EC
TS2861
TypeScriptERRORNotableTypeHIGH confidence

An object's '[Symbol.hasInstance]' method must return a boolean value for it ...

Production Risk

Build will fail; resolve before shipping.

What this means

A type-checking error (TS2861): An object's '[Symbol.hasInstance]' method must return a boolean value for it to be used on the right-hand side of an 'instanceof' expression.. This diagnostic is emitted by the TypeScript compiler when an object's '[Symbol.hasInstance]' method must return a boolean value for it to be used on the right-hand side of an 'instanceof' expression..

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

TypeScript compiler reports TS2861 during type checking.

trigger — this will error
trigger — this will error
// Triggers TS2861
// An object's '[Symbol.hasInstance]' method must return a boolean value for it to be used on the right-hand side of an 'instanceof' expression.

expected output

error TS2861: An object's '[Symbol.hasInstance]' method must return a boolean value for it to be used on the right-hand side of an 'instanceof' expression.

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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