useNullishCoalescing

JavaScript (and super languages)

Summary

Rule available since: v2.4.5
Diagnostic Category: lint/nursery/useNullishCoalescing
This rule has a safe fix.
The default severity of this rule is information.
This rule belongs to the following domains:
- types
Sources:
- Inspired from @typescript-eslint/prefer-nullish-coalescing

How to configure

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{
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  "linter": {
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    "rules": {
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      "nursery": {
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        "useNullishCoalescing": "error"
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      }
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    }
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  }
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}

Description

Enforce using the nullish coalescing operator (??) instead of logical or (||).

The ?? operator only checks for null and undefined, while || checks for any falsy value including 0, '', and false. This can prevent bugs where legitimate falsy values are incorrectly treated as missing.

For || expressions, this rule triggers when the left operand is possibly nullish (contains null or undefined in its type). A safe fix is only offered when type analysis confirms the left operand can only be truthy or nullish (not other falsy values like 0 or '').

For ||= assignment expressions, the same logic applies: a ||= b is flagged when a is possibly nullish and can be rewritten as a ??= b.

By default, || expressions in conditional test positions (if/while/for/ternary) are ignored, as the falsy-checking behavior is often intentional there. This can be disabled with the ignoreConditionalTests option.

Examples

Invalid

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declare const maybeString: string | null;
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const value = maybeString || 'default'; // should use ??

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declare const maybeNumber: number | undefined;
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const value = maybeNumber || 0; // should use ??

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declare let x: string | null;
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x ||= 'default'; // should use ??=

Valid

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// Already using ??
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declare const maybeString: string | null;
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const value = maybeString ?? 'default';

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// Type is not nullish - no null or undefined in union
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declare const definiteString: string;
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const value = definiteString || 'fallback';

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// In conditional test position (ignored by default)
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declare const cond: string | null;
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if (cond || 'fallback') {
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  console.log('in if');
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}

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// Already using ??=
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declare let y: string | null;
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y ??= 'default';