation. Promises are like a placeholder for some value that may not have been computed yet.
If the async operation failed, JavaScript will reject the promise. The catch() function tells JavaScript what function to call if the promise is rejected:

const p = Promise.reject(new Error('Oops!'));

p.catch(err => {
  err.message; // 'Oops!'
});

With Promise Chaining

The major benefit of .catch() is that you can catch errors that
occurred anywhere in a promise chain.

const p = Promise.resolve('Na');

return p.
  then(str => str + ' Na').
  then(str => str + ' Na').
  then(str => str + ' Na').
  then(() => { throw new Error('Batman!') }).
  then(() => console.log('Will not print')).
  catch(err => {
    err.message; // 'Batman!'
  });

This means you only need one catch() at the end of a promise chain
to handle any errors that occur in the promise chain!

Rethrow Errors for The Promise `catch()` Function in JavaScript

You can rethrow errors in .catch(), similar to rethrowing with try/catch.

const p = Promise.reject(new Error('Oops!'));

return p.
  catch(err => { throw err; }). // Rethrow the error
  catch(err => {
    err.message; // 'Oops!'
  });

This also means you should be very careful about uncaught errors in your .catch() error handlers. If the function you pass to .catch() throws and you don’t have another .catch() afterward, you’ll get an unhandled promise rejection.

Unwrapping Errors for The Promise `catch()` Function in JavaScript

If your .catch() returns a value, you can “unwrap” that value with
await or then().

const p = Promise.reject(new Error('Oops!'));

const answer = await p.catch(() => 42);
answer; // 42

return p.
  catch(() => 42).
  then(answer => {
    answer; // 42
  });

In other words, you can handle async errors in JavaScript using Golang-like syntax.

const p = Promise.reject(new Error('Oops!'));

const err = await p.catch(err => err);
err.message; // 'Oops!'

Versus `then()`

The catch() function is just a thin layer of syntactic sugar on
top of the Promise then() function.
Recall that then() takes 2 parameters:

onFulfilled(): JavaScript will call this function if the underlying async operation succeeded.
onRejected(): JavaScript will call this function if the underlying async operation failed.

So .catch(fn) is the same thing as .then(null, fn). In other words, below is a one-line polyfill for catch():

Promise.prototype.catch = function(onRejected) {
  return this.then(null, onRejected);
};

That means you can handle promise errors with .then() as well:

const p = Promise.reject(new Error('Oops!'));

return p.then(() => {}, function onRejected(err) {
  err.message; // 'Oops!'
});

JavaScript Fundamentals for The Promise `catch()` Function in JavaScript

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 var keyword.
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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