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prefer-nullish-coalescing.md

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Expected Behavior: prefer-nullish-coalescing Rule

This rule enforces the use of nullish coalescing operator (??) over logical OR (||) when dealing with null or undefined values, particularly when the right-hand side is a falsy but not nullish value.

Examples

Empty String Default

- const displayName = user.name || ''
+ const displayName = user.name ?? ''

- const message = input || ''
+ const message = input ?? ''

- const title = config.title || ''
+ const title = config.title ?? ''

Zero Default

- const count = items.length || 0
+ const count = items.length ?? 0

- const index = searchIndex || 0
+ const index = searchIndex ?? 0

- const timeout = settings.timeout || 0
+ const timeout = settings.timeout ?? 0

Boolean False Default

+ const enabled = feature.flag ?? false
- const enabled = feature.flag || false

- const visible = config.show || false
+ const visible = config.show ?? false

- const required = field.required || false
+ const required = field.required ?? false

Object Properties with Falsy Defaults

  const user = {
-   name: input.name || '',
+   name: input.name ?? '',
-   count: data.count || 0,
+   count: data.count ?? 0,
-   active: status.active || false
+   active: status.active ?? false
  }

Template Literals with Falsy Values

- const greeting = `Hello ${user.name || ''}!`
+ const greeting = `Hello ${user.name ?? ''}!`

- const status = `Count: ${items.length || 0}`
+ const status = `Count: ${items.length ?? 0}`

- const message = `Debug: ${config.debug || false}`
+ const message = `Debug: ${config.debug ?? false}`

Rule Scope

This rule only applies to:

  • ✅ Logical OR (||) operators where the right-hand side is a falsy but not nullish value:
    • Empty string ("")
    • Zero (0)
    • Boolean false

This rule does NOT apply to:

  • ❌ Logical OR (||) with non-falsy values ("hello", 1, true)
  • ❌ Logical AND (&&) operators
  • ❌ Nullish coalescing (??) operators
  • ❌ Other logical operators (!, &&, ??)

Auto-fix Behavior

The rule provides auto-fix suggestions that replace || with ??:

  • value || "" → value ?? ""
  • count || 0 → count ?? 0
  • flag || false → flag ?? false
  • (user?.name || "") || false → (user?.name ?? "") || false (first occurrence)