[javascript] How can I view array structure in JavaScript with alert()?

For readability purposes you can use:

alert(JSON.stringify(someArrayOrObj, '', 2));

More about JSON.stringify().

Example:

let user = {
  name: "John",
  age: 30,
  roles: {
    isAdmin: false,
    isEditor: true
  }
};

alert(JSON.stringify(user, "", 2));
/* Result:
{
  "name": "John",
  "age": 30,
  "roles": {
    "isAdmin": false,
    "isEditor": true
  }
} 
*/