EC
TS2406
TypeScriptERRORNotableTypeHIGH confidence

The left-hand side of a 'for...in' statement must be a variable or a property...

Production Risk

Build will fail; resolve before shipping.

What this means

A type-checking error (TS2406): The left-hand side of a 'for...in' statement must be a variable or a property access.. This diagnostic is emitted by the TypeScript compiler when the left-hand side of a 'for...in' statement must be a variable or a property access..

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

TypeScript compiler reports TS2406 during type checking.

trigger — this will error
trigger — this will error
// Triggers TS2406
// The left-hand side of a 'for...in' statement must be a variable or a property access.

expected output

error TS2406: The left-hand side of a 'for...in' statement must be a variable or a property access.

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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