编辑代码

function transformAbsolutePath(_url, _relativepath) {
    if (!_relativepath) return _relativepath
    let absolutePath = ''
    try {
        if (/^\/\//.test(_relativepath) || new URL(_relativepath))
            absolutePath = _relativepath
    } catch {
        const url = new URL(_url)
        const pathname = url.pathname
        const pathnameArr = pathname.split('/')
        const relativepathArr = _relativepath.split('/')
        let newUrl = ''
        if (/^\.\.\//.test(_relativepath)) {
            const relativepathFilterArr = relativepathArr.filter(s => s !== '..')
            const count = relativepathArr.length - relativepathFilterArr.length
            const pathnameSliceArr = pathnameArr.slice(0, pathnameArr.length - count)
            newUrl = pathnameSliceArr.concat(relativepathFilterArr).join('/')
        } else if (/^\/[^\/]/.test(_relativepath)) {
            newUrl = _relativepath
        } else {
            const relativepathFilterArr = relativepathArr.filter(s => s !== '.')
            newUrl = pathnameArr.concat(relativepathFilterArr).join('/')
        }
        absolutePath = url.origin + newUrl
    } finally {
        return absolutePath
    }
}

const url = 'https://www.demo.com/a/b/c/d'
// 返回上两级
const relativePath1 = '../../foo'
// 下一级
const relativePath2 = './bar'
// 根子级
const relativePath3 = '/foobar'
// 绝对路径
const remotePath1 = 'https://www.icon.com/a.png'
const remotePath2 = '//www.icon.com/a.png'

// 'https://www.demo.com/a/b/foo'
const absolutePath1 = transformAbsolutePath(url, relativePath1)

// 'https://www.demo.com/a/b/c/d/bar'
const absolutePath2 = transformAbsolutePath(url, relativePath2)

// 'https://www.demo.com/foobar
const absolutePath3 = transformAbsolutePath(url, relativePath3)

// 'https://www.icon.com/a.png'
const absoluteRemotePath1 = transformAbsolutePath(url, remotePath1)
// '//www.icon.com/a.png'
const absoluteRemotePath2 = transformAbsolutePath(url, remotePath2)

console.log({ absolutePath1, absolutePath2, absolutePath3, absoluteRemotePath1, absoluteRemotePath2 })