EC
TS6621
TypeScriptERRORNotableConfigHIGH confidence

Emit more compliant, but verbose and less performant JavaScript for iteration.

Production Risk

Build will fail; resolve before shipping.

What this means

A compiler configuration error (TS6621): Emit more compliant, but verbose and less performant JavaScript for iteration.. This diagnostic is emitted by the TypeScript compiler when emit more compliant, but verbose and less performant JavaScript for iteration..

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

TypeScript compiler reports TS6621 during type checking.

trigger — this will error
trigger — this will error
// Triggers TS6621
// Emit more compliant, but verbose and less performant JavaScript for iteration.

expected output

error TS6621: Emit more compliant, but verbose and less performant JavaScript for iteration.

Fix

Fix the tsconfig.json option

WHEN When a compiler option is invalid or missing

Fix the tsconfig.json option
// tsconfig.json
{
  "compilerOptions": {
    // Review and correct the flagged option
  }
}
// Run: npx tsc --showConfig to see effective config

Why this works

TypeScript validates tsconfig.json options; check the official tsconfig reference for valid values.

Sources
Official documentation ↗

TypeScript Compiler Diagnostics

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