134 lines
4.9 KiB
Plaintext
134 lines
4.9 KiB
Plaintext
---
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description: 'Disallow unnecessary equality comparisons against boolean literals.'
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---
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> 🛑 This file is source code, not the primary documentation location! 🛑
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>
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> See **https://typescript-eslint.io/rules/no-unnecessary-boolean-literal-compare** for documentation.
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Comparing boolean values to boolean literals is unnecessary: those comparisons result in the same booleans.
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Using the boolean values directly, or via a unary negation (`!value`), is more concise and clearer.
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This rule ensures that you do not include unnecessary comparisons with boolean literals.
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A comparison is considered unnecessary if it checks a boolean literal against any variable with just the `boolean` type.
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A comparison is **_not_** considered unnecessary if the type is a union of booleans (`string | boolean`, `SomeObject | boolean`, etc.).
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## Examples
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:::note
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Throughout this page, only strict equality (`===` and `!==`) are used in the examples.
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However, the implementation of the rule does not distinguish between strict and loose equality.
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Any example below that uses `===` would be treated the same way if `==` was used, and `!==` would be treated the same way if `!=` was used.
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:::
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<!--tabs-->
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### ❌ Incorrect
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```ts
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declare const someCondition: boolean;
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if (someCondition === true) {
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}
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```
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### ✅ Correct
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```ts
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declare const someCondition: boolean;
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if (someCondition) {
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}
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declare const someObjectBoolean: boolean | Record<string, unknown>;
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if (someObjectBoolean === true) {
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}
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declare const someStringBoolean: boolean | string;
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if (someStringBoolean === true) {
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}
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```
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## Options
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This rule always checks comparisons between a boolean variable and a boolean
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literal. Comparisons between nullable boolean variables and boolean literals
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are **not** checked by default.
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### `allowComparingNullableBooleansToTrue`
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Examples of code for this rule with `{ allowComparingNullableBooleansToTrue: false }`:
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<!--tabs-->
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#### ❌ Incorrect
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```ts option='{ "allowComparingNullableBooleansToTrue": false }'
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declare const someUndefinedCondition: boolean | undefined;
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if (someUndefinedCondition === true) {
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}
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declare const someNullCondition: boolean | null;
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if (someNullCondition !== true) {
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}
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```
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#### ✅ Correct
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```ts option='{ "allowComparingNullableBooleansToTrue": false }'
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declare const someUndefinedCondition: boolean | undefined;
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if (someUndefinedCondition) {
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}
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declare const someNullCondition: boolean | null;
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if (!someNullCondition) {
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}
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```
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### `allowComparingNullableBooleansToFalse`
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Examples of code for this rule with `{ allowComparingNullableBooleansToFalse: false }`:
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<!--tabs-->
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#### ❌ Incorrect
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```ts option='{ "allowComparingNullableBooleansToFalse": false }'
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declare const someUndefinedCondition: boolean | undefined;
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if (someUndefinedCondition === false) {
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}
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declare const someNullCondition: boolean | null;
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if (someNullCondition !== false) {
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}
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```
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#### ✅ Correct
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```ts option='{ "allowComparingNullableBooleansToFalse": false }'
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declare const someUndefinedCondition: boolean | undefined;
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if (someUndefinedCondition ?? true) {
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}
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declare const someNullCondition: boolean | null;
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if (!(someNullCondition ?? true)) {
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}
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```
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## Fixer
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| Comparison | Fixer Output | Notes |
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| :-------------------------------: | ------------------------------- | ----------------------------------------------------------------------------------- |
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| `booleanVar === true` | `booleanVar` | |
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| `booleanVar !== true` | `!booleanVar` | |
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| `booleanVar === false` | `!booleanVar` | |
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| `booleanVar !== false` | `booleanVar` | |
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| `nullableBooleanVar === true` | `nullableBooleanVar` | Only checked/fixed if the `allowComparingNullableBooleansToTrue` option is `false` |
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| `nullableBooleanVar !== true` | `!nullableBooleanVar` | Only checked/fixed if the `allowComparingNullableBooleansToTrue` option is `false` |
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| `!(nullableBooleanVar === false)` | `nullableBooleanVar ?? true` | Only checked/fixed if the `allowComparingNullableBooleansToFalse` option is `false` |
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| `!(nullableBooleanVar !== false)` | `!(nullableBooleanVar ?? true)` | Only checked/fixed if the `allowComparingNullableBooleansToFalse` option is `false` |
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## When Not To Use It
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Do not use this rule when `strictNullChecks` is disabled.
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ESLint is not able to distinguish between `false` and `undefined` or `null` values.
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This can cause unintended code changes when using autofix.
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