将一个字符串翻转，例 '123456' -> '654321'

// 翻转字符串算法
function fanzhuan( str ) {
    let result = ''
    for ( let i = str.length; i > 0; i-- ) {
        result += str.slice( i-1, i )
    }
    return result
}
let str = '123456'
let str2 = fanzhuan( str )      // 654321
let str3 = fanzhuan( '123456789abc' )   //cba987654321
let str4 = fanzhuan( '清欢' )     // 欢清

有a，b，c...等十个js文件，需要按照顺序引入，使用promise封装

// 封装 promise
function promiseOnloadJS ( url ) {
    return new Promise( ( resolve, reject )=> {
        const node = document.createElement('script')
        node.type = 'text/javascript'
        node.src = url
        node.onload = function() {  // 在js 文件加载完成 ，调用resolve
            resolve()
        }
        document.head.appendChild(node)
    })
}
promiseOnloadJS('js/temp/a.js').then(function() {
    console.log('---------------');
    return promiseOnloadJS('js/temp/b.js')
}).then(function() {
    console.log('---------------');
    return promiseOnloadJS('js/temp/c.js')
}).then(function() {
    console.log('---------------');
    return promiseOnloadJS('js/temp/d.js')
}).then(function() {
    console.log('---------------');
    return promiseOnloadJS('js/temp/e.js')
}).then(function() {
    console.log('---------------');
    return promiseOnloadJS('js/temp/f.js')
}).then(function() {
    console.log('---------------');
    return promiseOnloadJS('js/temp/g.js')
}).then(function() {
    console.log('---------------');
    return promiseOnloadJS('js/temp/h.js')
}).then(function() {
    console.log('---------------');
    return promiseOnloadJS('js/temp/i.js')
})