How to pass JSON data to a variable in Javascript?

Question

I'm trying to pass a JSON in a variable. The fetch() prints out the JSON correctly in the console, but the for loop shows that the variable is empty (undefined). What am I doing wrong?

var mydates;    
fetch("{% static 'dates.json' %}")
.then(function(u){return u.json();})
.then(function(json){mydates = json;})
.then(function(){console.log(mydates);})    //The json is printed in the console just fine

for (let year in mydates){                  // This should print in the console 2018, 2019, 2020
    console.log(year)                       // but instead mydates is marked as undefined
};

My dates.json file looks like this:

{
   "2018": {
       "January": ["week1","week2","week3","week4"],
       "February": ["week5","week6","week7"],
       "March": ["week8","week9","week10","week11"]
   },
   "2019": {
        "January": ["week1","week2","week3","week4"],
        "February": ["week5","week6","week7"],
        "March": ["week8","week9","week10","week11"]
    },
    "2020": {
        "January": ["week1","week2","week3","week4"],
        "February": ["week5","week6","week7"],
        "March": ["week8","week9","week10","week11"]
    }
}

Many thanks for your help!

`fetch()` is **asynchronous**. You are trying to eat a pizza before it gets delivered — charlietfl, Jan 03 '21 at 18:55
Thanks but after reading the original answer, I'm still completely lost. So what should be the right code ? Thanks — Joss, Jan 03 '21 at 20:06
Consume the data in the last `then()` when it actually exists — charlietfl, Jan 03 '21 at 20:07
ok, great hint thanks. Any chance you could show me what the last .then() should look like for me to finally understand ? — Joss, Jan 03 '21 at 20:14
`.then(function(json){mydates = json; console.log(mydates)})` — charlietfl, Jan 03 '21 at 20:15
Thanks, but in the for loop, mydates remains undefined... it's driving me nuts ;-) — Joss, Jan 03 '21 at 20:29
Let us [continue this discussion in chat](https://chat.stackoverflow.com/rooms/226786/discussion-between-joss-and-charlietfl). — Joss, Jan 03 '21 at 21:03

How to pass JSON data to a variable in Javascript?

0 Answers0