EC
TS1091
TypeScriptERRORNotableSyntaxHIGH confidence

Only a single variable declaration is allowed in a 'for...in' statement.

Production Risk

Build will fail; resolve before shipping.

What this means

A syntax error (TS1091): Only a single variable declaration is allowed in a 'for...in' statement.. This diagnostic is emitted by the TypeScript compiler when only a single variable declaration is allowed in a 'for...in' statement..

Why it happens
  1. 1Incorrect TypeScript syntax or type usage
  2. 2Type mismatch between declared and actual value
How to reproduce

TypeScript compiler reports TS1091 during type checking.

trigger — this will error
trigger — this will error
// Triggers TS1091
// Only a single variable declaration is allowed in a 'for...in' statement.

expected output

error TS1091: Only a single variable declaration is allowed in a 'for...in' statement.

Fix

Address the TypeScript diagnostic

WHEN When this error is reported by the compiler

Address the TypeScript diagnostic
// Run the TypeScript compiler for details:
npx tsc --noEmit
// Use the TypeScript playground to test your code:
// https://www.typescriptlang.org/play

Why this works

Review the full error message and location; the TypeScript handbook provides guidance on each error category.

Sources
Official documentation ↗

TypeScript Compiler Diagnostics

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