EC
TS1228
TypeScriptERRORNotableSyntaxHIGH confidence

A type predicate is only allowed in return type position for functions and me...

Production Risk

Build will fail; resolve before shipping.

What this means

A syntax error (TS1228): A type predicate is only allowed in return type position for functions and methods.. This diagnostic is emitted by the TypeScript compiler when a type predicate is only allowed in return type position for functions and methods..

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

TypeScript compiler reports TS1228 during type checking.

trigger — this will error
trigger — this will error
// Triggers TS1228
// A type predicate is only allowed in return type position for functions and methods.

expected output

error TS1228: A type predicate is only allowed in return type position for functions and methods.

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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