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 flagsfunctionusesEmptyObject(x:{}){if(x){console.log('truthy case')}else{console.log('truthy case')}}// becauseusesEmptyObject(false);// used to flag, no longer flagsfunctionusesNumberyObject(x:{toFixed:()=>string}){if(x){console.log('truthy case')}else{console.log('truthy case')}}// becauseusesNumberyObject(0);

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

constneverNull={};if(neverNull){}

That's due to TS's (questionable?) decision to giveneverNull type{} instead of typeobject, even when initializing aconst variable.

The following do still flag

constneverNull:object={};if(neverNull){}// this is a TS error tooif({}){}declarefunctionassert(x:unknown): assertsx;// flagged by no-unnecessary-condition and not a TS error.assert({});

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

kirkwaiblinger added4 commits

November 23, 2024 11:30

basics in place

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assignability

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November 23, 2024 12:56

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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

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I like this direction! Nicely done 🙂

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'};

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That's due to TS's (questionable?) decision to giveneverNull type{} instead of typeobject, even when initializing aconst 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.

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😩 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.

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

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

I hate the{} type so much.

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Regardingconst o = {}; // has type {}, started discord conversation athttps://discord.com/channels/508357248330760243/508357638677856287/1310419206260654081.

Regarding{} ->object assignments, filedmicrosoft/TypeScript#60582... and immediately learned this is a duplicate ofmicrosoft/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 forhttps://typescript-eslint.io/rules/no-empty-object-type/ as an outcome of these discussions 😆

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Nov 25, 2024

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

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Mar 19, 2025

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

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