fix: [no-unnecessary-condition] use assignability APIs in no-unnecessary-condition (POC) #10378

kirkwaiblinger · 2024-11-23T19:10:39Z

PR Checklist

Addresses an existing open issue: fixes #000
That issue was marked as accepting prs
Steps in Contributing were taken

Overview

This fundamentally changes the truthiness/falsiness algorithm used in no-unnecessary-condition. Up until now, the rule has checked various heuristics for possible falsiness: essentially whether the type is a known falsy literal and whether the type has TS's (inconsistent) PossiblyFalsy flag set. The proposed change is to use the assignability API to simply check whether one of JS's few falsy types is assignable to the type being investigated.

This has a few interesting consequences that improve correctness....

// used to flag, no longer flags
function usesEmptyObject(x: {}) {
  if (x) {
    console.log('truthy case')
  } else {
    console.log('truthy case')
  }
}
// because 
usesEmptyObject(false);

// used to flag, no longer flags
function usesNumberyObject(x: { toFixed: () => string }) {
  if (x) {
    console.log('truthy case')
  } else {
    console.log('truthy case')
  }
}
// because 
usesNumberyObject(0);

Annoyingly, this also has the drawback that the following no longer flags.

const neverNull = {};
if (neverNull) {
}

That's due to TS's (questionable?) decision to give neverNull type {} instead of type object, even when initializing a const variable.

The following do still flag

const neverNull: object = {};
if (neverNull) {
}

// this is a TS error too
if ({}) {
}

declare function assert(x: unknown): asserts x;
// flagged by no-unnecessary-condition and not a TS error.
assert({});

If that specific case is something we want to preserve, we could do so with scope analysis.

typescript-eslint · 2024-11-23T19:10:42Z

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codecov · 2024-11-23T19:28:21Z

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.67%. Comparing base (88e4c66) to head (0b8d6b8).
Report is 10 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10378      +/-   ##
==========================================
- Coverage   86.69%   86.67%   -0.03%     
==========================================
  Files         434      434              
  Lines       15227    15226       -1     
  Branches     4445     4442       -3     
==========================================
- Hits        13201    13197       -4     
- Misses       1673     1675       +2     
- Partials      353      354       +1

Flag	Coverage Δ
unittest	`86.67% <100.00%> (-0.03%)`	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...slint-plugin/src/rules/no-unnecessary-condition.ts	`98.00% <100.00%> (-1.21%)`	⬇️

JoshuaKGoldberg

I like this direction! Nicely done 🙂

JoshuaKGoldberg · 2024-11-24T18:13:59Z

packages/eslint-plugin/docs/rules/no-unnecessary-condition.mdx

@@ -107,7 +107,7 @@ function assert(condition: unknown): asserts condition {

 assert(false); // Unnecessary; condition is always falsy.

-const neverNull = {};
+const neverNull = { someProperty: 'someValue' };


That's due to TS's (questionable?) decision to give neverNull type {} instead of type object, even when initializing a const variable.

😩 this again... is there a TypeScript issue filed that we can reference? It feels like a bug to me.

I also don't mind it very much. It's not often that folks use {}. This docs change kind of makes it more realistic to me.

😩 this again... is there a TypeScript issue filed that we can reference? It feels like a bug to me.

That's a good question. I'll take an action item to investigate this.

I also don't mind it very much. It's not often that folks use {}. This docs change kind of makes it more realistic to me.

Agreed that the docs change is probably an improvement. But, the false negative here makes the rule a little less understandable to me, so I'd lean on the side of wanting it to be correct if we can get it to be.

Oh, dear, the moment I start playing around with this, I immediately find infuriating behavior around {}....

function takesObject(x: object) { if (x) { console.log('should always be true') } else { console.error('this should be unreachable'); } } const o: {} = 0; // no error here!?!?!?!? takesObject(o);

I hate the {} type so much.

Regarding const o = {}; // has type {}, started discord conversation at https://discord.com/channels/508357248330760243/508357638677856287/1310419206260654081.

Regarding {} -> object assignments, filed microsoft/TypeScript#60582... and immediately learned this is a duplicate of microsoft/TypeScript#56205, in which this behavior is deemed intentional 🫤.

Reminder to self: I'm pretty sure we'll have some good info to add to the motivation for https://typescript-eslint.io/rules/no-empty-object-type/ as an outcome of these discussions 😆

kirkwaiblinger added 4 commits November 23, 2024 11:30

basics in place

b6139da

simplify

cc887e5

add back comment

da1887c

assignability

c2b72da

kirkwaiblinger added 2 commits November 23, 2024 12:56

more tests

d8b30bc

comments

0b8d6b8

kirkwaiblinger changed the title ~~POC - using assignability APIs in no-unnecessary-condition~~ fix: [no-unnecessary-condition] use assignability APIs in no-unnecessary-condition (POC) Nov 23, 2024

JoshuaKGoldberg reviewed Nov 24, 2024

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kirkwaiblinger mentioned this pull request Nov 25, 2024

Rethinking relationships between {} type, object, and primitives microsoft/TypeScript#60582

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kirkwaiblinger added the DO NOT MERGE PRs which should not be merged yet label Dec 1, 2024

kirkwaiblinger mentioned this pull request Mar 19, 2025

Docs: [no-unnecessary-condition] mention subtleties of primitives' assignability #10970

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fix: [no-unnecessary-condition] use assignability APIs in no-unnecessary-condition (POC) #10378

fix: [no-unnecessary-condition] use assignability APIs in no-unnecessary-condition (POC) #10378

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fix: [no-unnecessary-condition] use assignability APIs in no-unnecessary-condition (POC) #10378

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fix: [no-unnecessary-condition] use assignability APIs in no-unnecessary-condition (POC) #10378

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