EC
TS2637
TypeScriptERRORNotableTypeHIGH confidence

Variance annotations are only supported in type aliases for object, function,...

Production Risk

Build will fail; resolve before shipping.

What this means

A type-checking error (TS2637): Variance annotations are only supported in type aliases for object, function, constructor, and mapped types.. This diagnostic is emitted by the TypeScript compiler when variance annotations are only supported in type aliases for object, function, constructor, and mapped types..

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

TypeScript compiler reports TS2637 during type checking.

trigger — this will error
trigger — this will error
// Triggers TS2637
// Variance annotations are only supported in type aliases for object, function, constructor, and mapped types.

expected output

error TS2637: Variance annotations are only supported in type aliases for object, function, constructor, and mapped types.

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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