EC
TS2802
TypeScriptERRORNotableTypeHIGH confidence

Type 'X' can only be iterated through when using the '--downlevelIteration' f...

Production Risk

Build will fail; resolve before shipping.

What this means

A type-checking error (TS2802): Type 'X' can only be iterated through when using the '--downlevelIteration' flag or with a '--target' of 'es2015' or higher.. This diagnostic is emitted by the TypeScript compiler when type 'X' can only be iterated through when using the '--downlevelIteration' flag or with a '--target' of 'es2015' or higher..

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

TypeScript compiler reports TS2802 during type checking.

trigger — this will error
trigger — this will error
// Triggers TS2802
// Type 'X' can only be iterated through when using the '--downlevelIteration' flag or with a '--target' of 'es2015' or higher.

expected output

error TS2802: Type 'X' can only be iterated through when using the '--downlevelIteration' flag or with a '--target' of 'es2015' or higher.

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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