fix: Prevent false positives with no-constant-condition #15486

captbaritone · 2022-01-04T22:59:11Z

Fixes #15467 by defining isConstant with inBooleanPosition set to false as meaning:

For both string and number, if coerced to that type, the value will be constant.

Prerequisites checklist

I have read the contributing guidelines.

What is the purpose of this pull request? (put an "X" next to an item)

[x] Bug fix (template)
[x] Changes an existing rule (template)

What changes did you make? (Give an overview)

Make no-constant-condition more correct. See #15467 for more information.

Is there anything you'd like reviewers to focus on?

Would ESLint treat this as a breaking change? It will remove some false positives, but also -- in a few rare cases -- introduce a few false negatives.

captbaritone · 2022-01-04T23:01:04Z

tests/lib/rules/no-constant-condition.js

@@ -262,8 +277,6 @@ ruleTester.run("no-constant-condition", rule, {

        // #5228 , typeof conditions
        { code: "if(typeof x){}", errors: [{ messageId: "unexpected", type: "UnaryExpression" }] },
-        { code: "if(`${typeof x}`){}", errors: [{ messageId: "unexpected", type: "TemplateLiteral" }] },
-        { code: "if(`${''}${typeof x}`){}", errors: [{ messageId: "unexpected", type: "TemplateLiteral" }] },


These are true errors that we won't catch any more.

captbaritone · 2022-01-04T23:02:44Z

tests/lib/rules/no-constant-condition.js

@@ -358,18 +371,6 @@ ruleTester.run("no-constant-condition", rule, {
            code: "if(''+[]) {}",
            errors: [{ messageId: "unexpected", type: "BinaryExpression" }]
        },
-        {
-            code: "if([a]==[a]) {}",


This is a true error that we won't catch any more, but it is equivalent to a false positive (if([a]==[b]) {}) which used to trigger.

Is if([a]==[b]) {} a false positive? Both sides are objects, so this should be comparison by reference, and thus always false (even when the elements are same)?

[1]==[1] // false

Ah, yes. Sorry, not sure what I was thinking.

That said, [a] cannot be considered "constant" for the purposes of == comparison unless we also know what we are comparing it to (and thus how it will be compared), and currently the rule does not try to determine that information.

[1] == 1 true [0] == 1 false

So, you're absolutely right that if([a]==[b]) {} is a constant condition, but I still think it's appropriate to treat [a] as not constant when comparing with inBooleanPosition: false.

Will no-constant-binary-expression catch these cases?

ArrayExpression === anything

ArrayExpression !== anything

ArrayExpression == ArrayExpression

ArrayExpression != ArrayExpression

I'm asking because these look like realistic mistakes that will be no longer reported by no-constant-condition, so it might be good to special-case them to retain that behavior. But if they will be reported by another rule, then there's no need for that.

These would get caught by no-constnat-binary-expression:

ArrayExpression === anything

ArrayExpression !== anything

These would not:

ArrayExpression == ArrayExpression

ArrayExpression != ArrayExpression

However, I suspect (hope!) that most people would be using some form of the eqeqeq rule to avoid the many many pitfalls and foot-guns exposed by testing for == equality.

These would not:

ArrayExpression == ArrayExpression

ArrayExpression != ArrayExpression

I've just left a question about this on the no-constant-binary-expression PR: #15296 (comment)

captbaritone · 2022-01-04T23:05:22Z

tests/lib/rules/no-constant-condition.js

-            errors: [{ messageId: "unexpected", type: "BinaryExpression" }]
-        },
-        {
-            code: "if([a] - '') {}",


This was a false positive. If a = 1 the condition is true, if a = 0 it is false.

captbaritone · 2022-01-04T23:05:29Z

tests/lib/rules/no-constant-condition.js

-            errors: [{ messageId: "unexpected", type: "BinaryExpression" }]
-        },
-        {
-            code: "if(+[a]) {}",


This was a false positive. If a = 1 the condition is true, if a = 0 it is false.

lib/rules/no-constant-condition.js

Fixes eslint#15467 by defining `isConstant` with `inBooleanPosition` set to `false` as meaning: > For both string and number, if coerced to that type, the value will be constant.

mdjermanovic

LGTM, thanks! I'll leave this open for a few days in case anyone else wants to review it before merging.

nzakas

LGTM

eslint-github-bot bot added triage An ESLint team member will look at this issue soon bug ESLint is working incorrectly labels Jan 4, 2022

captbaritone commented Jan 4, 2022

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captbaritone mentioned this pull request Jan 4, 2022

Bug: False positives with no-constant-condition #15467

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captbaritone force-pushed the fix-is-constant branch from bd5cc8c to feea6d1 Compare January 5, 2022 00:21

mdjermanovic added accepted There is consensus among the team that this change meets the criteria for inclusion rule Relates to ESLint's core rules and removed triage An ESLint team member will look at this issue soon labels Jan 6, 2022

mdjermanovic reviewed Jan 9, 2022

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fix: Prevent false positives with no-constant-condition

0456613

Fixes eslint#15467 by defining `isConstant` with `inBooleanPosition` set to `false` as meaning: > For both string and number, if coerced to that type, the value will be constant.

captbaritone force-pushed the fix-is-constant branch from feea6d1 to 0456613 Compare January 9, 2022 20:59

mdjermanovic approved these changes Jan 10, 2022

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nzakas approved these changes Jan 11, 2022

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nzakas merged commit 03ac8cf into eslint:main Jan 11, 2022

captbaritone mentioned this pull request Jan 11, 2022

feat: Add rule no-constant-binary-expression #15296

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sync-by-unito bot mentioned this pull request Feb 1, 2022

build(deps-dev): bump eslint from 7.32.0 to 8.8.0 numbersprotocol/capture-lite#1232

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eslint-github-bot bot added the archived due to age This issue has been archived; please open a new issue for any further discussion label Jul 11, 2022

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fix: Prevent false positives with no-constant-condition #15486

fix: Prevent false positives with no-constant-condition #15486

captbaritone commented Jan 4, 2022

captbaritone Jan 4, 2022

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fix: Prevent false positives with no-constant-condition #15486

fix: Prevent false positives with no-constant-condition #15486

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captbaritone commented Jan 4, 2022

Prerequisites checklist

What is the purpose of this pull request? (put an "X" next to an item)

What changes did you make? (Give an overview)

Is there anything you'd like reviewers to focus on?

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