TOOLS_JS异步调用的同步处理

js异步调用转同步

// 执行两个嵌套的异步请求
function test() {
    let promise_a = fetch('https://www.baidu.com/')

    promise_a.then(res_a => {
        res_a = "https://www.baidu.com/"
        let promise_b = fetch(res_a)
        promise_b.then(res_b => {
            console.log("res_b = ", res_b)
        }).catch(err => {
            console.log("promise_b err=>", err);
        })

    }).catch(err => {
        console.log("promise_a err=>", err);
    })

}

test()

// *******************************************
// 将这两个嵌套的异步请求修改为同步的处理
function test_async() {
    return new Promise(function(resolve, reject) {
        let promise_a = fetch('https://www.baidu.com/')

        promise_a.then(res_a => {
            res_a = "https://www.baidu.com/"
            let promise_b = fetch(res_a)
            promise_b.then(res_b => {
                resolve(res_b)
            }).catch(err => {
                // console.log("promise_b err=>", err);
                reject(err)
            })

        }).catch(err => {
            // console.log("promise_a err=>", err);
            reject(err)
        })

    })
}

// 将异步请求用一个promise包裹 然后放到一个异步函数中
async function test() {
    // console.log('test开始...');
    var value1 = await test_async();
    // console.log("value1 = ", value1);
    return value1;
}

// 调用时使用await阻塞即可
let res = await test()
console.log("res = ", res)