EC
TS1024
TypeScriptERRORNotableSyntaxHIGH confidence

'readonly' modifier can only appear on a property declaration or index signat...

Production Risk

Build will fail; resolve before shipping.

What this means

A syntax error (TS1024): 'readonly' modifier can only appear on a property declaration or index signature.. This diagnostic is emitted by the TypeScript compiler when 'readonly' modifier can only appear on a property declaration or index signature..

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

TypeScript compiler reports TS1024 during type checking.

trigger — this will error
trigger — this will error
// Triggers TS1024
// 'readonly' modifier can only appear on a property declaration or index signature.

expected output

error TS1024: 'readonly' modifier can only appear on a property declaration or index signature.

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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