Flint Plugin

Rules for writing third-party Flint plugins and custom rules. Meta!
This plugin is provided in a standalone @flint.fyi/plugin-flint npm package.

npm install @flint.fyi/plugin-flint

bun install @flint.fyi/plugin-flint

deno install @flint.fyi/plugin-flint

pnpm install @flint.fyi/plugin-flint

yarn install @flint.fyi/plugin-flint

Presets

`logical`

Rules that find bugs and enforce good practices for Flint plugins and rules.

import { 
const flint: Plugin<RuleAbout, string | undefined, [Rule<{
    readonly description: "Requires passing `sourceFile` to `getStart()` for better performance.";
    readonly id: "getStartSourceFile";
    readonly presets: ["logical"];
}, object, object, "missingSourceFile", undefined>, Rule<{
    readonly description: "Reports cases for invalid code that isn't formatted across lines.";
    readonly id: "invalidCodeLines";
    readonly presets: ["logical"];
} & {
    presets?: ("logical" | "logicalStrict" | "stylistic")[];
    url: string;
}, object, object, "singleLineTest", AnyOptionalSchema>, ... 4 more ..., Rule<...>]>
flint } from "@flint.fyi/plugin-flint";
import { 
function defineConfig(definition: ConfigDefinition): Config
Defines a new linter configuration for a Flint config file.
@see ― https://flint.fyi/configuration flint.fyi/configuration
defineConfig, 
const ts: Plugin<RuleAbout, "all" | "javascript" | "typescript", [Rule<{
    readonly description: "Reports getter and setter accessors for the same property that are not adjacent.";
    readonly id: "accessorPairGroups";
    readonly presets: ["stylistic", "stylisticStrict"];
} & {
    presets?: ("logical" | "logicalStrict" | "stylistic" | "stylisticStrict" | "untyped")[];
    url: string;
}, object, object, "notGrouped", AnyOptionalSchema>, Rule<{
    readonly description: "Reports mismatched types between getter and setter accessor pairs.";
    readonly id: "accessorPairTypes";
    readonly presets: [...];
} & {
    presets?: ("logical" | "logicalStrict" | "stylistic" | "stylisticStrict" | "untyped")[];
    url: string;
}, object, object, "mismatchedTypes", AnyOptionalSchema>, ... 290 more ..., Rule<...>]>
ts } from "flint";

export default 
function defineConfig(definition: ConfigDefinition): Config
Defines a new linter configuration for a Flint config file.
@see ― https://flint.fyi/configuration flint.fyi/configuration
defineConfig({
  
ConfigDefinition.use: ConfigUseDefinition[]
Specifies the files to be linted alongside the rules and settings to lint with.
@see ― https://flint.fyi/configuration#use flint.fyi/configuration#use
use: [
    {
      
ConfigUseDefinition.files: AnyLevelDeep<FilesValue>
A list of glob patterns describing which file(s) to lint.
@see ― https://flint.fyi/configuration#files flint.fyi/configuration#files
files: 
const ts: Plugin<RuleAbout, "all" | "javascript" | "typescript", [Rule<{
    readonly description: "Reports getter and setter accessors for the same property that are not adjacent.";
    readonly id: "accessorPairGroups";
    readonly presets: ["stylistic", "stylisticStrict"];
} & {
    presets?: ("logical" | "logicalStrict" | "stylistic" | "stylisticStrict" | "untyped")[];
    url: string;
}, object, object, "notGrouped", AnyOptionalSchema>, Rule<{
    readonly description: "Reports mismatched types between getter and setter accessor pairs.";
    readonly id: "accessorPairTypes";
    readonly presets: [...];
} & {
    presets?: ("logical" | "logicalStrict" | "stylistic" | "stylisticStrict" | "untyped")[];
    url: string;
}, object, object, "mismatchedTypes", AnyOptionalSchema>, ... 290 more ..., Rule<...>]>
ts.
Plugin<RuleAbout, "all" | "javascript" | "typescript", [Rule<{ readonly description: "Reports getter and setter accessors for the same property that are not adjacent."; readonly id: "accessorPairGroups"; readonly presets: [...]; } & { ...; }, object, object, "notGrouped", AnyOptionalSchema>, ... 291 more ..., Rule<...>]>.files: Record<"all" | "javascript" | "typescript", FilesValue>
Selectors of files this plugin suggests applying its rules to.
@see ― https://flint.fyi/glossary#files flint.fyi/glossary#files
files.
all: FilesValue
all,
      
ConfigUseDefinition.rules: AnyLevelDeep<ConfigRuleDefinition>
Any number of rules and/or presets of rules to enable for those files.
@see ― https://flint.fyi/configuration#rules flint.fyi/configuration#rules
rules: 
const flint: Plugin<RuleAbout, string | undefined, [Rule<{
    readonly description: "Requires passing `sourceFile` to `getStart()` for better performance.";
    readonly id: "getStartSourceFile";
    readonly presets: ["logical"];
}, object, object, "missingSourceFile", undefined>, Rule<{
    readonly description: "Reports cases for invalid code that isn't formatted across lines.";
    readonly id: "invalidCodeLines";
    readonly presets: ["logical"];
} & {
    presets?: ("logical" | "logicalStrict" | "stylistic")[];
    url: string;
}, object, object, "singleLineTest", AnyOptionalSchema>, ... 4 more ..., Rule<...>]>
flint.
Plugin<RuleAbout, string | undefined, [Rule<{ readonly description: "Requires passing `sourceFile` to `getStart()` for better performance."; readonly id: "getStartSourceFile"; readonly presets: [...]; }, object, object, "missingSourceFile", undefined>, ... 5 more ..., Rule<...>]>.presets: PluginPresets<RuleAbout, "logical" | "stylistic">
Preset lists of rules to enable on files.
@see ― https://flint.fyi/glossary#preset flint.fyi/glossary#preset
presets.
logical: Rule<RuleAbout, object, object, string, AnyOptionalSchema | undefined>[]
logical,
    },
  ],
});

`logicalStrict`

Additional logical rules that enforce best practices which are not always straightforward to implement. These rules are recommended for projects where a majority of developers are experienced with both JavaScript/TypeScript and writing plugins for a linter.

import { 
const flint: Plugin<RuleAbout, string | undefined, [Rule<{
    readonly description: "Requires passing `sourceFile` to `getStart()` for better performance.";
    readonly id: "getStartSourceFile";
    readonly presets: ["logical"];
}, object, object, "missingSourceFile", undefined>, Rule<{
    readonly description: "Reports cases for invalid code that isn't formatted across lines.";
    readonly id: "invalidCodeLines";
    readonly presets: ["logical"];
} & {
    presets?: ("logical" | "logicalStrict" | "stylistic")[];
    url: string;
}, object, object, "singleLineTest", AnyOptionalSchema>, ... 4 more ..., Rule<...>]>
flint } from "@flint.fyi/plugin-flint";
import { 
function defineConfig(definition: ConfigDefinition): Config
Defines a new linter configuration for a Flint config file.
@see ― https://flint.fyi/configuration flint.fyi/configuration
defineConfig, 
const ts: Plugin<RuleAbout, "all" | "javascript" | "typescript", [Rule<{
    readonly description: "Reports getter and setter accessors for the same property that are not adjacent.";
    readonly id: "accessorPairGroups";
    readonly presets: ["stylistic", "stylisticStrict"];
} & {
    presets?: ("logical" | "logicalStrict" | "stylistic" | "stylisticStrict" | "untyped")[];
    url: string;
}, object, object, "notGrouped", AnyOptionalSchema>, Rule<{
    readonly description: "Reports mismatched types between getter and setter accessor pairs.";
    readonly id: "accessorPairTypes";
    readonly presets: [...];
} & {
    presets?: ("logical" | "logicalStrict" | "stylistic" | "stylisticStrict" | "untyped")[];
    url: string;
}, object, object, "mismatchedTypes", AnyOptionalSchema>, ... 290 more ..., Rule<...>]>
ts } from "flint";

export default 
function defineConfig(definition: ConfigDefinition): Config
Defines a new linter configuration for a Flint config file.
@see ― https://flint.fyi/configuration flint.fyi/configuration
defineConfig({
  
ConfigDefinition.use: ConfigUseDefinition[]
Specifies the files to be linted alongside the rules and settings to lint with.
@see ― https://flint.fyi/configuration#use flint.fyi/configuration#use
use: [
    {
      
ConfigUseDefinition.files: AnyLevelDeep<FilesValue>
A list of glob patterns describing which file(s) to lint.
@see ― https://flint.fyi/configuration#files flint.fyi/configuration#files
files: 
const ts: Plugin<RuleAbout, "all" | "javascript" | "typescript", [Rule<{
    readonly description: "Reports getter and setter accessors for the same property that are not adjacent.";
    readonly id: "accessorPairGroups";
    readonly presets: ["stylistic", "stylisticStrict"];
} & {
    presets?: ("logical" | "logicalStrict" | "stylistic" | "stylisticStrict" | "untyped")[];
    url: string;
}, object, object, "notGrouped", AnyOptionalSchema>, Rule<{
    readonly description: "Reports mismatched types between getter and setter accessor pairs.";
    readonly id: "accessorPairTypes";
    readonly presets: [...];
} & {
    presets?: ("logical" | "logicalStrict" | "stylistic" | "stylisticStrict" | "untyped")[];
    url: string;
}, object, object, "mismatchedTypes", AnyOptionalSchema>, ... 290 more ..., Rule<...>]>
ts.
Plugin<RuleAbout, "all" | "javascript" | "typescript", [Rule<{ readonly description: "Reports getter and setter accessors for the same property that are not adjacent."; readonly id: "accessorPairGroups"; readonly presets: [...]; } & { ...; }, object, object, "notGrouped", AnyOptionalSchema>, ... 291 more ..., Rule<...>]>.files: Record<"all" | "javascript" | "typescript", FilesValue>
Selectors of files this plugin suggests applying its rules to.
@see ― https://flint.fyi/glossary#files flint.fyi/glossary#files
files.
all: FilesValue
all,
      
ConfigUseDefinition.rules: AnyLevelDeep<ConfigRuleDefinition>
Any number of rules and/or presets of rules to enable for those files.
@see ― https://flint.fyi/configuration#rules flint.fyi/configuration#rules
rules: 
const flint: Plugin<RuleAbout, string | undefined, [Rule<{
    readonly description: "Requires passing `sourceFile` to `getStart()` for better performance.";
    readonly id: "getStartSourceFile";
    readonly presets: ["logical"];
}, object, object, "missingSourceFile", undefined>, Rule<{
    readonly description: "Reports cases for invalid code that isn't formatted across lines.";
    readonly id: "invalidCodeLines";
    readonly presets: ["logical"];
} & {
    presets?: ("logical" | "logicalStrict" | "stylistic")[];
    url: string;
}, object, object, "singleLineTest", AnyOptionalSchema>, ... 4 more ..., Rule<...>]>
flint.
Plugin<RuleAbout, string | undefined, [Rule<{ readonly description: "Requires passing `sourceFile` to `getStart()` for better performance."; readonly id: "getStartSourceFile"; readonly presets: [...]; }, object, object, "missingSourceFile", undefined>, ... 5 more ..., Rule<...>]>.presets: PluginPresets<RuleAbout, "logical" | "stylistic">
Preset lists of rules to enable on files.
@see ― https://flint.fyi/glossary#preset flint.fyi/glossary#preset
presets.
any
logicalStrict,
    },
  ],
});

This preset’s rules are a superset of those in logical.

`stylistic`

Rules that enforce consistent styling and best stylistic practices for most-to-all Flint plugins.

import { 
const flint: Plugin<RuleAbout, string | undefined, [Rule<{
    readonly description: "Requires passing `sourceFile` to `getStart()` for better performance.";
    readonly id: "getStartSourceFile";
    readonly presets: ["logical"];
}, object, object, "missingSourceFile", undefined>, Rule<{
    readonly description: "Reports cases for invalid code that isn't formatted across lines.";
    readonly id: "invalidCodeLines";
    readonly presets: ["logical"];
} & {
    presets?: ("logical" | "logicalStrict" | "stylistic")[];
    url: string;
}, object, object, "singleLineTest", AnyOptionalSchema>, ... 4 more ..., Rule<...>]>
flint } from "@flint.fyi/plugin-flint";
import { 
function defineConfig(definition: ConfigDefinition): Config
Defines a new linter configuration for a Flint config file.
@see ― https://flint.fyi/configuration flint.fyi/configuration
defineConfig, 
const ts: Plugin<RuleAbout, "all" | "javascript" | "typescript", [Rule<{
    readonly description: "Reports getter and setter accessors for the same property that are not adjacent.";
    readonly id: "accessorPairGroups";
    readonly presets: ["stylistic", "stylisticStrict"];
} & {
    presets?: ("logical" | "logicalStrict" | "stylistic" | "stylisticStrict" | "untyped")[];
    url: string;
}, object, object, "notGrouped", AnyOptionalSchema>, Rule<{
    readonly description: "Reports mismatched types between getter and setter accessor pairs.";
    readonly id: "accessorPairTypes";
    readonly presets: [...];
} & {
    presets?: ("logical" | "logicalStrict" | "stylistic" | "stylisticStrict" | "untyped")[];
    url: string;
}, object, object, "mismatchedTypes", AnyOptionalSchema>, ... 290 more ..., Rule<...>]>
ts } from "flint";

export default 
function defineConfig(definition: ConfigDefinition): Config
Defines a new linter configuration for a Flint config file.
@see ― https://flint.fyi/configuration flint.fyi/configuration
defineConfig({
  
ConfigDefinition.use: ConfigUseDefinition[]
Specifies the files to be linted alongside the rules and settings to lint with.
@see ― https://flint.fyi/configuration#use flint.fyi/configuration#use
use: [
    {
      
ConfigUseDefinition.files: AnyLevelDeep<FilesValue>
A list of glob patterns describing which file(s) to lint.
@see ― https://flint.fyi/configuration#files flint.fyi/configuration#files
files: 
const ts: Plugin<RuleAbout, "all" | "javascript" | "typescript", [Rule<{
    readonly description: "Reports getter and setter accessors for the same property that are not adjacent.";
    readonly id: "accessorPairGroups";
    readonly presets: ["stylistic", "stylisticStrict"];
} & {
    presets?: ("logical" | "logicalStrict" | "stylistic" | "stylisticStrict" | "untyped")[];
    url: string;
}, object, object, "notGrouped", AnyOptionalSchema>, Rule<{
    readonly description: "Reports mismatched types between getter and setter accessor pairs.";
    readonly id: "accessorPairTypes";
    readonly presets: [...];
} & {
    presets?: ("logical" | "logicalStrict" | "stylistic" | "stylisticStrict" | "untyped")[];
    url: string;
}, object, object, "mismatchedTypes", AnyOptionalSchema>, ... 290 more ..., Rule<...>]>
ts.
Plugin<RuleAbout, "all" | "javascript" | "typescript", [Rule<{ readonly description: "Reports getter and setter accessors for the same property that are not adjacent."; readonly id: "accessorPairGroups"; readonly presets: [...]; } & { ...; }, object, object, "notGrouped", AnyOptionalSchema>, ... 291 more ..., Rule<...>]>.files: Record<"all" | "javascript" | "typescript", FilesValue>
Selectors of files this plugin suggests applying its rules to.
@see ― https://flint.fyi/glossary#files flint.fyi/glossary#files
files.
all: FilesValue
all,
      
ConfigUseDefinition.rules: AnyLevelDeep<ConfigRuleDefinition>
Any number of rules and/or presets of rules to enable for those files.
@see ― https://flint.fyi/configuration#rules flint.fyi/configuration#rules
rules: 
const flint: Plugin<RuleAbout, string | undefined, [Rule<{
    readonly description: "Requires passing `sourceFile` to `getStart()` for better performance.";
    readonly id: "getStartSourceFile";
    readonly presets: ["logical"];
}, object, object, "missingSourceFile", undefined>, Rule<{
    readonly description: "Reports cases for invalid code that isn't formatted across lines.";
    readonly id: "invalidCodeLines";
    readonly presets: ["logical"];
} & {
    presets?: ("logical" | "logicalStrict" | "stylistic")[];
    url: string;
}, object, object, "singleLineTest", AnyOptionalSchema>, ... 4 more ..., Rule<...>]>
flint.
Plugin<RuleAbout, string | undefined, [Rule<{ readonly description: "Requires passing `sourceFile` to `getStart()` for better performance."; readonly id: "getStartSourceFile"; readonly presets: [...]; }, object, object, "missingSourceFile", undefined>, ... 5 more ..., Rule<...>]>.presets: PluginPresets<RuleAbout, "logical" | "stylistic">
Preset lists of rules to enable on files.
@see ― https://flint.fyi/glossary#preset flint.fyi/glossary#preset
presets.
stylistic: Rule<RuleAbout, object, object, string, AnyOptionalSchema | undefined>[]
stylistic,
    },
  ],
});

Rules

Implemented: 5 of 12 (41.6%)

Flint Rule	Preset
`invalidCodeLines`Reports cases for invalid code that isn't formatted across lines.	logical
`missingPlaceholders`Reports `context.report()` calls missing data for message placeholders.	logical
`nodePropertyInChecks`Disallows using the `in` operator to check properties on TypeScript nodes.	logical
`ruleDescriptions`	logical
`rulePresets`	logical
`ruleURLs`	logical
`testCaseDuplicates`Reports test cases that are identical to previous test cases.	logical
`testCaseOnlies`	logical
`unusedMessageIds`	logical
`unusedPlaceholders`	logical
`messagePhrases`	logical (strict)
`testShorthands`Test cases with only a code property can use string shorthand syntax instead of object literal syntax.	stylistic

Made with ❤️‍🔥 around the world by the Flint team and contributors.