Run a JS promise exactly once

once.js

// Sentinel value that indicates Promise has not executed.
var empty = {};

function once(f) {
    var result = empty;
    var promise = null;

    return function() {
        // If we've saved something already, resolve it immediately.
        if (result !== empty) {
            return Promise.resolve(result);
        }

        if (promise == null) {
            // Create the promise and save the result afterwards.
            promise = f();
            promise.then(function(result_) { result = result_; });
        }

        // Return the promise so clients can wait on it.
        return promise;
    };
}