app/assets/javascripts/pictures.js
$( document ).on('turbolinks:load', function() {
$('#user_avatar, #team_avatar').on('change', function (evt) {
if (evt.target.files && evt.target.files.length === 1) {
var file = evt.target.files[0];
var fileReader = new FileReader();
fileReader.onload = function () {
$('#preview').attr('src', fileReader.result);
};
fileReader.readAsDataURL(file);
}
});
});