Difference between string enums and string literal types in TS

Well, there is a difference between string enums and literal types in the transpiled code.

Compare the Typescript Code

// with a String Literal Type 
type MyKeyType1 = 'foo' | 'bar' | 'baz';

// or with a String Enum   
enum MyKeyType2 {
   FOO = 'foo',
   BAR = 'bar',
   BAZ = 'baz'
}

With the transpiled JavaScript Code

// or with a String Enum   
var MyKeyType2;
(function (MyKeyType2) {
    MyKeyType2["FOO"] = "foo";
    MyKeyType2["BAR"] = "bar";
    MyKeyType2["BAZ"] = "baz";
})(MyKeyType2 || (MyKeyType2 = {}));

What you can see is, there is no generated code for the string literal. Because Typescripts Transpiler is only using for type safety while transpiling. At runtime string literals are "generated to dumb" strings. No references between the definition of the literal and the usages.

So there is a third alternative called const enum

Look at this

// with a String Literal Type 
type MyKeyType1 = 'foo' | 'bar' | 'baz';

// or with a String Enum   
enum MyKeyType2 {
   FOO = 'foo',
   BAR = 'bar',
   BAZ = 'baz'
}

// or with a Const String Enum   
const enum MyKeyType3 {
   FOO = 'foo',
   BAR = 'bar',
   BAZ = 'baz'
}

var a : MyKeyType1 = "bar" 
var b: MyKeyType2 = MyKeyType2.BAR
var c: MyKeyType3 = MyKeyType3.BAR

will be transpiled to

// or with a String Enum   
var MyKeyType2;
(function (MyKeyType2) {
    MyKeyType2["FOO"] = "foo";
    MyKeyType2["BAR"] = "bar";
    MyKeyType2["BAZ"] = "baz";
})(MyKeyType2 || (MyKeyType2 = {}));
var a = "bar";
var b = MyKeyType2.BAR;
var c = "bar" /* BAR */;

For further playing you can check this link

I prefer the const enum case, because of the convenient way of typing Enum.Value. Typescript will do the rest for me to get the highest performance when transpiling.

The key thing to understand is that the values of string enums are opaque.

The intended use case for a string enum is that you don't want other code to know or care what the literal string backing MyKeyType.FOO is. This means that you won't be able to, say, pass the literal string "bar" to a function accepting a MyKeyType -- you'll have to write MyKeyType.BAR instead.