const callbacks = {}

/**
 * 加载一个远程脚本
 * @param {String} src 一个远程脚本
 * @param {Function} callback 回调
 */
function loadScript(src, callback) {
 const existingScript = document.getElementById(src)
 const cb = callback || (() => {})
 if (!existingScript) {
 callbacks[src] = []
 const $script = document.createElement('script')
 $script.src = src
 $script.id = src
 $script.async = 1
 document.body.appendChild($script)
 const onEnd = 'onload' in $script ? stdOnEnd.bind($script) : ieOnEnd.bind($script)
 onEnd($script)
 }

 callbacks[src].push(cb)

 function stdOnEnd(script) {
 script.onload = () => {
 this.onerror = this.onload = null
 callbacks[src].forEach(item => {
 item(null, script)
 })
 delete callbacks[src]
 }
 script.onerror = () => {
 this.onerror = this.onload = null
 cb(new Error(`Failed to load ${src}`), script)
 }
 }

 function ieOnEnd(script) {
 script.onreadystatechange = () => {
 if (this.readyState !== 'complete' && this.readyState !== 'loaded') return
 this.onreadystatechange = null
 callbacks[src].forEach(item => {
 item(null, script)
 })
 delete callbacks[src]
 }
 }
}

/**
 * 顺序加载一组远程脚本
 * @param {Array} list 一组远程脚本
 * @param {Function} cb 回调
 */
export function loadScriptQueue(list, cb) {
 const first = list.shift()
 list.length ? loadScript(first, () => loadScriptQueue(list, cb)) : loadScript(first, cb)
}

export default loadScript