JS.TS.NO.UNNECESSARY.BOOLEAN.LITERAL.COMPARE

Comparing boolean values to boolean literals is unnecessary, those comparisons result in the same booleans. Using the boolean values directly, or via a unary negation (!value), is more concise and clearer.

Rule Details

This rule ensures that you do not include unnecessary comparisons with boolean literals. A comparison is considered unnecessary if it checks a boolean literal against any variable with just the boolean type. A comparison is not considered unnecessary if the type is a union of booleans (string | boolean, someObject | boolean).

Warning: Do not use this rule when strictNullChecks is disabled. ESLint is not able to distinguish between false and undefined or null values. This can cause unintended code changes when using autofix.

Note: Throughout this page, only strict equality (=== and !==) are used in the examples. However, the implementation of the rule does not distinguish between strict and loose equality. Any example below that uses === would be treated the same way if == was used, and any example below that uses !== would be treated the same way if != was used.

Examples of code for this rule:

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Incorrect

declare const someCondition: boolean;
if (someCondition === true) {
}

Correct

declare const someCondition: boolean;
if (someCondition) {
}

declare const someObjectBoolean: boolean | Record<string, unknown>;
if (someObjectBoolean === true) {
}

declare const someStringBoolean: boolean | string;
if (someStringBoolean === true) {
}

Options

The rule accepts an options object with the following properties.

type Options = {
  // if false, comparisons between a nullable boolean variable to `true` will be checked and fixed
  allowComparingNullableBooleansToTrue?: boolean;
  // if false, comparisons between a nullable boolean variable to `false` will be checked and fixed
  allowComparingNullableBooleansToFalse?: boolean;
};

Defaults

This rule always checks comparisons between a boolean variable and a boolean literal. Comparisons between nullable boolean variables and boolean literals are not checked by default.

const defaults = {
  allowComparingNullableBooleansToTrue: true,
  allowComparingNullableBooleansToFalse: true,
};

allowComparingNullableBooleansToTrue

Examples of code for this rule with { allowComparingNullableBooleansToTrue: false }:

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Incorrect

declare const someUndefinedCondition: boolean | undefined;
if (someUndefinedCondition === true) {
}

declare const someNullCondition: boolean | null;
if (someNullCondition !== true) {
}

Correct

declare const someUndefinedCondition: boolean | undefined;
if (someUndefinedCondition) {
}

declare const someNullCondition: boolean | null;
if (!someNullCondition) {
}

allowComparingNullableBooleansToFalse

Examples of code for this rule with { allowComparingNullableBooleansToFalse: false }:

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Incorrect

declare const someUndefinedCondition: boolean | undefined;
if (someUndefinedCondition === false) {
}

declare const someNullCondition: boolean | null;
if (someNullCondition !== false) {
}

Correct

declare const someUndefinedCondition: boolean | undefined;
if (someUndefinedCondition ?? true) {
}

declare const someNullCondition: boolean | null;
if (!(someNullCondition ?? true)) {
}

Fixer

| Comparison | Fixer Output | Notes | | :----------------------------: | ------------------------------- | ----------------------------------------------------------------------------------- | | booleanVar === true | booleanVar | | | booleanVar !== true | !booleanVar | | | booleanVar === false | !booleanVar | | | booleanVar !== false | booleanVar | | | nullableBooleanVar === true | nullableBooleanVar | Only checked/fixed if the allowComparingNullableBooleansToTrue option is false | | nullableBooleanVar !== true | !nullableBooleanVar | Only checked/fixed if the allowComparingNullableBooleansToTrue option is false | | nullableBooleanVar === false | nullableBooleanVar ?? true | Only checked/fixed if the allowComparingNullableBooleansToFalse option is false | | nullableBooleanVar !== false | !(nullableBooleanVar ?? true) | Only checked/fixed if the allowComparingNullableBooleansToFalse option is false |