封装一个图片懒加载方法在imagesPreLoader.js里：

export const preLoadImg = source => {
    const pr = []
    const images = []
    source.forEach(url => {
        let p = loadImage(url)
            .then(img => images.push(img))
            .catch(err => console.log(err))
        pr.push(p)
    })
    return Promise.all(pr)
}

const loadImage = url => {
    return new Promise((resolve, reject) => {
        let img = new Image()
        img.onload = () => resolve(img)
        img.onerror = reject
        img.src = url
    })
}

在VUE中使用，在main.js里引用：

import { preLoadImg } from './utils/imagesPreLoader'
;(async () => {
    await preLoadImg([require('./assets/images/bg.jpg'), require('./assets/images/bg-rules.png'), require('./assets/images/bg-prize.png')])
    new Vue({
        el: '#app',
        components: { App },
        template: '<App/>'
    })
})()