Typescript enum as a parameter type allows invalid values

Question

In TypeScript, I'm defining an enum, then I want a function to take a parameter whose value is one of the enum's values. However, TypeScript doesn't seem to do any validation on the value, and allows values outside of the enum. Is there a way to do this?

Example

enum myenum {
    hello = 1,
    world = 2,
}

const myfunc = (num:myenum):void => console.log(`num=${num}`);

myfunc(1);            // num=1 (expected)
myfunc(myenum.hello); // num=1 (expected)

//THE ISSUE: I'm expecting next line to be a TS compile error, but it is not
myfunc(7); // num=7

Alternative

If I use a type instead of enum I can get something similar to what I'm looking for, but I lose some of the functionality of enum.

type mytype = 1|2;
const myfunc = (num:mytype):void => console.log(`num=${num}`);

myfunc(1);
myfunc(7);  //TS Compile Error: Argument of type '7' is not assignable to a parameter of type 'mytype'

Have you seen this question and its answers? stackoverflow.com/questions/43804805/… — Jonah Bishop
– Jonah Bishop, Commented Apr 23, 2020 at 1:22
@JonahBishop Apparently I had, since I had upvoted the question and an answer. :) That doesn't quite help me. I can add a condition in my function like if(!Object.values(myenum).includes(num)) throw('Illegal Argument'); but that makes it a runtime error when I'd rather it be something typescript can identify at compilation. — Kip
– Kip, Commented Apr 23, 2020 at 17:14

Odinn · Accepted Answer · 2020-04-22 19:57:28Z

1

you might be expecting too much from enum in TS... :)

enum MyCoolEnum {
  A = 1,
  B = 2,
}

function test(data: MyCoolEnum) {
  console.log(`Test = ${typeof data}`)
}

test(1)
test(MyCoolEnum.A)
test(500)

If you run this code above, you will see that all the lines print number that means that behind the scenes it is translated to a number and that's why it accepts everything... Enums are usually just a good practice to avoid magic numbers and make the code more readable

On the other hand, if you stop using numeric enums and actually change the values for A and B to strings you will get:

TSError: ⨯ Unable to compile TypeScript:
dima.ts:10:6 - error TS2345: Argument of type '1' is not assignable to parameter of type 'MyCoolEnum'.

10 test(1)
        ~
dima.ts:12:6 - error TS2345: Argument of type '500' is not assignable to parameter of type 'MyCoolEnum'.

12 test(500)
        ~~~

    at createTSError (/Users/odinn/.nvm/versions/node/v10.15.1/lib/node_modules/ts-node/src/index.ts:423:12)
    at reportTSError (/Users/odinn/.nvm/versions/node/v10.15.1/lib/node_modules/ts-node/src/index.ts:427:19)
    at getOutput (/Users/odinn/.nvm/versions/node/v10.15.1/lib/node_modules/ts-node/src/index.ts:554:36)
    at Object.compile (/Users/odinn/.nvm/versions/node/v10.15.1/lib/node_modules/ts-node/src/index.ts:760:32)
    at Module.m._compile (/Users/odinn/.nvm/versions/node/v10.15.1/lib/node_modules/ts-node/src/index.ts:839:43)
    at Module._extensions..js (internal/modules/cjs/loader.js:700:10)
    at Object.require.extensions.(anonymous function) [as .ts] (/Users/odinn/.nvm/versions/node/v10.15.1/lib/node_modules/ts-node/src/index.ts:842:12)
    at Module.load (internal/modules/cjs/loader.js:599:32)
    at tryModuleLoad (internal/modules/cjs/loader.js:538:12)
    at Function.Module._load (internal/modules/cjs/loader.js:530:3)

So what I'm trying to say is that if it's a numeric enum it's just cosmetics and it's treated as a number but if you make a string enum for example then it's actually tries to match the type

answered Apr 22, 2020 at 19:57

Odinn

8085 silver badges23 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

Kip Over a year ago

thanks, this all kind of confirms what i've already seen. i'm working with some code where the enum was defined 2 years ago and is used in a lot of places, so i can't easily change to a type. but in my new function, i'd like to be able to have typescript verify that i'm passing in a value that is actually in the enum. is there a way to do that, or am i just out of luck?

Odinn Over a year ago

not that I know of, only thing I can think of is adding some validation logic... or maybe if you change the enum and add a string value to it which will stop making it a numeric enum, maybe that can help... sort of a hack though

Kip Over a year ago

seems this is "by design" for now- github.com/microsoft/TypeScript/issues/36756

Collectives™ on Stack Overflow

Typescript enum as a parameter type allows invalid values

Example

Alternative

1 Answer 1

3 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

Example

Alternative

1 Answer 1

3 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related