Create Promise that takes parameter

Question

The Promise() constructor only takes resolve and reject methods. How do you create a Promise that takes parameters?

You don't want to create a promise, which is just a plain non-callable object. You want to create *a function* which may return a promise. — Bergi, Aug 03 '19 at 09:37

score 0 · Accepted Answer · answered Aug 03 '19 at 05:57

The example at https://developers.google.com/web/fundamentals/primers/promises does this using closures. It creates a function that returns a Promise. The Promise uses the function's parameter via closure.

function get(url) {
  // Return a new promise.
  return new Promise(function(resolve, reject) {
    // Do the usual XHR stuff
    var req = new XMLHttpRequest();
    req.open('GET', url);

    req.onload = function() {
      // This is called even on 404 etc
      // so check the status
      if (req.status == 200) {
        // Resolve the promise with the response text
        resolve(req.response);
      }
      else {
        // Otherwise reject with the status text
        // which will hopefully be a meaningful error
        reject(Error(req.statusText));
      }
    };

    // Handle network errors
    req.onerror = function() {
      reject(Error("Network Error"));
    };

    // Make the request
    req.send();
  });
}

Create Promise that takes parameter

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