EC
TS1274
TypeScriptERRORNotableSyntaxHIGH confidence

'X' modifier can only appear on a type parameter of a class, interface or typ...

Production Risk

Build will fail; resolve before shipping.

What this means

A syntax error (TS1274): 'X' modifier can only appear on a type parameter of a class, interface or type alias. This diagnostic is emitted by the TypeScript compiler when 'X' modifier can only appear on a type parameter of a class, interface or type alias.

Why it happens
  1. 1Incorrect number or type of generic type arguments
How to reproduce

TypeScript compiler reports TS1274 during type checking.

trigger — this will error
trigger — this will error
// Triggers TS1274
// 'X' modifier can only appear on a type parameter of a class, interface or type alias

expected output

error TS1274: 'X' modifier can only appear on a type parameter of a class, interface or type alias

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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