How to debug TypeScript by printing the inferred variable type?

Question

Often TS fails for me because the type it infers is different from what I expect it to be. I could force typecast variables, but I'd prefer to figure out why TS isn't inferring the types correctly.

Is there a way to print the type that TS believes a variable is?

E.g.:

const obj = [
  { a: 'foo' },
  { b: 'bar' },
];

printTsType(obj[0].a);

I would expect this to be the literal 'foo', but I think TS just infers string.

"Is there a way to print the type that TS believes a variable is?" no. The type system exists at compile time, not when the code is running. — VLAZ
– VLAZ, Commented Jun 17, 2020 at 15:58
@VLAZ when TS is compiling, I want the compiler to dump whatever type it inferred when it hits a particular line. I guess it should be a compiler directive and not a JS function — Leo Jiang
– Leo Jiang, Commented Jun 17, 2020 at 16:01
Your editor should be able to tell you what something is if you hover your mouse over it, for example. If you want it programmatically, then you'd need to get a hold of the intermediate representation that TS produces. This should be exposed somehow as people have wrote plugins using that information. — VLAZ
– VLAZ, Commented Jun 17, 2020 at 16:03
@LeoJiang You could force TS to infer type of obj[0].a to be 'foo' by using {a: 'foo' as const} or const obj = [...] as const depending on your requirements. — Yury Tarabanko
– Yury Tarabanko, Commented Jun 17, 2020 at 16:06

Igor · Accepted Answer · 2020-06-17 19:29:47Z

13

One trick to learn what is the inferred type is to pass that variable to a function that expects a different type (e.g. never) to generate a compiler error which will tell you "inferred type is not assignable to never". TS Playground example: here

answered Jun 17, 2020 at 19:29

Igor

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2 Comments

SystemParadox Over a year ago

Even shorter way is let foo: never = objectWithUnknownType

Nathan Chappell Over a year ago

For now I prefer this to installing some ide or intellisense to use typescript.

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