"in" check returns false when using String enums

Question

I'm using TypeScript, and have the following test Enum

enum Colors {
    Red = "RED",
    Green = "GREEN",
    Blue = "BLUE",
    Other = 1
}

Note: This enum is just for demonstration. I don't actually mix numbers and strings, but it's just easier than to write two enums just to show what's wrong.

Now, in my code, I want to check if a specific value is in the enum, however, when checking against string Enums it's always false

'RED' in Colors // false
"RED" in Colors // false
'GREEN' in Colors // false
"GREEN" in Colors // false
1 in Colors // true

I checked the compiled code, and this is how it looks

var Colors;
(function (Colors) {
    Colors["Red"] = "RED";
    Colors["Green"] = "GREEN";
    Colors["Blue"] = "BLUE";
    Colors[Colors["Other"] = 1] = "Other";
})(Colors || (Colors = {}));

Isn't it quite strange that the generated code for an integer looks different than for a string?

My TypeScript version is 2.4.1 and according to this blog, it's available since 2.4, so I should be good to go, right?

I tried to only use one value (string/number) in the enum, but still the same errors. How can I check if a value exists in a string based Enum?

I compile my code to ES5, but with ES6 the same problem occurs.

Why would you want to mix number/string values to begin with? — Nitzan Tomer
– Nitzan Tomer, Commented Jun 29, 2017 at 10:52
Was just an example to show that numbers work, but strings don't. I don't have them mixed. I also don't see how this is relevant? — Musterknabe
– Musterknabe, Commented Jun 29, 2017 at 10:53

Hitmands · Accepted Answer · 2017-06-29 14:15:53Z

3

You are accessing by property value, but, instead, in operator is used to access by property name.

your data looks like:

var Colors = {
  Red: "RED"
}

so you can test if the key Red is in Colors: 'Red' in Colors // true

if you need to check if one of the keys has "RED" (all upper) as value, then you need to iterate over alle the object.

var Colors = {
  Red: "RED"
}

function valueInColors(needed) {
  for(let k in Colors) {
    if(Colors.hasOwnProperty(k) && Colors[k] === needed) {
      return true;
    }
  }
  
  return false;
}

console.log(valueInColors('RED'))

Ok, so you mean, when I'm searching for 1 in Colors it doesn't find the element with the value 1, but instead it finds the element with the key 1 in the generated array?

Colors[Colors["Other"] = 1] = "Other";

// Assignment operator returns the value, so, the previous expression is like:

Colors["Other"] = 1
Colors[1] = "Other";

Ah, I understand. So technically speaking, I don't even need the values? I don't need to use "Red: Red, Green: Green", I could just go with "Red, Green", etc.?

Sometimes you need, sometimes no. In this specific case I think you don't need values. You can find further info here: https://www.typescriptlang.org/docs/handbook/enums.html

edited Jun 29, 2017 at 14:15

answered Jun 29, 2017 at 10:55

Hitmands

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7 Comments

Musterknabe Over a year ago

Okay, but why does it work that way for strings, but not for integers? I can check 1 in Colors and it returns true. For example in this answer: stackoverflow.com/questions/43804805/… They show how to search for value, not by property name. Same for this answer: stackoverflow.com/questions/40598296/… It shows for getting a value

Musterknabe Over a year ago

Thanks for your answer. Upvoted you. I'm still wondering, however, why it works different for integers.

Hitmands Over a year ago

They are not looking by value, all the example you have linked are looking by key, the thing is that Enums are also indexed by int: typescriptlang.org/docs/handbook/enums.html

Musterknabe Over a year ago

Ok, so you mean, when I'm searching for 1 in Colors it doesn't find the element with the value 1, but instead it finds the element with the key 1 in the generated array?

Musterknabe Over a year ago

Thanks, but what I'm not getting is why it's different for a string? Why isn't it `Colors[Colors["RED"] = 'Red'] = 'Red' and then I have both as the key and value?

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