javascript - 如何通过递归使用 setTimeout 来等待在 JavaScript 中定义变量？

Question

我想等到storage.get('session')!=null，然后执行回调.

我遇到的问题是，我的递归 setTimeout 方法正在呈指数级运行，而不是每秒检查一次是否定义了变量。

结果是 waitForElement 每秒执行数千次，这是我不希望的。我希望它每 1 秒执行一次，直到 storage.get('session')!=null

waitForElement(function(){
    console.log("DONE!");
});

function waitForElement(callback){
    if(storage.get('session')!=null)
    {
        console.log("session exists now");
        if(typeof callback=="function")
        {
            callback();
        }
    }
    else
    {
        console.log("session still does not exist. Checking again in 1 second");

        //ISSUE: THIS RUNS IMMEDIATELY AND FOREVER!
        setTimeout(waitForElement(function(cb){
                if(typeof cb == "function"){
                    cb();
                }
        }), 1000);
    }
}

Answer 1

你根本不应该使用超时——Promises 是如今这种异步处理的首选模型，例如

function login() {
    return new Promise((resolve, reject) => {
        // do something that creates the session
        if (successful) {
            resolve();
        } else {
            reject();
        }
    })
}

// promise that will eventually be resolve when the user logs in
var loggedIn = login();

// multiple (potentially parallel) actions
loggedIn.then(doSomething);
loggedIn.then(doSomethingElse);

// serial actions
loggedIn.then(doFirstThing).then(doSecondThing);