How do I run a function once session storage is != undefined

Question

I am in a situation wherein I have a an external JavaScript file that makes an ajax call and saves certain data to session storage. The JavaScript file loads from an external site and therefore, I cannot edit it.

I need to run a function to consume the saved data as soon as it loads into session storage. How can I trigger a function once this data loads?

Answer 1

Maybe this can help you:

window.onstorage = function(e) {
  console.log('The ' + e.key + ' key has been changed from ' + e.oldValue + ' to ' + e.newValue + '.');
};

More info here and here

So you could subscribe to your session key changes with (not tested):

window.onstorage = function(e) {
    if ( 
        e.storageArea === sessionStorage && 
        e.key === "<your_key>" && 
        e.oldValue === undefined &&
        e.newValue !== undefined
    ) {
        console.log("my key is not undefined anymore")
    } 
};

UPDATE: So it seems that it is not working for you. Then you can try something like this (using interval to check if sessionStorage changes):

var INTERVAL = 2000;
// Then, in your code you can check every `INTERVAL` milleconds
// if the sessionKey you need is not null
console.log("Program starts");
// You can limit your intents too
var limit = 5;
var intervalId = setInterval(function() {
  console.log("Checking sessionStorage");
  var test = sessionStorage.getItem("test");
  if (test !== null) {
    console.log("'test' is now defined: " + test);
    clearInterval(intervalId);
  }

  if (--limit <= 0) {
    console.log("'test' has not changed");
    clearInterval(intervalId);
  }
}, INTERVAL);

Test here: https://jsbin.com/tukubutoje/edit?js,console