How to verify if a variable is Enum?

Question

I wish to check if a variable is a valid Google App Script Enum type.

function myFunc(BORDER_COLOR) {
    if (typeof BORDER_COLOR !== "Enum") {    // This does not work
        // do something for this exception
    }
    // rest of the function
}

I use typeof and instanceof to check it. But something strange happens. The Enum is an object. But it is not an instance of Object. This contradicts my understandings.

• I suppose all primitive types (string, boolean, etc.) are non-objects.
• I suppose all non-primitive (Array, user-defined types, etc.) types are objects.
• (Why?)

.

Logger.log(typeof SpreadsheetApp.BorderStyle.SOLID);             // object
Logger.log(SpreadsheetApp.BorderStyle.SOLID instanceof Object);  // false <-- unexpected
Logger.log("");

var value = "text";
Logger.log(typeof value);               // string
Logger.log(value instanceof Object);    // false
Logger.log("");

var value = new String("text");
Logger.log(typeof value);               // object
Logger.log(value instanceof Object);    // true
Logger.log("");

Logger.log(Array instanceof Object);    // true
Logger.log(Object instanceof Object);   // true

---

Added:

var value = 123;
Logger.log(typeof value);               // number
Logger.log(TYPEOF(value));              // number
Logger.log(value instanceof Object);    // false
Logger.log("");

var value = [];
Logger.log(typeof value);               // object
Logger.log(TYPEOF(value));              // Array[0]
Logger.log(value instanceof Object);    // true
Logger.log("");

var value = {};
Logger.log(typeof value);               // object
Logger.log(TYPEOF(value));              // Object
Logger.log(value instanceof Object);    // true
Logger.log("");

function TYPEOF(value) {
  if (typeof value !== 'object')    // not an object, it is a primitive type
    return typeof value;

  var details = '';
  if (value instanceof Array) {            // if this is an object, may be this is an Array
    details += '[' + value.length + ']';
    if (value[0] instanceof Array)
      details += '[' + value[0].length + ']';
  }

  var className = value.constructor ? value.constructor.name : value.constructor;   // if this is not an Array, get the (tailor-made) constructor name
  return className + details;
}

Answer 1

Use Object.keys() to see if there's anything you can use for verification. The most valuable I found are name & ordinal. So then the easiest check will be to see if you can call name() on the Enum.

function test() {
  Logger.log(Object.keys(SpreadsheetApp.BorderStyle.DASHED)); // [name, toString, compareTo, ordinal]
  Logger.log(SpreadsheetApp.BorderStyle.DASHED.name()); // DASHED
  Logger.log("name" in SpreadsheetApp.BorderStyle.DASHED); // true

  try {  
    if ("name" in SpreadsheetApp.BorderStyle.DASHED) { // This is true
      Logger.log("SpreadsheetApp.BorderStyle.DASHED is valid enum"); // This should print
    }
  } catch (err) {}; 

  try {
    if ("name" in SpreadsheetApp.BorderStyle.NOT_VALID) { // This errors
      Logger.log("SpreadsheetApp.BorderStyle.NOT_VALID is not a valid enum"); // Will not print
    }
  } catch (err) {
    Logger.log("Errored");
  };
}

As such, you can then create a more generalized function to test enum values in the SpreadsheetApp (or adapt for other services). In fact, because the property name is being passed, we can more confidently check that it is an enum by comparing the names instead of just checking in, which also returns true for properties in the prototype chain.

/**
 * Check if the type and property are a valid Enum of the SpreadsheetApp.
 * @param {String} type
 * @param {String} property
 * @returns {Boolean}
 */
function isSpreadsheetEnum(type, property) {
  try {
    return SpreadsheetApp[type][property].name() === property;
  } catch (err) {
    return false;
  }
}

Which you could use like this:

function testEnum() {
  Logger.log(isSpreadsheetEnum("DataValidationCriteria", "DATE_ON_OR_AFTER"));
}