Microsoft/TypeScript

Non-null assertion should work on possibly-uninitialized auto-typed `let` variables

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#52,439 opened on Jan 26, 2023

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Description

Bug Report

🔎 Search Terms

non-null postfix bang implicit any

🕗 Version & Regression Information

  • This is the behavior in every version I tried

⏯ Playground Link

This is the link

💻 Code

let foo;
if (Math.random() > 0.5) {
  foo = "hello";
}
// bar: string | undefined, seems fine
let bar = foo;
/* -- pretend I know foo is defined for other reasons -- */
// Error, foo is implicitly any
let baz = foo!;

🙁 Actual behavior

Implicit any on foo!

🙂 Expected behavior

Since the inferred type of foo (as seen in bar) is string | undefined, foo! should trivially have the type string

Contributor guide

Research direction
Reproduce the bug using the playground link, then set up the TypeScript compiler locally and trace the type checker's control flow analysis for non null assertions on uninitialized variables. Focus on the logic that infers `string | undefined` for `foo` but then raises an implicit any error on `foo!`. Look at the binder and checker code related to control flow and non null assertions.
Tech stack
typescript
Domain
developer experience
Issue type
Bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
GitNode.js
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
70