Microsoft/TypeScript

String.replace does not accept string | (string, ...any[]) => string replacer

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#22,378 opened on Mar 7, 2018

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 (9 comments) (9 reactions) (0 assignees)TypeScript (48,455 stars) (6,726 forks)batch import
BugDomain: lib.d.tsHelp Wanted

Description

TypeScript Version: 2.7.2

Search Terms: String.replace overload union parameter

Code

type ReplacerFn = () => string;
type Replacer = string | ReplacerFn;

function replaceFoo(str: string, replacer: Replacer) {
    return str.replace(new RegExp("foo"), replacer);
}

Expected behavior: Compiles successfully.

Perhaps the change is as simple as combining the two overloads into a union, though the implications of that change are possibly unpleasant as far as tooling is concerned.

Actual behavior: bad.ts(5,43): error TS2345: Argument of type 'Replacer' is not assignable to parameter of type '(substring: string, ...args: any[]) => string'. Type 'string' is not assignable to type '(substring: string, ...args: any[]) => string'.

Playground Link: playground link

Related Issues: #20432 (Array.from) #20215 (new Date) #14107 (underlying issue)

Contributor guide

Tech stack
typescriptjavascript
Domain
developer experience
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.
half day
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 type systemfunction overloadsunion types
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
40
Research direction
Investigate the current type definitions for String.prototype.replace in lib.es5.d.ts. The proposed fix is to combine two overloads into a union. Review related issues #20432 and #20215 for analogous changes. Consider implications for tooling and type inference.