EC
TS1055
TypeScriptERRORNotableSyntaxHIGH confidence

Type 'X' is not a valid async function return type in ES5 because it does not...

Production Risk

Build will fail; resolve before shipping.

What this means

A syntax error (TS1055): Type 'X' is not a valid async function return type in ES5 because it does not refer to a Promise-compatible constructor value.. This diagnostic is emitted by the TypeScript compiler when type 'X' is not a valid async function return type in ES5 because it does not refer to a Promise-compatible constructor value..

Why it happens
  1. 1await used outside an async function, or async/await return type mismatch
How to reproduce

TypeScript compiler reports TS1055 during type checking.

trigger — this will error
trigger — this will error
// Triggers TS1055
// Type 'X' is not a valid async function return type in ES5 because it does not refer to a Promise-compatible constructor value.

expected output

error TS1055: Type 'X' is not a valid async function return type in ES5 because it does not refer to a Promise-compatible constructor value.

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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