JavaScript Create Promise

JavaScript Create Promise

JavaScript Create Promise

In this post, we will give you information about JavaScript Create Promise. Here we will give you detail about JavaScript Create Promise And how to use it also give you a demo for it if it is necessary.

In general, there are 4 ways to create a new promise in JavaScript:

  • Using the Promise constructor
  • Using the static helpers Promise.resolve() and Promise.reject()
  • Chaining with the then() function or catch() function
  • Call an async function

Using the Promise Constructor for  JavaScript Create Promise

The Promise constructor takes a single parameter, an executor function. When you call new Promise(executor), JavaScript immediately executes your executor function with 2 arguments: resolve() and reject().

function executor(resolve, reject) {
  typeof resolve; // 'function'
  typeof reject; // 'function'
}

new Promise(executor);

The executor() function is then responsible for calling resolve() to mark
the promise as fulfilled (successful) or rejected (failed).

const success = new Promise(function executor(resolve) {
  resolve('OK');
});

const fail = new Promise(function executor(resolve, reject) {
  reject(new Error('Oops'));
});

const str = await success;
str; // 'OK'

const err = await fail.catch(err => err);
err.message; // Oops

Using Static Helpers for JavaScript Create Promise

The Promise.resolve() function lets you create a new promise that is immediately fulfilled.

const p = Promise.resolve(42);
p.then(v => {
  v; // 42
});

You can think of Promise.resolve(v) as short for new Promise(resolve => resolve(v)).

Similarly, the Promise.reject() function lets you create a new promise that is immediately rejected.

const p = Promise.reject(new Error('Oops!'));
p.catch(err => {
  err.message; // 'Oops!'
});

Be careful with Promise.reject(): if you don’t immediately add a .catch() handler to your new promise, you’ll get an unhandled promise rejection.

then() and catch()

When you call .then() or .catch(), JavaScript creates a new promise.

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

const p2 = p.then(str => '${str} World');

p2 instanceof Promise; // true
p2 === p; // false

Async Functions for JavaScript Create Promise

When you call an async function, JavaScript returns a new promise. No matter what you return from an async function, JavaScript always returns a promise, so make sure you await on async function calls!

async function test() {
  return 42;
}

test() instanceof Promise; // true

Without Executing for JavaScript Create Promise

JavaScript promises are “hot” in the sense that JavaScript executes the executor function immediately.

If you find yourself wanting a “cold” promise in the sense that your promise doesn’t execute until you await on it, you should just use an
async function. Calling an async function returns a new promise every time.

async function getAnswer() {
  return 42;
}

const p1 = getAnswer();
p1 instanceof Promise; // true

const p2 = getAnswer();
p2 instanceof Promise; // true
p2 === p1; // false

Another common alternative is the deferred pattern, where you create a promise that has resolve() and reject() functions that you can call outside the executor() function.

Promise.deferred = function() {
  let resolve = null;
  let reject = null;
  const p = new Promise((_resolve, _reject) => {
    resolve = _resolve;
    reject = _reject;
  });
  return Object.assign(p, { resolve, reject });
};

const p = Promise.deferred();

p.then(v => {
  v; // 42
});

p.resolve(42);

However, the deferred pattern is considered an antipattern. That’s because synchronous errors that occur outside the executor function won’t reject the promise!

// JavaScript catches any errors that occur in the promise executor
// and treats them as a promise rejection.
const p1 = new Promise(() => { throw new Error('Oops!'); });
p1.catch(err => {
  err.message; // 'Oops!'
});

// With 'deferred', you're responsible for handling errors that
// occur outside the executor. If you forget, your promise will
// be pending forever like 'p2' below.
const p2 = Promise.deferred();
throw new Error('Oops!');

 

JavaScript Fundamentals for JavaScript Create Promise

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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