A promise is a feature introduced in ES6 (ECMAScript 2015), which allows us to work with asynchronous events in a more synchronous way. It's a way to handle the results of an operation that may take some time to complete.

Promises are objects in JavaScript that represent the eventual completion (or failure) of an asynchronous operation and its resulting value. A promise has three states:

• pending: The initial state, neither fulfilled nor rejected.
• fulfilled: Meaning that the operation completed successfully, and the promise has a resulting value (also known as resolve).
• rejected: Meaning that the operation failed, and the promise has a reason for that failure (also known as rejected).

A promise is always created with the new Promise() constructor. The constructor takes a function as a parameter, which is called the executor function. This executor function takes two parameters which both are functions resolve and reject. The resolve function is called, when the operation is completed, and the promise has a value to return. The reject function is called, when the operation fails, and the promise has a reason for that failure.

javascript
 
 
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const promise = new Promise((resolve, reject) => {
  const computation = Math.random() * 100;
  setTimeout(() => {
    if (computation < 50) {
      resolve(computation);
    } else {
      reject(`Computation is too high: ${computation}`);
    }
  }, 1000);
});

promise.then(result => {
  console.log(`Computation result: ${result}`);
}).catch(error => {
  console.error(`Error occurred: ${error}`);
});

In the example above, we created a promise, which resolves after a random time delay and returns the computed number unless the result is too high. If the computation is lower than 50, the promise resolves and logs the Computation result. However, when the computation is higher than 50, the promise is rejected and logs the Error occurred.

Promises can also be chained. Multiple then() function can be called, connecting them with each other. Each then() function receives the result of the previous one.

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const endpoint = "https://jsonplaceholder.typicode.com/todos/1";
fetch(endpoint)
  .then(response => response.json())
  .then(data => console.log(data))
  .catch(error => console.log(error));

In the code above, we first call fetch() to load data from an API, then we parse the response using .json(), and finally, we log the resulting JSON object. If at any point during the chain an error occurs, we can catch it using .catch() function which will go to the error block and log the exception.

Using Promises can make our code more readable especially when dealing with complex asynchronous operations.

In TypeScript, a Promise<number> is a type definition that represents a promise that will eventually return a number. A Promise is an object that represents the eventual completion (or failure) of an asynchronous operation, and is commonly used to handle asynchronous operations like network requests, database queries, and other operations that may take time.

Here's an example of how you can define and use a Promise<number> in TypeScript:

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function fetchNumber(): Promise<number> {
  return new Promise<number>((resolve, reject) => {
    // some asynchronous operation, e.g., fetch a number from server
    const randomNum = Math.floor(Math.random() * 10);
    if (randomNum >= 5) {
      resolve(randomNum); // operation succeeded, return the number
    } else {
      reject("Failed to fetch a number."); // operation failed, return an error
    }
  });
}

// usage:
fetchNumber()
  .then((num) => {
    console.log(`The number is: ${num}`);
  })
  .catch((error) => {
    console.error(`An error occurred: ${error}`);
  });

In this example, fetchNumber() is a function that returns a Promise<number>. It creates a new promise and defines the code to execute when the operation succeeds (resolve) or fails (reject). Then, we use fetchNumber() by calling it and chaining .then() and .catch() methods to handle the eventual completion of the promise. If the operation succeeds, the .then() method is called with the number value as its argument; otherwise, the .catch() method is called with the error message as its argument.

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const promise = new Promise((resolve, reject) => {
    if (someCondition) {
        resolve('the promise was fulfilled');
    }
    reject('the promse was rejected');
});