loader.js

// For things that must only happen once
export function loader(promiseCreator) {
    let promise

    return (...args) => {
        if (!promise) {
            promise = promiseCreator(...args).then(res => {
                promise.loaded = true
                return res
            })
        }

        return promise
    }
}

export const bodyLoader = loader(() => {
    return new Promise(resolve => {
        if (document.readyState === 'complete') {
            return resolve()
        }

        window.addEventListener('load', () => {
            resolve()
        })
    })
})

export const interactiveLoader = loader(() => {
    return new Promise(resolve => {
        if (document.readyState !== 'loading') {
            return resolve()
        }

        document.addEventListener('readystatechange', () => {
            if (document.readyState !== 'loading') {
                return resolve()
            }
        })
    })
})