Microsoft/TypeScript

Suggestion: stricter operators

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#7,989 opened on Apr 10, 2016

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 (14 comments) (15 reactions) (0 assignees)TypeScript (48,455 stars) (6,726 forks)batch import
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Description

Currently operators like "+" are defined such that they match their semantics in JS. The below are all allowed by the compiler and produce the shown values, even with --strictNullChecks on.

  • 2 + 'a' => "2a"
  • null + 'a' => "nulla" (!)
  • 2 - null => 2

I propose letting users opt in (maybe via some --strictOperators) to strict operator behavior. Concretely I think this means:

  • restrict +, and += to just number and string, e.g. for the former only declare

    function +(a: number, b: number): number;
function +(a: string, b: string): string;

  • restrict - and -= to just number

(any should continue to work as normal, of course.)

Relevant spec section: https://github.com/Microsoft/TypeScript/blob/master/doc/spec.md#419-binary-operators

See also "Expression operators" in the strictNullTypes change: https://github.com/Microsoft/TypeScript/pull/7140 and in particular this rationale: https://github.com/Microsoft/TypeScript/pull/7140#issuecomment-186432250

This would fall under of "stricter" TypeScript, https://github.com/Microsoft/TypeScript/issues/274 .

Contributor guide

Tech stack
typescript
Domain
developer experience
Issue type
feature
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
4
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
over 1 week
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 systemcompiler architecturestrictNullChecks implementation
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
15
Research direction
Review the TypeScript specification section on binary operators (https://github.com/Microsoft/TypeScript/blob/master/doc/spec.md#419 binary operators). Examine the existing strictNullChecks implementation in PR #7140 and the related rationale in the comments. Understand the compiler's type checking for operators in the checker.ts file. Propose a design that extends the strict mode to include operator restrictions while maintaining compatibility with 'any' type.