enumValueDuplicates

Reports enum members with duplicate values.

✅ This rule is included in the ts logical presets.

TypeScript allows multiple enum members to have the same value. However, this is usually unintentional and can lead to bugs that are hard to track down.

When enum members have duplicate values:

• Accessing the enum by value will return only one of the duplicate members
• The enum’s reverse mapping becomes unreliable
• Code that relies on unique values may behave unexpectedly

This rule reports when enum members are initialized with literal values (strings or numbers) that have already been used by another member.

Examples

❌ Incorrect

enum Status {
  Active = 1,
  Inactive = 1,
}

enum Color {
  Red = "r",
  Blue = "r",
}

✅ Correct

enum Status {
  Active = 1,
  Inactive = 2,
}

enum Color {
  Red = "red",
  Blue = "blue",
}

enum Direction {
  Up,
  Down,
  Left,
  Right,
}

Scope of Detection

This rule only checks enum members initialized with literal values:

• Numeric literals: 1, -5, +3
• String literals: "value", 'value'
• Template literals without substitutions: `value`

Members without initializers or with computed expressions are not checked:

• Implicit values: enum E { A, B } (not checked)
• Computed values: enum E { A = getValue() } (not checked)

Options

This rule is not configurable.

When Not To Use It

It can sometimes be useful to include duplicate enum members for specific use cases, such as when renaming an enum member and keeping the old name as an alias until a breaking change. You might consider using ESLint disable comments for those specific situations instead of completely disabling this rule.

Further Reading

• TypeScript Handbook: Enums

Equivalents in Other Linters

• ESLint: @typescript-eslint/no-duplicate-enum-values
• Oxlint: typescript/no-duplicate-enum-values