EC
TS1239
TypeScriptERRORNotableSyntaxHIGH confidence

Unable to resolve signature of parameter decorator when called as an expression.

Production Risk

Build will fail; resolve before shipping.

What this means

A syntax error (TS1239): Unable to resolve signature of parameter decorator when called as an expression.. This diagnostic is emitted by the TypeScript compiler when unable to resolve signature of parameter decorator when called as an expression..

Why it happens
  1. 1A decorator is applied incorrectly or to an unsupported target
  2. 2The 'experimentalDecorators' flag may need to be enabled
How to reproduce

TypeScript compiler reports TS1239 during type checking.

trigger — this will error
trigger — this will error
// Triggers TS1239
// Unable to resolve signature of parameter decorator when called as an expression.

expected output

error TS1239: Unable to resolve signature of parameter decorator when called as an expression.

Fix

Enable experimentalDecorators

WHEN Using legacy decorator syntax

Enable experimentalDecorators
// tsconfig.json
{
  "compilerOptions": {
    "experimentalDecorators": true,
    "emitDecoratorMetadata": true
  }
}

Why this works

Decorators require the 'experimentalDecorators' flag; TypeScript 5.0+ also supports the TC39 Stage 3 decorator proposal without the flag.

Sources
Official documentation ↗

TypeScript Compiler Diagnostics

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