104 lines
2.5 KiB
Plaintext
104 lines
2.5 KiB
Plaintext
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---
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description: 'Require all enum members to be literal values.'
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---
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> 🛑 This file is source code, not the primary documentation location! 🛑
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>
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> See **https://typescript-eslint.io/rules/prefer-literal-enum-member** for documentation.
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TypeScript allows the value of an enum member to be many different kinds of valid JavaScript expressions.
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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.
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For example:
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```ts
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const imOutside = 2;
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const b = 2;
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enum Foo {
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outer = imOutside,
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a = 1,
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b = a,
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c = b,
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// does c == Foo.b == Foo.c == 1?
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// or does c == b == 2?
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}
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```
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> The answer is that `Foo.c` will be `1` at runtime [[TypeScript playground](https://www.typescriptlang.org/play/#src=const%20imOutside%20%3D%202%3B%0D%0Aconst%20b%20%3D%202%3B%0D%0Aenum%20Foo%20%7B%0D%0A%20%20%20%20outer%20%3D%20imOutside%2C%0D%0A%20%20%20%20a%20%3D%201%2C%0D%0A%20%20%20%20b%20%3D%20a%2C%0D%0A%20%20%20%20c%20%3D%20b%2C%0D%0A%20%20%20%20%2F%2F%20does%20c%20%3D%3D%20Foo.b%20%3D%3D%20Foo.c%20%3D%3D%201%3F%0D%0A%20%20%20%20%2F%2F%20or%20does%20c%20%3D%3D%20b%20%3D%3D%202%3F%0D%0A%7D)].
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Therefore, it's often better to prevent unexpected results in code by requiring the use of literal values as enum members.
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This rule reports when an enum member is given a value that is not a literal.
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## Examples
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<!--tabs-->
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### ❌ Incorrect
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```ts
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const str = 'Test';
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enum Invalid {
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A = str, // Variable assignment
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B = {}, // Object assignment
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C = `A template literal string`, // Template literal
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D = new Set(1, 2, 3), // Constructor in assignment
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E = 2 + 2, // Expression assignment
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}
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```
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### ✅ Correct
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```ts
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enum Valid {
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A,
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B = 'TestStr', // A regular string
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C = 4, // A number
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D = null,
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E = /some_regex/,
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}
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```
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<!--/tabs-->
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## Options
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### `allowBitwiseExpressions`
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When set to `true` will allow you to use bitwise expressions in enum initializer (default: `false`).
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Examples of code for the `{ "allowBitwiseExpressions": true }` option:
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<!--tabs-->
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### ❌ Incorrect
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```ts option='{ "allowBitwiseExpressions": true }'
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const x = 1;
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enum Foo {
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A = x << 0,
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B = x >> 0,
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C = x >>> 0,
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D = x | 0,
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E = x & 0,
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F = x ^ 0,
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G = ~x,
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}
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```
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### ✅ Correct
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```ts option='{ "allowBitwiseExpressions": true }'
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enum Foo {
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A = 1 << 0,
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B = 1 >> 0,
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C = 1 >>> 0,
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D = 1 | 0,
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E = 1 & 0,
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F = 1 ^ 0,
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G = ~1,
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}
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```
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## When Not To Use It
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If you want use anything other than simple literals as an enum value, this rule might not be for you.
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