JavaScript How to get value of variable assigned i

Question

I'm whiting firefox extension. I got function that's reading contents of file:

var HelloWorld = {...
getData: function () {
        var env = Components.classes["@mozilla.org/processenvironment;1"].getService(Components.interfaces.nsIEnvironment);
        var path = env.get("TEMP");
        path = path + "\\lastcall.txt"
        alert(path);
        Components.utils.import("resource://gre/modules/osfile.jsm");
        let decoder = new TextDecoder(); 
        let promise = OS.File.read(path); 
        var line = null;
        promise = promise.then(
            function onSuccess(array) {
            line = decoder.decode(array)
            alert(line);
            return line;       
            }
        );
        alert("ducky:"+line+"duck");
    },
...};

I except that line will be the same because is declared outside function. From inner alert I got proper value but from outer one I get duckynullduck. how to fix it

Answer 1

getData: function () {
        var env = Components.classes["@mozilla.org/processenvironment;1"].getService(Components.interfaces.nsIEnvironment);
        var path = env.get("TEMP");
        path = path + "\\lastcall.txt"
        alert(path);
        Components.utils.import("resource://gre/modules/osfile.jsm");
        return OS.File.read(path).then(function(array) {
          let decoder = new TextDecoder(); 
          return decoder.decode(array);
        });
    },
...};

Instead of returning line you return promise for line, then the caller can do:

var line = getData();

// When you finally need the actual line, unwrap it:

line.then(function(actualLine) {

});

Answer 2

how to fix it

Don't use the outer alert.

That's how asynchronous code works, you can only access the data in the callbacks that are executed later. However, with promise chaining, it doesn't need to go all in the same callback or in nested callbacks.

let decoder = new TextDecoder(); 
let promise = OS.File.read(path); 
return promise.then(function onSuccess(array) {
    var line = decoder.decode(array);
    alert(line);
    return line;       
}).then(function onSuccess2(line) {
    alert("ducky:"+line+"duck");
    return line;
}); // return the promise for the line!