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novemberborn opened this issue Jan 26, 2018 · 6 comments
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Disallow implicit boolean casting of strings and numbers #49

novemberborn opened this issue Jan 26, 2018 · 6 comments
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enhancement: new plugin rule New rule request for eslint-plugin has pr there is a PR raised to close this package: eslint-plugin Issues related to @typescript-eslint/eslint-plugin

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@novemberborn
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It'd be great to have a lint rule that disallows (possible) strings and numbers from being implicitly converted to booleans:

const str: string | null = ''
if (str) {
  // ignores the empty string, the programmer likely attempted to guard against null
}

const num: number | undefined = 0
if (!num) {
  // ignores the 0 value, but the programmer likely attempted to guard against undefined
}

Besides if statements this would apply to ternary statements as well as &&, || and ! operators.

For the above examples passing code could be:

const str: string | null = ''
if (typeof str === 'string') {
  // …
}
if (str !== null) {
  // …
}

const num: number | undefined = 0
if (typeof num !== 'undefined') {
  // ...
}

I can see a stricter version of this rule which enforces explicit conversions even if a value can only be a string or number.
@j-f1
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j-f1 commented Jan 26, 2018

We’re interested in doing this in GitHub Desktop: desktop/desktop#3538. I tried to add a rule in desktop/desktop#3550, but the type information available was limited, and @iAmWillShepherd suggested that we try to write this rule with TSLint instead.

@novemberborn
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the type information available was limited

I suppose you'd need to use the Language Service API to access accurate type information, especially when you're accessing properties that were defined in a separate file. Not sure how easy it'd be to integrate that into ESLint's lifecycle.

@Coder-256
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Bump! I would love to see this too. This could be added as an option for the no-implicit-coercion rule's boolean property, because if (x) does an implicit boolean conversion, just the same as !!x.

@bradzacher
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flow supports this via their sketchy-null rules.
https://flow.org/en/docs/linting/rule-reference/#toc-sketchy-null

Speaking from experience over my last few months at facebook, it's actually really, really annoying; most engineers I've spoken to hate it!

But if there is demand from the community, it's a rule we could look into.

@JamesHenry JamesHenry transferred this issue from bradzacher/eslint-plugin-typescript Jan 18, 2019
@bradzacher bradzacher added enhancement New feature or request package: eslint-plugin Issues related to @typescript-eslint/eslint-plugin enhancement: new plugin rule New rule request for eslint-plugin labels Jan 18, 2019
@SpainTrain
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As a temporary workaround, one can use @typescript-eslint/eslint-plugin-tslint with tslint rule strict-boolean-expressions (https://github.com/palantir/tslint/blob/master/src/rules/strictBooleanExpressionsRule.ts).

I miss this feature from Flow and would love to see this as a rule. Chatter I saw on the TypeScript tracker indicated it is unlikely to be added to the compiler.

@novemberborn
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strict-boolean-expressions is now available: https://github.com/typescript-eslint/typescript-eslint/blob/master/packages/eslint-plugin/docs/rules/strict-boolean-expressions.md

@bradzacher bradzacher added has pr there is a PR raised to close this and removed enhancement New feature or request labels Jul 12, 2019
@typescript-eslint typescript-eslint locked as resolved and limited conversation to collaborators Feb 21, 2020
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enhancement: new plugin rule New rule request for eslint-plugin has pr there is a PR raised to close this package: eslint-plugin Issues related to @typescript-eslint/eslint-plugin
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@novemberborn @SpainTrain @bradzacher @Coder-256 @j-f1