Can I get TypeScript to reveal what type it has determined for an expression?

Question

I have some code like the following where I want to determine what type TypeScript has inferred for an expression:

var timer = window.setTimeout(...);
/* QUESTION: What is the inferred type of "timer" here? */

When I use the mypy typechecker for Python, I can insert the special expression reveal_type(my_expression) into code to have the typechecker print a fake error containing the inferred type for expression my_expression.

Is there a way I can ask the TypeScript tsc type-checker for similar information about the inferred type of an expression?

Answer 1

I don't know of an equivalent to reveal_type, but you can force an invalid assignment and inspect the resulting error message. Assignment to never always fails¹, so in your example:

var timer = window.setTimeout(...);
const revealType : never = timer;

Should cause the compiler to print:

Type 'number' is not assignable to type 'never'

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¹ Unless the type you're checking is also never