EC
TS1331
TypeScriptERRORNotableSyntaxHIGH confidence

A property of a class whose type is a 'unique symbol' type must be both 'stat...

Production Risk

Build will fail; resolve before shipping.

What this means

A syntax error (TS1331): A property of a class whose type is a 'unique symbol' type must be both 'static' and 'readonly'.. This diagnostic is emitted by the TypeScript compiler when a property of a class whose type is a 'unique symbol' type must be both 'static' and 'readonly'..

Why it happens
  1. 1Attempting to modify a property or variable declared as readonly
How to reproduce

TypeScript compiler reports TS1331 during type checking.

trigger — this will error
trigger — this will error
// Triggers TS1331
// A property of a class whose type is a 'unique symbol' type must be both 'static' and 'readonly'.

expected output

error TS1331: A property of a class whose type is a 'unique symbol' type must be both 'static' and 'readonly'.

Fix

Remove the readonly modifier or create a mutable copy

WHEN When you need to mutate a readonly property

Remove the readonly modifier or create a mutable copy
// Create a mutable copy
const mutable = { ...readonlyObject };
mutable.property = newValue;
// Or remove readonly from the type definition
type MyType = { property: string }; // not readonly

Why this works

readonly properties enforce immutability at the type level; create a copy to modify.

Sources
Official documentation ↗

TypeScript Compiler Diagnostics

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