prefer-literal-enum-member
Require all enum members to be literal values.
Extending "plugin:@typescript-eslint/strict"
in an ESLint configuration enables this rule.
TypeScript allows the value of an enum member to be many different kinds of valid JavaScript expressions. However, because enums create their own scope whereby each enum member becomes a variable in that scope, developers are often surprised at the resultant values. For example:
const imOutside = 2;
const b = 2;
enum Foo {
outer = imOutside,
a = 1,
b = a,
c = b,
// does c == Foo.b == Foo.c == 1?
// or does c == b == 2?
}
The answer is that
Foo.c
will be1
at runtime [TypeScript playground].
Therefore, it's often better to prevent unexpected results in code by requiring the use of literal values as enum members. This rule reports when an enum member is given a value that is not a literal.
module.exports = {
"rules": {
"@typescript-eslint/prefer-literal-enum-member": "error"
}
};
Try this rule in the playground ↗
Examples
- ❌ Incorrect
- ✅ Correct
const str = 'Test';
const string1 = 'string1';
const string2 = 'string2';
enum Invalid {
A = str, // Variable assignment
B = `Interpolates ${string1} and ${string2}`, // Template literal with interpolation
C = 2 + 2, // Expression assignment
D = C, // Assignment to another enum member
}
Open in Playgroundenum Valid {
A, // No initializer; initialized with ascending integers starting from 0
B = 'TestStr', // A regular string
C = `A template literal string`, // A template literal without interpolation
D = 4, // A number
}
Open in PlaygroundOptions
This rule accepts the following options:
type Options = [
{
/** Whether to allow using bitwise expressions in enum initializers. */
allowBitwiseExpressions?: boolean;
},
];
const defaultOptions: Options = [{ allowBitwiseExpressions: false }];
allowBitwiseExpressions
Whether to allow using bitwise expressions in enum initializers (default: false
).
Examples of code for the { "allowBitwiseExpressions": true }
option:
- ❌ Incorrect
- ✅ Correct
const x = 1;
enum Foo {
A = x << 0,
B = x >> 0,
C = x >>> 0,
D = x | 0,
E = x & 0,
F = x ^ 0,
G = ~x,
}
Open in Playgroundenum Foo {
A = 1 << 0,
B = 1 >> 0,
C = 1 >>> 0,
D = 1 | 0,
E = 1 & 0,
F = 1 ^ 0,
G = ~1,
}
Open in PlaygroundWhen Not To Use It
If you want use anything other than simple literals as an enum value, this rule might not be for you.