場景：
1. 設計稿按照 750px 設計
1. html 基準字型大小為 100px
1. 頁面最大寬度為 750px
1. 頁面最小寬度為 320px

REM 自適應工具類

1.安裝throttle-debounce，執行：cnpm install -S throttle-debounce
2.複製如下程式碼（typescript）：

declare const window: any
declare const document: any

const debounce = require('throttle-debounce/debounce')

export class RemResponsiveUtils
 
 {
    static baseFontSize = 100
    static baseDocWidth = 750
    static maxDocWidth = 750
    static minDocWidth = 320

    static onResize () {
        let docWidth: number = document.documentElement.clientWidth
        if (docWidth > RemResponsiveUtils.maxDocWidth) {
            docWidth = RemResponsiveUtils.maxDocWidth
        } else
 
 if (docWidth < RemResponsiveUtils.minDocWidth) {
            docWidth = RemResponsiveUtils.minDocWidth
        }
        document.querySelector('html').style.fontSize = (docWidth / RemResponsiveUtils.baseDocWidth) * RemResponsiveUtils.baseFontSize + 'px'
    }

    static init () {
        RemResponsiveUtils.onResize()
        window.onresize = debounce(500
 
, RemResponsiveUtils.onResize)
    }
}

3.執行 RemResponsive.init()