Uncaught (in promise) TypeError: Cannot read properties of null (reading 'iso3166_1Alpha2') / Trying to skip null but cannot

I have a global list of mountains. I want to filter all French mountains. To do this, I need to check if iso3166_1Alpha2 is set to FR. The problem is that not all mountains have a value. The script dies after it hits a null value I think because this is the error:

Uncaught (in promise) TypeError: Cannot read properties of null (reading 'iso3166_1Alpha2')

This is my script. Seems my check of !== null is not working and do not know why.

function addJSON() {
  let url = "https://development.example.com/admin/mtn/json/mtn_areas.geojson";
  fetch(url)
    .then(function (response) {
      return response.json();
    })
    .then(function (data) {
      let mtn = data.features;
      for (var i = 0; i < mtn.length; i++) {
        if (mtn[i].properties.location.iso3166_1Alpha2 !== null) {
          x = mtn[i].properties.location.iso3166_1Alpha2;
          console.log(x);
        }
      }
    });
}

>Solution :

It seems like your are getting an item which doesn’t have a location property, in which case your condition won’t help preventing it. Try changin your function as below. Notice I’m using Optional chaining, the ?. syntax, to avoid any null or undifined property.

function addJSON() {
  let url = "https://development.example.com/admin/mtn/json/mtn_areas.geojson";
  fetch(url)
    .then(function (response) {
      return response.json();
    })
    .then(function (data) {
      let mtn = data.features;
      for (var i = 0; i < mtn.length; i++) {
        if (mtn[i]?.properties?.location?.iso3166_1Alpha2) {
          x = mtn[i].properties.location.iso3166_1Alpha2;
          console.log(x);
        }
      }
    });
}