wait for a javascript function to return

Question

I'm doing a function that makes a synchronized ajax call to a server. The problem is that when I call the function, the main doesn't wait for the data propagation.

I explain myself better:

I need to return some data from a function:

var firstdate = getLastPaycheck();

this function is:

function getLastPaycheck() {
    let returnValue  = sessionStorage.getItem('last_paycheck');

    if (returnValue == "" || returnValue == null) {
        const message = { "cookie": getCookie("userCookie") };
        var xhr = new XMLHttpRequest();
        xhr.addEventListener('load', complete, false);
        xhr.open("POST", '/getLastPaycheck', false);
        xhr.setRequestHeader("Content-type", "application/json");
        xhr.send(JSON.stringify(message));

        function complete(e) {//Call a function when the state changes.
                const response = JSON.parse(xhr.responseText);
                console.log("Value taken from the server");
                returnValue = moment(response.value[0].last_paycheck).format(getDateFormat("iso"));
                return returnValue;
            console.log("Returned value");
            } // if readyStatus = 200;

    } else {
        console.log("Value taken from the session");
        return returnValue;
    }
}

the problem is that the value of "firstdate" is always undefined when the data is taken from the server.

Could someone explain me how to do that?

Thank you. I apreciate it.

Your connection may be synchronous but you're still relying on events, which are not. Additionally, your return statements inside the callback function would not trigger a return from the outer function. — Fissure King
– Fissure King, Commented Apr 30, 2018 at 12:48

bthe0 · Accepted Answer · 2018-04-30 13:17:24Z

2

You may also do it using callbacks:

    function getLastPaycheck(callback) {
        let returnValue  = sessionStorage.getItem('last_paycheck');

        if (returnValue == "" || returnValue == null) {
            const message = { "cookie": getCookie("userCookie") };
            var xhr = new XMLHttpRequest();
            xhr.addEventListener('load', complete, false);
            xhr.open("POST", '/getLastPaycheck', false);
            xhr.setRequestHeader("Content-type", "application/json");
            xhr.send(JSON.stringify(message));

            function complete(e) {//Call a function when the state changes.
                    const response = JSON.parse(xhr.responseText);
                    console.log("Value taken from the server");
                    returnValue = moment(response.value[0].last_paycheck).format(getDateFormat("iso"));
                    callback(returnValue);
                console.log("Returned value");
                } // if readyStatus = 200;

        } else {
            console.log("Value taken from the session");
            callback(returnValue);
        }
    }

    getLastPaycheck(function(data) {
      // data is accessible here
    });

edited Apr 30, 2018 at 13:17

answered Apr 30, 2018 at 12:44

bthe0

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3 Comments

MrDjToto Over a year ago

Hi, thank you but seems to don't work :/ I did getLastPaycheck(function (data) { startdate.value = data; }); but it seems to don't work. This is I made the function: function getLastPaycheck(callback) { let returnValue = sessionStorage.getItem('last_paycheck'); ...

bthe0 Over a year ago

Well, it could be because it's not getting inside the if statement. Inside else, also do callback(returnValue). Check the edit.

MrDjToto Over a year ago

Allright! It works, I forgot to change "return" with "callback"! Thank you so much, I have a due of a brew!

TheCog19 · Accepted Answer · 2018-04-30 12:41:44Z

1

You can use ES6 async-await and then simply call

var firstdate = await getLastPaycheck();

answered Apr 30, 2018 at 12:41

TheCog19

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