8000 Rule proposal: Detect useless default in function parameters and object destructuring · Issue #6106 · typescript-eslint/typescript-eslint · GitHub
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Rule proposal: Detect useless default in function parameters and object destructuring #6106

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RunDevelopment opened this issue Nov 27, 2022 · 9 comments
Open
6 tasks done

Rule proposal: Detect useless default in function parameters and object destructuring #6106

RunDevelopment opened this issue Nov 27, 2022 · 9 comments
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accepting prs Go ahead, send a pull request that resolves this issue enhancement: new plugin rule New rule request for eslint-plugin package: eslint-plugin Issues related to @typescript-eslint/eslint-plugin

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@RunDevelopment
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Before You File a Proposal Please Confirm You Have Done The Following...

My proposal is suitable for this project

  • My proposal specifically checks TypeScript syntax, or it proposes a check that requires type information to be accurate.
  • My proposal is not a "formatting rule"; meaning it does not just enforce how code is formatted (whitespace, brace placement, etc).
  • I believe my proposal would be useful to the broader TypeScript community (meaning it is not a niche proposal).

Description

Default parameters and default values are only used if the parameter or property is undefined (either because its value is undefined or because it is missing). So if we know that a function parameter or property can never be undefined, then we also know that the default is never actually used. This can be used to detect useless defaults.

Fail Cases

// React props destructuring
function Bar({ foo = "" }: { foo: string }) { /* ... */ }

// destructuring assignment
const { foo = "" } = { foo: "bar" }

// default parameter
[1, 2, 3].map((a = 42) => a + 1)

Pass Cases

// React props destructuring
function Bar({ foo }: { foo: string }) { /* ... */ }
function Bar({ foo = "" }: { foo?: string }) { /* ... */ }

// destructuring assignment
const { foo } = { foo: "bar" }

// default parameter
[1, 2, 3].map((a) => a + 1)
[1, 2, 3, undefined].map((a = 42) => a + 1)

Additional Info

This rule requires strict null checks to be enabled.
@RunDevelopment RunDevelopment added enhancement: new plugin rule New rule request for eslint-plugin package: eslint-plugin Issues related to @typescript-eslint/eslint-plugin triage Waiting for team members to take a look labels Nov 27, 2022
@JoshuaKGoldberg

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@JoshuaKGoldberg JoshuaKGoldberg added accepting prs Go ahead, send a pull request that resolves this issue and removed triage Waiting for team members to take a look labels Nov 27, 2022
@JoshuaKGoldberg
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Yeah this would be useful, +1. Thanks for filing!

Note that the considerations mentioned in #5748 around making sure users enable strictNullChecks should be followed in this rule.

@ronami ronami mentioned this issue Oct 1, 2024
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@ronami
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ronami commented Oct 4, 2024

Would it make sense for this to be split into two rules (e.g. no-useless-default-parameters and no-useless-default-values)?

These two examples check for unnecessary destructuring default values:

function Bar({ foo = "" }: { foo: string }) { /* ... */ }

const { foo = "" } = { foo: "bar" }

While this is for redundant default function parameters:

[1, 2, 3].map((a = 42) => a + 1)

@bradzacher
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I don't see a reason you'd want one check but not the other.
I'd probably name it something like no-useless-default-assignment and cover both cases in one rule.
Similarly as I mentioned here

could just expand this to an extension rule for no-useless-assignment.

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