Class Completer<T>

Create a Promise with a completion marker

Remarks

Completer is inspired by dart, it can be used to return the same piece of data for the same asynchronous request. For example, you have a database singleton that only returns the same connection when the database is actually requested.

Example

Suppose we have a connect function that creates a connection to the database and its signature looks like this function connectDB():Promise<DB>. We can use the Completer wrapper to create only one connection

class Helper {
    private constructor() { }
    private static db_?: Completer<DB>
    static async db(): Promise<DB> {
        const db = Helper.db_
        if (db) {
            // connecting, return the promise of the connection
            return db.promise
        }
        // no connection, create a new connection
        const completer = new Completer<DB>()
        Helper.db_ = completer // locked
        try {
            const val = await connectDB()
            completer.resolve(val) // complete success
        } catch (e) {
            Helper.db_ = undefined // unlocked to allow subsequent calls to retry the operation

            completer.reject(e) // complete error
        }
        return completer.promise
    }
}