Typescript enum assignment compiler breaking change

Question

The following code used to work fine with TypeScript 2.0:

function enumDemo() {
   enum temperature{
      cold,
      hot
   };

   let temp = temperature.cold;

   switch (temp) {
      case temperature.cold:
         console.log("Brrr...");
         break;
      case temperature.hot:
         console.log("yikes!");
         break;
   }
}

enumDemo();

However, it is producing the below error in tsc 2.3.4 compiler versions:

Type 'temperature.hot' is not comparable to type 'temperature.

What has been changed between TypeScript 2.0 and 2.3?

Nieminen · Accepted Answer · 2017-06-11 04:23:12Z

2

Edit

https://github.com/Microsoft/TypeScript/wiki/Breaking-Changes#literal-types-are-inferred-by-default-for-const-variables-and-readonly-properties

This is the list of breaking changes per version, linked to what I think is the change you're looking for. In summary:

String, numeric, boolean and enum literal types are not inferred by default for const declarations and readonly properties. This means your variables/properties an have more narrowed type than before. This could manifest in using comparison operators such as === and !==.

const DEBUG = true; // Now has type `true`, previously had type `boolean`

if (DEBUG === false) { /// Error: operator '===' can not be applied to 'true' and 'false'
    ...
}

So don't forget to declare you types, it is typescript after all. If you change temp to temp: number like so:

function enumDemo() {
   enum temperature{
      cold,
      hot
   };

   let temp: number = temperature.cold;

   switch (temp) {
      case temperature.cold:
         console.log("Brrr...");
         break;
      case temperature.hot:
         console.log("yikes!");
         break;
   }
}

It should work fine. What's happening is the compiler is trying to assign the enum type, rather than the number the enum represents.

And alternative method would be to make a class with static members instead of an enum

export class temperature {
    public static cold: number = 0;
    public static hot: number = 1;
}

edited Jun 11, 2017 at 4:23

answered Jun 11, 2017 at 3:40

Nieminen

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3 Comments

Allan Xu Over a year ago

This code used to work without error in TypeScript 2.0. The question is "What has been changed between TypeScript 2.0 and 2.3?"

user663031 Over a year ago

Shouldn't it be let temp: temperature = temperature.cold;?

Nieminen Over a year ago

@torazaburo that creates the same compilation error. You would think it would be fine, and I think it would look and feel nicer than type of number, but it doesn't seem to work that way. I just pulled it up in my VS Code, and it's showing the temp type as 'temperature.cold', even though the type definition was for the enum, and not the value (let temp: temperature = temperature.cold)

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