EC
TS8009
TypeScriptERRORNotableStrictHIGH confidence

The 'X' modifier can only be used in TypeScript files.

Production Risk

Build will fail; resolve before shipping.

What this means

A strict mode error (TS8009): The 'X' modifier can only be used in TypeScript files.. This diagnostic is emitted by the TypeScript compiler when the 'X' modifier can only be used in TypeScript files..

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

TypeScript compiler reports TS8009 during type checking.

trigger — this will error
trigger — this will error
// Triggers TS8009
// The 'X' modifier can only be used in TypeScript files.

expected output

error TS8009: The 'X' modifier can only be used in TypeScript files.

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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