Is it possible to assert compilation error in TypeScript?

Question

For example in Scala one can do following (ScalaTest):

assertDoesNotCompile("val a: String = 1")
assertTypeError("val a: String = 1")
assertCompiles("val a: Int = 1")

Does something similar exist in TypeScript world?

Edit:
I mean context-aware compilation. For example code from this question How do I write a scala unit test that ensures compliation fails?:

import shapeless.test.illTyped

//this version won't even compile
illTyped("getIdx(C.Ooga)")

//We can have multiple enum unions exist side by side
import Union_B_C._
B.values().foreach {b => Union_B_C.getIdx(b) should be (b.ordinal())}
C.values().foreach {c => Union_B_C.getIdx(c) should be (c.ordinal() + 2)}

//Though A exists in some union type, Union_B_C still doesn't know about it,
// so this won't compile
illTyped("""
  A.values().foreach {a => Union_B_C.getIdx(a) should be (a.ordinal())}
""")

artem · Accepted Answer · 2017-06-12 16:57:13Z

3

It is not a feature of Scala, it's a feature of ScalaTest which uses scala compiler at runtime as a library.

You can use typescript compiler as a library, it has rather complicated API documented here.

I have a node module published on github that simplifies things a bit, you can use it like this:

import {createCompiler, CompileResult} from 'tsc-simple';

const compiler= createCompiler({defaultLibLocation:'node_modules/typescript/lib'});

const r: CompileResult = compiler.compile('let x = 3 + 2');

assert.lengthOf(r.diagnostics, 0);

(using assert from chai module)

answered Jun 12, 2017 at 16:57

artem

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4 Comments

monnef Over a year ago

I think Scala has some way with types to crash when the compiler resolves them. But yes, this case is not a Scala feature directly, Scala only allows to use compiler. Your library with few helper functions could do the same thing as ScalaTest. Thank you for answering :).

monnef Over a year ago

It occurred to me that your solution probably will work only without context, that's not what I meant. I'll update the question.

artem Over a year ago

Then the answer is definitely no, nothing like scala macros exists in typescript.

atsu85 Over a year ago

@monnef it seems to me that this answer deserves an upvote. It seems that (at least for some situations) it is possible to assert compilation error using tsc-simple lib that artem shared

Mingwei Samuel · Accepted Answer · 2020-09-08 07:23:03Z

1

This isn't exactly your use case, but TypeScript 3.9 has the // @ts-expect-error comment which may be useful for people:

const x = 5;
// @ts-expect-error
const y: string = x;

Note that this comment will let you break things if you are not careful, so e.g. you should not use y in the example above even though the compiler will let you.

answered Sep 8, 2020 at 7:23

Mingwei Samuel

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