no-extra-boolean-cast

Disallow unnecessary boolean casts

In contexts such as an if statement’s test where the result of the expression will already be coerced to a Boolean, casting to a Boolean via double negation (!!) or a Boolean call is unnecessary. For example, these if statements are equivalent:

if (!!foo) {
    // ...
}

if (Boolean(foo)) {
    // ...
}

if (foo) {
    // ...
}
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Rule Details

This rule disallows unnecessary boolean casts.

Examples of incorrect code for this rule:

/*eslint no-extra-boolean-cast: "error"*/

const foo = !!!bar;

const foo1 = !!bar ? baz : bat;

const foo2 = Boolean(!!bar);

const foo3 = new Boolean(!!bar);

if (!!foo) {
    // ...
}

if (Boolean(foo)) {
    // ...
}

while (!!foo) {
    // ...
}

do {
    // ...
} while (Boolean(foo));

for (; !!foo; ) {
    // ...
}
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Examples of correct code for this rule:

/*eslint no-extra-boolean-cast: "error"*/

const foo = !!bar;
const foo1 = Boolean(bar);

function qux() {
    return !!bar;
}

foo = bar ? !!baz : !!bat;
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Options

This rule has an object option:

• "enforceForInnerExpressions" when set to true, in addition to checking default contexts, checks whether extra boolean casts are present in expressions whose result is used in a boolean context. See examples below. Default is false, meaning that this rule by default does not warn about extra booleans cast inside inner expressions.

Deprecated: The object property enforceForLogicalOperands is deprecated (eslint#18222). Please use enforceForInnerExpressions instead.

enforceForInnerExpressions

Examples of incorrect code for this rule with "enforceForInnerExpressions" option set to true:

/*eslint no-extra-boolean-cast: ["error", {"enforceForInnerExpressions": true}]*/

if (!!foo || bar) {
    //...
}

while (!!foo && bar) {
    //...
}

if ((!!foo || bar) && !!baz) {
    //...
}

const foo = new Boolean(!!bar || baz);

foo && Boolean(bar) ? baz : bat;

const ternaryBranches = Boolean(bar ? !!baz : bat);

const nullishCoalescingOperator = Boolean(bar ?? Boolean(baz));

const commaOperator = Boolean((bar, baz, !!bat));

// another comma operator example
for (let i = 0; console.log(i), Boolean(i < 10); i++) {
    // ...
}
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Examples of correct code for this rule with "enforceForInnerExpressions" option set to true:

/*eslint no-extra-boolean-cast: ["error", {"enforceForInnerExpressions": true}]*/

// Note that `||` and `&&` alone aren't a boolean context for either operand
// since the resultant value need not be a boolean without casting.
const foo = !!bar || baz;

if (foo || bar) {
    //...
}

while (foo && bar) {
    //...
}

if ((foo || bar) && baz) {
    //...
}

const foo1 = new Boolean(bar || baz);

foo && bar ? baz : bat;

const ternaryBranches = Boolean(bar ? baz : bat);

const nullishCoalescingOperator = Boolean(bar ?? baz);

const commaOperator = Boolean((bar, baz, bat));

// another comma operator example
for (let i = 0; console.log(i), i < 10; i++) {
    // ...
}

// comma operator in non-final position
Boolean((Boolean(bar), baz, bat));
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Version

This rule was introduced in ESLint v0.4.0.

Resources

• Rule source
• Tests source