An issue arises when using enums in TypeScript

Let's analyze this demonstration. Initially, an enum is created as follows:

enum myEnum {
  a = 'a', 
  b = 'b'
}

The next step involves creating a similar enum but with the addition of one more numeric value! This alteration is crucial.

enum myExtendedEnum {
    a = "a", 
    b = "b", 
    //any number can be assigned here
    c = 11, 
}

Now observe the following code snippet.

const returnsMyEnum = function() : myEnum {
    return 48;  
}

const returnMyExtendedEnum = function() : extendedEnum {
    return 48; 
}

Can you determine which function above is problematic for TypeScript? One might assume both are flawed, however, surprisingly only the returnsMyEnum function raises issues. Do you comprehend what's happening here or should I pursue opening a bug report in the TypeScript repository?

Answer №1

This question brings up an intriguing concept known as heterogeneous enums.

However, the issue at hand stems from the usage of numeric values.

For instance, the following example will compile without any errors:

enum NumericEnum {
  a = 23
}

const numeric = (): NumericEnum => 4;

When using an enum with a numeric value, the type will widen to number. This behavior is designed to support operations involving bitwise operators, common in scenarios where people assign power of 2 values for numeric enums and use bitwise to indicate multiple values. Consider this example:

enum Flags {
 value1 = 1,
 value2 = 2,
 value3 = 4,
 value4 = 8
}

You can now combine values like this: Flags.value1 | Flags.value4, which signifies having both value1 and value4. Due to its widespread utility, TypeScript broadens the type to number.

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