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JavaScript Promises

Promises are the foundation of asynchronous JavaScript. Understanding them deeply is essential for modern web development.

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Promise Basics

  • Promises start in a pending state, neither fulfilled or rejected
  • When the operation is completed, a promise will become fulfilled with a value
  • If the operation fails, a promise will get rejected with an error

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Creating Promises

The function passed to the Promise constructor will execute synchronously.

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From Values

// Resolving with a value
Promise.resolve(42).then(val => console.log(val)); // 42

// Rejecting with an error
Promise.reject(new Error('Failed')).catch(err => console.log(err.message)); // 'Failed'

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From Async Operations

// Resolving with a value, rejecting with an error
new Promise((resolve, reject) => {
  performOperation((err, val) => {
    if (err) reject(err);
    else resolve(val);
  });
});

// Resolving without value, no need for reject
const delay = (ms) => new Promise(resolve => setTimeout(resolve, ms));
If you put a fulfilled promise into a fulfilled promise, they will collapse into one.

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Handling Promises

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Promise.prototype.then()

Promise.prototype.then() accepts two optional arguments (onFulfilled, onRejected): 
promisedOperation()
  .then(
    val => val + 1,   // Called once the promise is fulfilled
    err => {            // Called if the promise is rejected
      if (err === someKnownErr) return defaultVal;
      else throw err;
    }
  );
  • Calls onFulfilled once the promise is fulfilled
  • Calls onRejected if the promise is rejected
  • Passes errors through if onRejected is undefined

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Promise.prototype.catch()

Promise.prototype.catch() accepts one argument (onRejected): 
promisedOperation()
  .then(val => val + 1)
  .catch(err => console.log(err)); // Called if the promise is rejected
  • Behaves like Promise.prototype.then() when onFulfilled is omitted
  • Passes fulfilled values through

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Promise.prototype.finally()

Promise.prototype.finally() accepts one argument (onFinally): 
promisedOperation()
  .then(val => val + 1)
  .catch(err => console.log(err))
  .finally(() => console.log('Done')); // Called once any outcome is available
  • Calls onFinally with no arguments once any outcome is available
  • Passes through input promise
All three methods will not be executed at least until the next tick, even for promises that already have an outcome.

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Combining Promises

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Promise.all()

Turns an array of promises into a promise of an array: 
Promise
  .all([ p1, p2, p3 ])
  .then(([ v1, v2, v3 ]) => {
    // Values always correspond to the order of promises,
    // not the order they resolved in (i.e. v1 corresponds to p1)
  });
If any promise is rejected, Promise.all() will immediately reject with that error.

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Promise.race()

Passes through the first settled promise: 
Promise
  .race([ p1, p2, p3 ])
  .then(val => {
    // val will take the value of the first resolved promise
  });

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Promise.allSettled()

Waits for all promises to settle (fulfilled or rejected): 
Promise.allSettled([p1, p2, p3]).then(results => {
  results.forEach(result => {
    if (result.status === 'fulfilled') {
      console.log('Value:', result.value);
    } else {
      console.log('Error:', result.reason);
    }
  });
});

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async/await

The modern way to work with promises.

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Basic Usage

async () => {
  try {
    let val = await promisedValue();
    // Do stuff here
  } catch (err) {
    // Handle error
  }
}

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Key Points

  • Calling an async function always results in a promise
  • (async () => value)() will resolve to value
  • (async () => throw err)() will reject with an error
  • await waits for a promise to be fulfilled and returns its value
  • await can only be used in async functions
  • await also accepts non-promise values
  • await always waits at least until the next tick before resolving

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Practical Example

async function fetchUserData(userId) {
  try {
    const response = await fetch(`/api/users/${userId}`);
    const user = await response.json();
    return user;
  } catch (error) {
    console.error('Failed to fetch user:', error);
    throw error;
  }
}

// Usage
fetchUserData(123)
  .then(user => console.log(user))
  .catch(err => console.error(err));

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Running Promises in Series

JavaScript promises are asynchronous, meaning they execute in parallel. To execute promises one after another (sequentially), chain them using Array.prototype.reduce(): 
const runPromisesInSeries = ps =>
  ps.reduce((p, next) => p.then(next), Promise.resolve());

const delay = d => new Promise(r => setTimeout(r, d));
runPromisesInSeries([() => delay(1000), () => delay(2000)]);
// Executes each promise sequentially, taking a total of 3 seconds to complete
Each promise in the chain returns the next promise when resolved, using Promise.prototype.then().

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Common Patterns

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Timeout with Promise.race()

const timeout = (ms) => new Promise((_, reject) =>
  setTimeout(() => reject(new Error('Timeout')), ms)
);

const fetchWithTimeout = (url, ms) =>
  Promise.race([
    fetch(url),
    timeout(ms)
  ]);

// Fails if fetch takes longer than 5 seconds
fetchWithTimeout('/api/data', 5000)
  .then(response => response.json())
  .catch(err => console.error(err));

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Retry Logic

const retry = async (fn, retries = 3, delay = 1000) => {
  try {
    return await fn();
  } catch (error) {
    if (retries === 0) throw error;
    await new Promise(resolve => setTimeout(resolve, delay));
    return retry(fn, retries - 1, delay);
  }
};

// Usage
retry(() => fetch('/api/data'), 3, 1000)
  .then(response => response.json())
  .catch(err => console.error('Failed after 3 retries:', err));

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Parallel with Limit

const parallelLimit = async (tasks, limit) => {
  const results = [];
  const executing = [];

  for (const [index, task] of tasks.entries()) {
    const promise = Promise.resolve().then(() => task());
    results.push(promise);

    if (limit <= tasks.length) {
      const executing = promise.then(() =>
        executing.splice(executing.indexOf(executing), 1)
      );
      executing.push(executing);
      if (executing.length >= limit) {
        await Promise.race(executing);
      }
    }
  }

  return Promise.all(results);
};

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Best Practices

Use .catch() or try/catch with async/await to handle errors. Unhandled promise rejections can cause issues.
async/await is more readable and easier to debug. Use .then() for simple chains.
A common mistake is forgetting to await async functions, which returns a promise instead of the value.
When operations don’t depend on each other, run them in parallel with Promise.all() for better performance.

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