Promises and Static Values

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Async can throw a real wrench into the cogs of our programming workflows, all despite the fact that async is the modern JavaScript pattern. While async/await helps, there’s sometimes confusion about the way to have a single function that returns a value whether it exists or needs a Promise to retrieve.

To return an existing (or cached) value or the result of a promise from a single function, you employ a quick new Promise(resolve):

async function getValueOrFetch(ojbOrInfo) {
  // If the value exists, immediately return it
  if(ojbOrInfo) {
    return new Promise(resolve => resolve(ojbOrInfo));
  }
  // Return the promise-based info
  return asyncFunctionToGetInfo(ojbOrInfo);
}

Here’s an example of using a cache with the Promise API:

const cache = {
  /* url: content */
};

// Async function that returns cached content or retrieves fresh content
async function getInfo(url) {
  // Check for value in cache
  if (cache[url]) {
    // Return a value from cache with a quick promise
    return new Promise(resolve => resolve(cache[url]));
  }
  // Get the content fresh
  const content = await fetch("https://www.facebook.com").then(r => r.text());
  cache[url] = content;
  return new Promise(resolve => resolve(content));
}

My main goal with this post is making you understand that return new Promise(resolve => resolve(data)) is the way to return static or known data from a Promise-based interaction. The return value from an async function needs to be a Promise and so new Promise(resolve => resolve(data)) is the best way to return known data!

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