no-unnecessary-boolean-literal-compare.md

Flags unnecessary equality comparisons against boolean literals (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).

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 incorrect code for this rule:

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

Examples of correct code for this rule

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 incorrect code for this rule with { allowComparingNullableBooleansToTrue: false }:

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

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

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

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

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

allowComparingNullableBooleansToFalse

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

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

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

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

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

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

Fixer

Comparison	Fixer Output	Notes
booleanVar === true	booleanLiteral
booleanVar !== true	!booleanLiteral
booleanVar === false	!booleanLiteral
booleanVar !== false	booleanLiteral
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

Related to

• TSLint: no-boolean-literal-compare