@vtbag/utensil-drawer/lib/switchable-promise.js

Pull out just what you need to craft seamless transitions. The Utensil Drawer holds reusable functions to help you build websites with view transitions. It is a bit sparse right now, but like the one in your kitchen, it is bound to fill up over time.

export function createPromiseSubstitute() {
    const saved = [];
    let delegate;
    const save = (kind, resolve, reject, callback) => {
        if (delegate) {
            return kind === 'then' ? delegate.then(resolve, reject) : delegate.finally(callback);
        }
        const promise = createPromiseSubstitute();
        saved.push({ kind, resolve, reject, callback, promise });
        return promise;
    };
    return {
        then(resolve, reject) {
            return save('then', resolve, reject);
        },
        catch(reject) {
            return save('then', undefined, reject);
        },
        finally(callback) {
            return save('finally', undefined, undefined, callback);
        },
        switch(to) {
            if (delegate)
                throw new Error('Already switched to another promise');
            saved.forEach((e) => e.promise.switch(e.kind === 'then' ? to.then(e.resolve, e.reject) : to.finally(e.callback)));
            delegate = to;
        },
    };
}