forEach循环如何才能在回调函数完成再进行下次循环？

首先，需要把 printPdf 封装成 Promise，比如

if (it.printType == 0 || it.printType == 1) {
    return new Promise(resolve => {
        clearInterval(this.scrollTimer);
        window['extendObj'].printPdf(it.result, function (el) {
            if (!el) {
                resolve("完成")
            } else {
                resolve("失败")
        }, it.printerName, 60, it.printType);
}

forEach() 是做不到了，可以自己写个 asyncForEach() 来干：

// 模拟 printPdf
function printPdf(p, cb) {
    const ms = 200 + ~~(Math.random() * 1000);
    setTimeout(() => cb(ms % 2), ms);
// 模拟 res
const res = Array.from({ length: 10 }, (_, i) => ({ result: i, printType: i % 3 }));
// 封装扩展一个 asyncForEach，需要每个 callback 返回一个 Promise 来等
Array.prototype.asyncForEach = async function (fn) {
    const length = this.length;
    for (let i = 0; i < length; i++) {
        await fn(this[i], i, this);
// 封装 printPdf 为 Promise，别忘了 resolve
res.asyncForEach(it => {
    if (it.printType == 0 || it.printType == 1) {
        return new Promise(resolve => {
            printPdf(it.result, function (el) {
                if (!el) {
                    console.log("完成" + it.result);
                } else {
                    console.log("失败" + it.result);
                resolve();