EC
TS6720
TypeScriptERRORNotableConfigHIGH confidence

Built-in iterators are instantiated with a 'TReturn' type of 'undefined' inst...

Production Risk

Build will fail; resolve before shipping.

What this means

A compiler configuration error (TS6720): Built-in iterators are instantiated with a 'TReturn' type of 'undefined' instead of 'any'.. This diagnostic is emitted by the TypeScript compiler when built-in iterators are instantiated with a 'TReturn' type of 'undefined' instead of 'any'..

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

TypeScript compiler reports TS6720 during type checking.

trigger — this will error
trigger — this will error
// Triggers TS6720
// Built-in iterators are instantiated with a 'TReturn' type of 'undefined' instead of 'any'.

expected output

error TS6720: Built-in iterators are instantiated with a 'TReturn' type of 'undefined' instead of 'any'.

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.

What not to do

✕ Suppress with @ts-ignore instead of fixing the type

ts-ignore hides real type errors and makes refactoring unsafe.

Sources
Official documentation ↗

TypeScript Compiler Diagnostics

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