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https://github.com/danielnixon/eslint-plugin-total-functions
An ESLint plugin to enforce the use of total functions (and prevent the use of partial functions) in TypeScript.
https://github.com/danielnixon/eslint-plugin-total-functions
eslint eslint-plugin functional-programming partial-functions type-safety typescript
Last synced: 1 day ago
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An ESLint plugin to enforce the use of total functions (and prevent the use of partial functions) in TypeScript.
- Host: GitHub
- URL: https://github.com/danielnixon/eslint-plugin-total-functions
- Owner: danielnixon
- License: mit
- Created: 2020-05-30T09:07:50.000Z (over 4 years ago)
- Default Branch: master
- Last Pushed: 2024-04-13T20:26:37.000Z (9 months ago)
- Last Synced: 2024-04-14T10:53:55.769Z (9 months ago)
- Topics: eslint, eslint-plugin, functional-programming, partial-functions, type-safety, typescript
- Language: TypeScript
- Homepage:
- Size: 3.02 MB
- Stars: 86
- Watchers: 4
- Forks: 5
- Open Issues: 39
-
Metadata Files:
- Readme: README.md
- License: LICENSE
Awesome Lists containing this project
- awesome-list - eslint-plugin-total-functions
README
# TypeScript Total Functions - ESLint Plugin
[](https://github.com/danielnixon/eslint-plugin-total-functions/actions/workflows/node.js.yml)
[](https://github.com/plantain-00/type-coverage)
[](https://codecov.io/gh/danielnixon/eslint-plugin-total-functions)
[](https://dashboard.stryker-mutator.io/reports/github.com/danielnixon/eslint-plugin-total-functions/master)
[](https://snyk.io/test/github/danielnixon/eslint-plugin-total-functions?targetFile=package.json)
[](https://www.codacy.com/gh/danielnixon/eslint-plugin-total-functions/dashboard?utm_source=github.com&utm_medium=referral&utm_content=danielnixon/eslint-plugin-total-functions&utm_campaign=Badge_Grade)
[](https://www.npmjs.com/package/eslint-plugin-total-functions)
An ESLint plugin to enforce the use of total functions (and prevent the use of [partial functions](https://wiki.haskell.org/Partial_functions)) in TypeScript. If you like your types to tell the truth, this is the ESLint plugin for you.
## Version Matrix
| TypeScript | ESLint | eslint-plugin-total-functions | Suppported? |
| :--------------: | :-----: | :---------------------------: | :---------: |
| 5.0.2 | 8.36.0 | 7.0.0 | |
| 4.9.5 | 8.35.0 | 6.2.0 | No |
| 4.7.3 | 8.17.0 | 6.0.0 | No |
| 4.5.4 | 8.5.0 | 5.0.0 | No |
| 4.4.2 | 7.32.0 | 4.10.1 | No |
| 4.3.5 | 7.30.0 | 4.8.0 | No |
| 4.1.2 | 7.12.0 | 4.7.2 | No |
| 4.0.2 | 7.9.0 | 3.3.0 | No |
## Installation
```sh
yarn add --dev eslint-plugin-total-functions \
@typescript-eslint/parser \
eslint \
typescript
```
## Setup
### Option 1
Use [eslint-config-typed-fp](https://github.com/danielnixon/eslint-config-typed-fp) which includes this plugin among others.
### Option 2
1. Turn on TypeScript's [strict mode](https://www.typescriptlang.org/tsconfig#strict) and [noUncheckedIndexedAccess](https://www.typescriptlang.org/tsconfig#noUncheckedIndexedAccess) option.
2. Set up [ESLint + TypeScript](https://github.com/typescript-eslint/typescript-eslint/blob/master/docs/getting-started/linting/README.md).
3. Turn on [eslint-plugin-functional](https://github.com/jonaskello/eslint-plugin-functional) (recommended). Its rules related to mutation and OO are more important than this plugin's rules and they'll help keep your types honest.
4. Update your `.eslintrc.js`:
```diff
module.exports = {
parser: "@typescript-eslint/parser",
parserOptions: {
project: "./tsconfig.json",
ecmaVersion: 2018,
sourceType: "module"
},
extends: [
+ "plugin:total-functions/recommended",
],
plugins: [
+ "total-functions",
],
};
```
Alternatively you can configure individual rules separately (see below).
## Rules
| Rule | Recommended | All | Fixer? |
| :-------------------------------------: | :----------: | :---: | :----: |
| require-strict-mode | ✅ | ✅ | |
| no-unsafe-type-assertion | ✅ | ✅ | |
| no-unsafe-readonly-mutable-assignment | ✅ | ✅ | |
| no-unsafe-mutable-readonly-assignment | | ✅ | |
| no-enums | ✅ | ✅ | |
| no-partial-url-constructor | ✅ | ✅ | |
| no-partial-division | ✅ | ✅ | |
| no-partial-string-normalize | ✅ | ✅ | |
| no-premature-fp-ts-effects | ✅ | ✅ | |
| no-nested-fp-ts-effects | | ✅ | |
| no-partial-array-reduce | ✅ | ✅ | |
| no-hidden-type-assertions | | ✅ | |
### Deprecated rules
* no-unsafe-optional-property-assignment
* no-unsafe-enum-assignment (No longer required as of TypeScript 5, see https://devblogs.microsoft.com/typescript/announcing-typescript-5-0/#all-enums-are-union-enums)
### total-functions/require-strict-mode
The world is a very strange place when [strict mode](https://www.typescriptlang.org/tsconfig#strict) is disabled. This rule enforces strict mode and [noUncheckedIndexedAccess](https://devblogs.microsoft.com/typescript/announcing-typescript-4-1-beta/#no-unchecked-indexed-access) mode (which is sadly not included under the strict umbrella).
### total-functions/no-unsafe-type-assertion
Bans unsafe type assertions, for example:
```typescript
type Foo = { readonly bar: number };
const foo = {} as Foo; // This compiles
foo.bar.toString(); // This explodes at runtime
```
This is similar to the [consistent-type-assertions](https://github.com/typescript-eslint/typescript-eslint/blob/master/packages/eslint-plugin/docs/rules/consistent-type-assertions.md) rule from [typescript-eslint](https://github.com/typescript-eslint/typescript-eslint/tree/master/packages/eslint-plugin), however:
1. this rule is _weaker_ than `consistent-type-assertions` with its `assertionStyle` option set to `never` -- this rule will permit type assertions that it considers safe as opposed to blanket banning all type assertions, and
2. this rule is _stronger_ than `consistent-type-assertions` with its `objectLiteralTypeAssertions` option set to `never`, for example:
```typescript
type Foo = { readonly bar: number };
const foo = {};
const foo2 = foo as Foo; // Flagged by this rule, but not by consistent-type-assertions (unless you set assertionStyle to never)
foo2.bar.toString(); // This explodes at runtime
```
For examples of type assertions that this rule considers valid and invalid, see [no-unsafe-type-assertion.test.ts](https://github.com/danielnixon/eslint-plugin-total-functions/blob/master/src/rules/no-unsafe-type-assertion.test.ts).
See [TypeScript issue #7481](https://github.com/microsoft/TypeScript/issues/7481) for a request to fix this at the language level.
### total-functions/no-unsafe-readonly-mutable-assignment
Bans unsafe assignment of readonly values to mutable values (which can lead to surprising mutation in the readonly value). This includes passing readonly values as arguments to functions that expect mutable parameters.
For examples of assignment that this rule considers valid and invalid, see [no-unsafe-readonly-mutable-assignment.test.ts](https://github.com/danielnixon/eslint-plugin-total-functions/blob/master/src/rules/no-unsafe-readonly-mutable-assignment.test.ts).
See [TypeScript issue #13347](https://github.com/microsoft/TypeScript/issues/13347) for a request to fix this at the language level.
### total-functions/no-unsafe-mutable-readonly-assignment
The inverse counterpart to no-unsafe-readonly-mutable-assignment. This rule bans unsafe assignment of mutable values to readonly values (which just like the inverse can lead to surprising mutation in the readonly value).
This rule is often noisy in practice so, unlike no-unsafe-readonly-mutable-assignment, is excluded from the `recommended` config.
Note that the following is considered an assignment from mutable to readonly:
```ts
type ReadonlyA = { readonly a: string };
const readonlyA: ReadonlyA = { a: "" };
```
The solution is to append `as const` to the RHS:
```ts
type ReadonlyA = { readonly a: string };
const readonlyA: ReadonlyA = { a: "" } as const;
```
For examples of assignment that this rule considers valid and invalid, see [no-unsafe-mutable-readonly-assignment.test.ts](https://github.com/danielnixon/eslint-plugin-total-functions/blob/master/src/rules/no-unsafe-mutable-readonly-assignment.test.ts).
### total-functions/no-enums
Enums have a number of issues, including unsoundness issues (which are especially relevant here). This rule bans the declaration of enums entirely. Use an alternative such as a union of strings instead.
### total-functions/no-unsafe-enum-assignment
*Deprecated* No longer required as of TypeScript 5 (https://devblogs.microsoft.com/typescript/announcing-typescript-5-0/#all-enums-are-union-enums).
If you do use an enum (or are forced to by a library), this rule flags unsafe assignment that the TypeScript compiler (prior to version 5) permits. For example:
```typescript
enum ZeroOrOne {
Zero = 0,
One = 1,
}
// This compiles (prior to TypeScript 5) but is flagged by no-unsafe-enum-assignment
const zeroOrOne: ZeroOrOne = 2;
// This is not flagged by no-unsafe-enum-assignment
const zeroOrOne: ZeroOrOne = ZeroOrOne.Zero;
```
### total-functions/no-partial-url-constructor
The URL constructor can throw (i.e. it is partial).
```typescript
// This compiles and foo appears to be a URL. It isn't.
const foo: URL = new URL(""); // Throws TypeError [ERR_INVALID_URL]: Invalid URL
```
Instead, you should use a wrapper that catches that error and returns `URL | undefined` or similar (perhaps using an `Option` type).
URL also happens to be mutable, which will be flagged by [prefer-immutable-types](https://github.com/eslint-functional/eslint-plugin-functional/blob/main/docs/rules/prefer-immutable-types.md). The [readonly-types](https://github.com/agiledigital/readonly-types) package provides a `readonlyURL` function that solves both of these issues.
### total-functions/no-partial-division
Division by zero is undefined. That makes the division operator partial.
In the case of `number`, it results in `Infinity` (IEEE 754...).
In the case of `bigint` it throws a `RangeError`.
The latter is much more indisputably partial than the former.
```
> 1 / 0
Infinity
> 1n / 0n
Uncaught RangeError: Division by zero
```
This rule flags division unless the denominator is provably non-zero. If you need division, you should wrap it in a wrapper that returns undefined when the denominator is zero. Alternatively, consider using branded types / refinements, such as https://github.com/gcanti/io-ts/blob/master/index.md#branded-types--refinements or https://gcanti.github.io/newtype-ts/modules/NonZero.ts.html
# See Also
* [TypeScript for Functional Programmers](https://www.typescriptlang.org/docs/handbook/typescript-in-5-minutes-func.html)
* https://github.com/danielnixon/eslint-config-typed-fp
* https://github.com/agiledigital/readonly-types
* https://github.com/eslint-functional/eslint-plugin-functional
* https://github.com/RebeccaStevens/is-immutable-type
* https://github.com/gcanti/fp-ts
* https://github.com/plantain-00/type-coverage
* https://github.com/immutable-js/immutable-js
* https://github.com/shian15810/eslint-plugin-typescript-enum