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MapView doesnt work with async/await

See original GitHub issue

This probably applies to all interfaces that inherits from corePromise If you try to return MapMiew instance in a promise using async& await you get compilation error: Return expression in async function does not have a valid callable 'then' member.

Issue Analytics

  • State:closed
  • Created 6 years ago
  • Comments:5 (1 by maintainers)

github_iconTop GitHub Comments

eric-parsonscommented, Jun 27, 2017

You can convert any promise-like object such as IPromise into an ES6 promise by using Promise.resolve:

await Promise.resolve(mapView);

However, there is another issue that you will immediately run into. Several objects in the JavaScript API are promises that fullfill with themselves when resolved. When attempting to resolve such promises, promise implementations that follow the A+ promise specification must recurse forever (or optionally detect the cycle and reject with a TypeError). This applies to most browser’s native implementations and several popular polyfills such as Bluebird. If you run the above code in e.g. Google Chrome, it will simply hang forever. To get around this, I created the following helper function (written in TypeScript, which I infer is what you’re using):

 * Returns a promise that is fulfilled when the given Esri API object is ready.
 * @param object The object to wait for.
export function ready(object: any): Promise<void> {
    return new Promise<void>((resolve, reject) => {
        if (object && typeof object.then === "function") {
            object.then(() => resolve(), reject);
        } else {

The trick is that it returns a Promise<void> instead of e.g. Promise<MapView>. The usage would then be:

await ready(mapView);

You might also find this helper useful as well:

 * Loads the given Esri API object and returns a promise that is fulfilled when
 * the object is loaded.
 * @param object The object to load.
export async function load(object: any): Promise<void> {
    if (object && typeof object.load === "function") {
        object = object.load();

    await ready(object);

Then you can do this if you need to load e.g. a layer:

await load(layer);
dasacommented, Jan 2, 2018
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