EC
TS1252
TypeScriptERRORNotableSyntaxHIGH confidence

Function declarations are not allowed inside blocks in strict mode when targe...

Production Risk

Build will fail; resolve before shipping.

What this means

A syntax error (TS1252): Function declarations are not allowed inside blocks in strict mode when targeting 'ES5'. Modules are automatically in strict mode.. This diagnostic is emitted by the TypeScript compiler when function declarations are not allowed inside blocks in strict mode when targeting 'ES5'. Modules are automatically in strict mode..

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

TypeScript compiler reports TS1252 during type checking.

trigger — this will error
trigger — this will error
// Triggers TS1252
// Function declarations are not allowed inside blocks in strict mode when targeting 'ES5'. Modules are automatically in strict mode.

expected output

error TS1252: Function declarations are not allowed inside blocks in strict mode when targeting 'ES5'. Modules are automatically in strict mode.

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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