Uploading a JSON file and using it

Question

How can I upload a JSON file on some click on a button on my web page say \"import\", and use it to store in a variable to use and update it using JavaScript.

I have

Answer 1

Basic upload File:

  document.getElementById('contentFile').onchange = function(evt) {
        try {
            let files = evt.target.files;
            if (!files.length) {
                alert('No file selected!');
                return;
            }
            let file = files[0];
            let reader = new FileReader();
            const self = this;
            reader.onload = (event) => {
                console.log('FILE CONTENT', event.target.result);
            };
            reader.readAsText(file);
        } catch (err) {
            console.error(err);
        }
    }

                     `