Microsoft/TypeScript

Strange error: string is not a string type

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#41,918 opened on Dec 10, 2020

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 (6 comments) (0 reactions) (0 assignees)TypeScript (48,455 stars) (6,726 forks)batch import
BugDomain: Error MessagesEffort: ModerateHelp Wanted

Description

TypeScript Version: 4.1.2

Search Terms: downlevelIteration, string, spread

Code

function test(name: string) {
    for (const _char of [...name]) {
    }
}

Run tsc on just this ts file with no extra flags (I couldn't find the right combination of playground checkboxes to simulate this.

Expected behavior: Assuming the code is wrong, the error message should probably not say that string is not a string type.

Actual behavior:

error TS2569: Type 'string' is not an array type or a string type. Use compiler option '--downlevelIteration' to allow iterating of 
iterators.

Contributor guide

Tech stack
typescript
Domain
developer experiencetooling
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
2
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.
stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
TypeScript basicscompiler error handling
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
65
Research direction
The error message 'Type 'string' is not an array type or a string type' is emitted by the TypeScript checker (likely in src/compiler/checker.ts). The fix involves updating the message to correctly reflect that string is iterable but requires downlevelIteration. Look for error code TS2569 and adjust the corresponding string. The issue comments provide additional context, including a suggestion to improve the message clarity.
Strange error: string is not a string type · Microsoft/TypeScript#41918 | Good First Issue