节点在继续之前等待异步功能

Question

我有一个使用一些异步函数的节点应用程序.

在继续执行其余应用程序流程之前,如何等待异步函数完成？

下面有一个简单的例子.

var a = 0;
var b = 1;
a = a + b;

// this async function requires at least 30 sec
myAsyncFunction({}, function(data, err) {
    a = 5;
});

// TODO wait for async function

console.log(a); // it must be 5 and not 1
return a;

在该示例中,a要返回的元素" "必须为5而不是1.如果应用程序不等待异步函数,则它等于1.

谢谢

Answer 1

 使用回调机制:

function operation(callback) {

    var a = 0;
    var b = 1;
    a = a + b;
    a = 5;

    // may be a heavy db call or http request?
    // do not return any data, use callback mechanism
    callback(a)
}

operation(function(a /* a is passed using callback */) {
    console.log(a); // a is 5
})

 使用异步等待

async function operation() {
    return new Promise(function(resolve, reject) {
        var a = 0;
        var b = 1;
        a = a + b;
        a = 5;

        // may be a heavy db call or http request?
        resolve(a) // successfully fill promise
    })
}

async function app() {
    var a = await operation() // a is 5
}

app()