Microsoft/TypeScript

URLSearchParams/Headers constructor also receives iterables including Map

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#19,806 opened on Nov 7, 2017

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 (2 comments) (13 reactions) (0 assignees)TypeScript (48,455 stars) (6,726 forks)batch import
Domain: lib.d.tsExperience EnhancementHelp WantedSuggestion

Description

TypeScript Version: master

Code

new Headers(new Map([['abc', 'bcd']])).get('abc')
new Headers({ [Symbol.iterator]() { return new Map([['abc', 'bcd']]).entries() } }).get('abc')

Expected behavior: Both returns bcd so should pass without errors

Actual behavior: 'not assignable' error

Contributor guide

Tech stack
typescript
Domain
tooling
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
TypeScript knowledgeFamiliarity with iterablesUnderstanding of lib.dom.d.ts
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
55
Research direction
The issue describes that Headers and URLSearchParams constructors should accept iterables (e.g., Map) but currently produce a type error. To fix, locate the type definitions in lib.dom.d.ts and modify the constructor signatures to accept Iterable<[string, string]> or a similar generic iterable type. Check the existing comments on the issue for any suggested approaches or prior attempts.