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feat(lint): add ignoreBooleanCoercion option to useNullishCoalescing
- suppresses || and ||= when the value is boolean-coerced by an enclosing Boolean() call - walks out through parens and logical parents, so chained and && cases inside Boolean() are covered too - partially addresses #9232, follows on from #9974
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‎.changeset/add-ignore-boolean-coercion.md‎

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---
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"@biomejs/biome": patch
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---
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Added `ignoreBooleanCoercion` to [useNullishCoalescing](https://biomejs.dev/linter/rules/use-nullish-coalescing/), partially addressing [#9232](https://github.com/biomejs/biome/issues/9232). When enabled, Biome ignores `||` and `||=` used inside a `Boolean()` call, where coalescing on falsy values is intentional.

‎crates/biome_js_analyze/src/lint/nursery/use_nullish_coalescing.rs‎

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@@ -7,9 +7,10 @@ use biome_diagnostics::Severity;
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use biome_js_factory::make;
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use biome_js_syntax::{
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AnyJsAssignmentPattern, AnyJsExpression, JsAssignmentExpression, JsAssignmentOperator,
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JsBinaryOperator, JsConditionalExpression, JsDoWhileStatement, JsForStatement,
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JsIfStatement, JsLogicalExpression, JsLogicalOperator, JsParenthesizedExpression,
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JsSyntaxKind, JsWhileStatement, OperatorPrecedence, T,
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JsBinaryOperator, JsCallArgumentList, JsCallArguments, JsCallExpression,
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JsConditionalExpression, JsDoWhileStatement, JsForStatement, JsIfStatement,
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JsLogicalExpression, JsLogicalOperator, JsParenthesizedExpression, JsSyntaxKind,
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JsWhileStatement, OperatorPrecedence, T,
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};
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use biome_js_type_info::{ConditionalType, TypeData};
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use biome_rowan::{
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/// assigned ||= b && 'fallback';
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/// ```
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///
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/// ### ignoreBooleanCoercion
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///
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/// Ignore `||` and `||=` used inside a `Boolean()` call, where coalescing on
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/// falsy values is intentional. Default: `false`.
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///
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/// ```json,options
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/// {
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/// "options": {
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/// "ignoreBooleanCoercion": true
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/// }
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/// }
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/// ```
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///
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/// #### Invalid
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///
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/// `||` and `||=` outside a `Boolean()` call are still reported.
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///
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/// ```ts,expect_diagnostic,use_options,file=invalid-boolean-coercion.ts
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/// declare const maybeString: string | null;
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/// const value = maybeString || 'default';
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/// ```
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///
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/// #### Valid
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///
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/// `||` and `||=` inside a `Boolean()` call are not reported.
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///
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/// ```ts,use_options,file=valid-boolean-coercion.ts
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/// declare const a: string | null;
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/// declare const b: string;
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/// const r = Boolean(a || b);
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/// ```
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///
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pub UseNullishCoalescing {
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version: "2.4.5",
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name: "useNullishCoalescing",
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return None;
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}
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if options.ignore_boolean_coercion()
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&& is_in_boolean_call_context(&AnyJsLogicalOrLikeExpression::from(logical.clone()))
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{
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return None;
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}
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let left = logical.left().ok()?;
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let left_ty = ctx.type_of_expression(&left);
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return None;
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}
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if options.ignore_boolean_coercion()
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&& is_in_boolean_call_context(&AnyJsLogicalOrLikeExpression::from(assignment.clone()))
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{
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return None;
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}
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let left = assignment.left().ok()?;
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let left_ty = match &left {
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AnyJsAssignmentPattern::AnyJsAssignment(assign) => {
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})
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}
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/// Returns `true` when the `||`/`||=` expression is boolean-coerced by an
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/// enclosing `Boolean(...)` call, where coalescing on falsy values is the
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/// intended behavior.
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///
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/// It skips parenthesized and logical-expression ancestors (both `||` and `&&`,
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/// since the coerced value still flows into the call) so that
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/// `Boolean(a || b || c)` and `Boolean(x && (a || b))` are both treated as
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/// coerced, then inspects the first ancestor that is neither. That ancestor must
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/// be the argument list of a `Boolean(...)` call for the expression to be
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/// considered coerced.
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fn is_in_boolean_call_context(node: &AnyJsLogicalOrLikeExpression) -> bool {
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node.syntax()
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.ancestors()
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.skip(1)
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.find(|ancestor| {
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!(JsParenthesizedExpression::can_cast(ancestor.kind())
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|| JsLogicalExpression::can_cast(ancestor.kind()))
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})
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.and_then(JsCallArgumentList::cast)
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.and_then(|list| list.parent::<JsCallArguments>())
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.and_then(|args| args.parent::<JsCallExpression>())
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.is_some_and(|call| call.has_callee("Boolean"))
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}
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fn is_in_test_position(logical: &JsLogicalExpression) -> bool {
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let logical_range = logical.syntax().text_trimmed_range();
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‎crates/biome_js_analyze/tests/specs/nursery/useNullishCoalescing/ignoreBooleanCoercionDisabled.options.json‎

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{
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"$schema": "../../../../../../packages/@biomejs/biome/configuration_schema.json",
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"linter": {
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"rules": {
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"nursery": {
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"useNullishCoalescing": {
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"level": "error",
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"options": {
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"ignoreBooleanCoercion": false
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}
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}
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}
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}
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}
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}

‎crates/biome_js_analyze/tests/specs/nursery/useNullishCoalescing/ignoreBooleanCoercionDisabled.ts‎

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// Tests for ignoreBooleanCoercion: false
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// `||` and `||=` inside a Boolean() call SHOULD still generate diagnostics.
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declare const a: string | null;
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declare const b: string;
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declare const c: string;
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// || directly inside Boolean()
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const r1 = Boolean(a || b);
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// chained || inside Boolean()
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const r2 = Boolean(a || b || c);
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// ||= inside Boolean()
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declare let assigned: string | null;
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const r3 = Boolean(assigned ||= b);

‎crates/biome_js_analyze/tests/specs/nursery/useNullishCoalescing/ignoreBooleanCoercionDisabled.ts.snap‎

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---
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source: crates/biome_js_analyze/tests/spec_tests.rs
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expression: ignoreBooleanCoercionDisabled.ts
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---
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# Input
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```ts
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// Tests for ignoreBooleanCoercion: false
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// `||` and `||=` inside a Boolean() call SHOULD still generate diagnostics.
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declare const a: string | null;
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declare const b: string;
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declare const c: string;
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// || directly inside Boolean()
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const r1 = Boolean(a || b);
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// chained || inside Boolean()
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const r2 = Boolean(a || b || c);
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// ||= inside Boolean()
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declare let assigned: string | null;
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const r3 = Boolean(assigned ||= b);
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```
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# Diagnostics
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```
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ignoreBooleanCoercionDisabled.ts:9:22 lint/nursery/useNullishCoalescing ━━━━━━━━━━━━━━━━━━━━━━━━━━━━
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i Use ?? instead of ||.
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8 │ // || directly inside Boolean()
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> 9 │ const r1 = Boolean(a || b);
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│ ^^
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10 │
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11 │ // chained || inside Boolean()
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i The || operator checks for all falsy values (including 0, '', and false), while ?? only checks for null and undefined.
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i This rule is still being actively worked on, so it may be missing features or have rough edges. Visit https://github.com/biomejs/biome/issues/8043 for more information or to report possible bugs.
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i This rule belongs to the nursery group, which means it is not yet stable and may change in the future. Visit https://biomejs.dev/linter/#nursery for more information.
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```
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```
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ignoreBooleanCoercionDisabled.ts:12:22 lint/nursery/useNullishCoalescing ━━━━━━━━━━━━━━━━━━━━━━━━━━━
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i Use ?? instead of ||.
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11 │ // chained || inside Boolean()
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> 12 │ const r2 = Boolean(a || b || c);
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│ ^^
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13 │
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14 │ // ||= inside Boolean()
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i The || operator checks for all falsy values (including 0, '', and false), while ?? only checks for null and undefined.
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i This rule is still being actively worked on, so it may be missing features or have rough edges. Visit https://github.com/biomejs/biome/issues/8043 for more information or to report possible bugs.
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i This rule belongs to the nursery group, which means it is not yet stable and may change in the future. Visit https://biomejs.dev/linter/#nursery for more information.
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```
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```
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ignoreBooleanCoercionDisabled.ts:16:29 lint/nursery/useNullishCoalescing ━━━━━━━━━━━━━━━━━━━━━━━━━━━
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i Use ??= instead of ||=.
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14 │ // ||= inside Boolean()
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15 │ declare let assigned: string | null;
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> 16 │ const r3 = Boolean(assigned ||= b);
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│ ^^^
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17 │
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i The ||= operator assigns when the left side is falsy, while ??= only assigns when it is null or undefined.
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i This rule is still being actively worked on, so it may be missing features or have rough edges. Visit https://github.com/biomejs/biome/issues/8043 for more information or to report possible bugs.
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i This rule belongs to the nursery group, which means it is not yet stable and may change in the future. Visit https://biomejs.dev/linter/#nursery for more information.
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```

‎crates/biome_js_analyze/tests/specs/nursery/useNullishCoalescing/ignoreBooleanCoercionEnabled.options.json‎

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{
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"$schema": "../../../../../../packages/@biomejs/biome/configuration_schema.json",
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"linter": {
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"rules": {
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"nursery": {
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"useNullishCoalescing": {
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"level": "error",
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"options": {
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"ignoreBooleanCoercion": true
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}
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}
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}
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}
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}
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}

‎crates/biome_js_analyze/tests/specs/nursery/useNullishCoalescing/ignoreBooleanCoercionEnabled.ts‎

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// Tests for ignoreBooleanCoercion: true
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// `||` and `||=` inside a Boolean() call should NOT generate diagnostics.
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declare const a: string | null;
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declare const b: string;
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declare const c: string;
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// || directly inside Boolean()
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const r1 = Boolean(a || b);
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// chained || inside Boolean()
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const r2 = Boolean(a || b || c);
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// || inside parenthesized argument
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const r3 = Boolean((a || b));
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// ||= inside Boolean()
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declare let assigned: string | null;
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const r4 = Boolean(assigned ||= b);
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// || nested inside Boolean() within another call: still suppressed because the
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// value still flows into the Boolean() coercion.
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declare function identity<T>(value: T): T;
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const r4b = identity(Boolean(a || 'default'));
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// || mixed with && inside Boolean(): the whole argument is boolean-coerced,
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// so the walk steps through the && parent and suppresses the ||.
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declare const d: string;
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const r4c = Boolean(d && (a || 'default'));
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// These SHOULD still generate diagnostics (not inside Boolean()).
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const r5 = a || 'default';
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declare const n: number | undefined;
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const r6 = n || 0;
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declare let assignPlain: string | null;
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assignPlain ||= 'default';
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// Inside a non-Boolean call: SHOULD generate a diagnostic.
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const r7 = identity(a || 'default');

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