3 While ESLint is designed to be run on the command line, it's possible to use ESLint programmatically through the Node.js API. The purpose of the Node.js API is to allow plugin and tool authors to use the ESLint functionality directly, without going through the command line interface.
5 **Note:** Use undocumented parts of the API at your own risk. Only those parts that are specifically mentioned in this document are approved for use and will remain stable and reliable. Anything left undocumented is unstable and may change or be removed at any point.
10 * [constructor()][eslint-constructor]
11 * [lintFiles()][eslint-lintfiles]
12 * [lintText()][eslint-linttext]
13 * [getRulesMetaForResults()][eslint-getrulesmetaforresults]
14 * [calculateConfigForFile()][eslint-calculateconfigforfile]
15 * [isPathIgnored()][eslint-ispathignored]
16 * [loadFormatter()][eslint-loadformatter]
17 * [static version][eslint-version]
18 * [static outputFixes()][eslint-outputfixes]
19 * [static getErrorResults()][eslint-geterrorresults]
20 * [LintResult type][lintresult]
21 * [LintMessage type][lintmessage]
22 * [EditInfo type][editinfo]
23 * [Formatter type][formatter]
24 * [SourceCode](#sourcecode)
25 * [splitLines()](#sourcecodesplitlines)
27 * [verify()](#linterverify)
28 * [verifyAndFix()](#linterverifyandfix)
29 * [defineRule()](#linterdefinerule)
30 * [defineRules()](#linterdefinerules)
31 * [getRules()](#lintergetrules)
32 * [defineParser()](#linterdefineparser)
33 * [version](#linterversionlinterversion)
34 * [RuleTester](#ruletester)
35 * [Customizing RuleTester](#customizing-ruletester)
41 The `ESLint` class is the primary class to use in Node.js applications.
43 This class depends on the Node.js `fs` module and the file system, so you cannot use it in browsers. If you want to lint code on browsers, use the [Linter](#linter) class instead.
45 Here's a simple example of using the `ESLint` class:
48 const { ESLint } = require("eslint");
50 (async function main() {
51 // 1. Create an instance.
52 const eslint = new ESLint();
55 const results = await eslint.lintFiles(["lib/**/*.js"]);
57 // 3. Format the results.
58 const formatter = await eslint.loadFormatter("stylish");
59 const resultText = formatter.format(results);
62 console.log(resultText);
63 })().catch((error) => {
69 And here is an example that autofixes lint problems:
72 const { ESLint } = require("eslint");
74 (async function main() {
75 // 1. Create an instance with the `fix` option.
76 const eslint = new ESLint({ fix: true });
78 // 2. Lint files. This doesn't modify target files.
79 const results = await eslint.lintFiles(["lib/**/*.js"]);
81 // 3. Modify the files with the fixed code.
82 await ESLint.outputFixes(results);
84 // 4. Format the results.
85 const formatter = await eslint.loadFormatter("stylish");
86 const resultText = formatter.format(results);
89 console.log(resultText);
90 })().catch((error) => {
96 ### ◆ new ESLint(options)
99 const eslint = new ESLint(options);
102 Create a new `ESLint` instance.
106 The `ESLint` constructor takes an `options` object. If you omit the `options` object then it uses default values for all options. The `options` object has the following properties.
108 ##### File Enumeration
110 * `options.cwd` (`string`)<br>
111 Default is `process.cwd()`. The working directory. This must be an absolute path.
112 * `options.errorOnUnmatchedPattern` (`boolean`)<br>
113 Default is `true`. Unless set to `false`, the [`eslint.lintFiles()`][eslint-lintfiles] method will throw an error when no target files are found.
114 * `options.extensions` (`string[] | null`)<br>
115 Default is `null`. If you pass directory paths to the [`eslint.lintFiles()`][eslint-lintfiles] method, ESLint checks the files in those directories that have the given extensions. For example, when passing the `src/` directory and `extensions` is `[".js", ".ts"]`, ESLint will lint `*.js` and `*.ts` files in `src/`. If `extensions` is `null`, ESLint checks `*.js` files and files that match `overrides[].files` patterns in your configuration.<br>**Note:** This option only applies when you pass directory paths to the [`eslint.lintFiles()`][eslint-lintfiles] method. If you pass glob patterns like `lib/**/*`, ESLint will lint all files matching the glob pattern regardless of extension.
116 * `options.globInputPaths` (`boolean`)<br>
117 Default is `true`. If `false` is present, the [`eslint.lintFiles()`][eslint-lintfiles] method doesn't interpret glob patterns.
118 * `options.ignore` (`boolean`)<br>
119 Default is `true`. If `false` is present, the [`eslint.lintFiles()`][eslint-lintfiles] method doesn't respect `.eslintignore` files or `ignorePatterns` in your configuration.
120 * `options.ignorePath` (`string | null`)<br>
121 Default is `null`. The path to a file ESLint uses instead of `$CWD/.eslintignore`. If a path is present and the file doesn't exist, this constructor will throw an error.
125 * `options.allowInlineConfig` (`boolean`)<br>
126 Default is `true`. If `false` is present, ESLint suppresses directive comments in source code. If this option is `false`, it overrides the `noInlineConfig` setting in your configurations.
127 * `options.baseConfig` (`ConfigData | null`)<br>
128 Default is `null`. [Configuration object], extended by all configurations used with this instance. You can use this option to define the default settings that will be used if your configuration files don't configure it.
129 * `options.overrideConfig` (`ConfigData | null`)<br>
130 Default is `null`. [Configuration object], overrides all configurations used with this instance. You can use this option to define the settings that will be used even if your configuration files configure it.
131 * `options.overrideConfigFile` (`string | null`)<br>
132 Default is `null`. The path to a configuration file, overrides all configurations used with this instance. The `options.overrideConfig` option is applied after this option is applied.
133 * `options.plugins` (`Record<string, Plugin> | null`)<br>
134 Default is `null`. The plugin implementations that ESLint uses for the `plugins` setting of your configuration. This is a map-like object. Those keys are plugin IDs and each value is implementation.
135 * `options.reportUnusedDisableDirectives` (`"error" | "warn" | "off" | null`)<br>
136 Default is `null`. The severity to report unused eslint-disable directives. If this option is a severity, it overrides the `reportUnusedDisableDirectives` setting in your configurations.
137 * `options.resolvePluginsRelativeTo` (`string` | `null`)<br>
138 Default is `null`. The path to a directory where plugins should be resolved from. If `null` is present, ESLint loads plugins from the location of the configuration file that contains the plugin setting. If a path is present, ESLint loads all plugins from there.
139 * `options.rulePaths` (`string[]`)<br>
140 Default is `[]`. An array of paths to directories to load custom rules from.
141 * `options.useEslintrc` (`boolean`)<br>
142 Default is `true`. If `false` is present, ESLint doesn't load configuration files (`.eslintrc.*` files). Only the configuration of the constructor options is valid.
146 * `options.fix` (`boolean | (message: LintMessage) => boolean`)<br>
147 Default is `false`. If `true` is present, the [`eslint.lintFiles()`][eslint-lintfiles] and [`eslint.lintText()`][eslint-linttext] methods work in autofix mode. If a predicate function is present, the methods pass each lint message to the function, then use only the lint messages for which the function returned `true`.
148 * `options.fixTypes` (`("directive" | "problem" | "suggestion" | "layout")[] | null`)<br>
149 Default is `null`. The types of the rules that the [`eslint.lintFiles()`][eslint-lintfiles] and [`eslint.lintText()`][eslint-linttext] methods use for autofix.
153 * `options.cache` (`boolean`)<br>
154 Default is `false`. If `true` is present, the [`eslint.lintFiles()`][eslint-lintfiles] method caches lint results and uses it if each target file is not changed. Please mind that ESLint doesn't clear the cache when you upgrade ESLint plugins. In that case, you have to remove the cache file manually. The [`eslint.lintText()`][eslint-linttext] method doesn't use caches even if you pass the `options.filePath` to the method.
155 * `options.cacheLocation` (`string`)<br>
156 Default is `.eslintcache`. The [`eslint.lintFiles()`][eslint-lintfiles] method writes caches into this file.
157 * `options.cacheStrategy` (`string`)<br>
158 Default is `"metadata"`. Strategy for the cache to use for detecting changed files. Can be either `"metadata"` or `"content"`.
160 ### ◆ eslint.lintFiles(patterns)
163 const results = await eslint.lintFiles(patterns);
166 This method lints the files that match the glob patterns and then returns the results.
170 * `patterns` (`string | string[]`)<br>
171 The lint target files. This can contain any of file paths, directory paths, and glob patterns.
175 * (`Promise<LintResult[]>`)<br>
176 The promise that will be fulfilled with an array of [LintResult] objects.
178 ### ◆ eslint.lintText(code, options)
181 const results = await eslint.lintText(code, options);
184 This method lints the given source code text and then returns the results.
186 By default, this method uses the configuration that applies to files in the current working directory (the `cwd` constructor option). If you want to use a different configuration, pass `options.filePath`, and ESLint will load the same configuration that [`eslint.lintFiles()`][eslint-lintfiles] would use for a file at `options.filePath`.
188 If the `options.filePath` value is configured to be ignored, this method returns an empty array. If the `options.warnIgnored` option is set along with the `options.filePath` option, this method returns a [LintResult] object. In that case, the result may contain a warning that indicates the file was ignored.
192 The second parameter `options` is omittable.
194 * `code` (`string`)<br>
195 The source code text to check.
196 * `options.filePath` (`string`)<br>
197 Optional. The path to the file of the source code text. If omitted, the `result.filePath` becomes the string `"<text>"`.
198 * `options.warnIgnored` (`boolean`)<br>
199 Optional. If `true` is present and the `options.filePath` is a file ESLint should ignore, this method returns a lint result contains a warning message.
203 * (`Promise<LintResult[]>`)<br>
204 The promise that will be fulfilled with an array of [LintResult] objects. This is an array (despite there being only one lint result) in order to keep the interfaces between this and the [`eslint.lintFiles()`][eslint-lintfiles] method similar.
206 ### ◆ eslint.getRulesMetaForResults(results)
209 const results = await eslint.lintFiles(patterns);
210 const rulesMeta = eslint.getRulesMetaForResults(results);
213 This method returns an object containing meta information for each rule that triggered a lint error in the given `results`.
217 * `results` (`LintResult[]`)<br>
218 An array of [LintResult] objects returned from a call to `ESLint#lintFiles()` or `ESLint#lintText()`.
223 An object whose property names are the rule IDs from the `results` and whose property values are the rule's meta information (if available).
225 ### ◆ eslint.calculateConfigForFile(filePath)
228 const config = await eslint.calculateConfigForFile(filePath);
231 This method calculates the configuration for a given file, which can be useful for debugging purposes.
233 * It resolves and merges `extends` and `overrides` settings into the top level configuration.
234 * It resolves the `parser` setting to absolute paths.
235 * It normalizes the `plugins` setting to align short names. (e.g., `eslint-plugin-foo` → `foo`)
236 * It adds the `processor` setting if a legacy file extension processor is matched.
237 * It doesn't interpret the `env` setting to the `globals` and `parserOptions` settings, so the result object contains the `env` setting as is.
241 * `filePath` (`string`)<br>
242 The path to the file whose configuration you would like to calculate. Directory paths are forbidden because ESLint cannot handle the `overrides` setting.
246 * (`Promise<Object>`)<br>
247 The promise that will be fulfilled with a configuration object.
249 ### ◆ eslint.isPathIgnored(filePath)
252 const isPathIgnored = await eslint.isPathIgnored(filePath);
255 This method checks if a given file is ignored by your configuration.
259 * `filePath` (`string`)<br>
260 The path to the file you want to check.
264 * (`Promise<boolean>`)<br>
265 The promise that will be fulfilled with whether the file is ignored or not. If the file is ignored, then it will return `true`.
267 ### ◆ eslint.loadFormatter(nameOrPath)
270 const formatter = await eslint.loadFormatter(nameOrPath);
273 This method loads a formatter. Formatters convert lint results to a human- or machine-readable string.
277 * `nameOrPath` (`string | undefined`)<br>
278 The path to the file you want to check. The following values are allowed:
279 * `undefined`. In this case, loads the `"stylish"` built-in formatter.
280 * A name of [built-in formatters][builtin-formatters].
281 * A name of [third-party formatters][thirdparty-formatters]. For examples:
282 * `"foo"` will load `eslint-formatter-foo`.
283 * `"@foo"` will load `@foo/eslint-formatter`.
284 * `"@foo/bar"` will load `@foo/eslint-formatter-bar`.
285 * A path to the file that defines a formatter. The path must contain one or more path separators (`/`) in order to distinguish if it's a path or not. For example, start with `./`.
289 * (`Promise<Formatter>`)<br>
290 The promise that will be fulfilled with a [Formatter] object.
295 const version = ESLint.version;
298 The version string of ESLint. E.g. `"7.0.0"`.
300 This is a static property.
302 ### ◆ ESLint.outputFixes(results)
305 await ESLint.outputFixes(results);
308 This method writes code modified by ESLint's autofix feature into its respective file. If any of the modified files don't exist, this method does nothing.
310 This is a static method.
314 * `results` (`LintResult[]`)<br>
315 The [LintResult] objects to write.
319 * (`Promise<void>`)<br>
320 The promise that will be fulfilled after all files are written.
322 ### ◆ ESLint.getErrorResults(results)
325 const filteredResults = ESLint.getErrorResults(results);
328 This method copies the given results and removes warnings. The returned value contains only errors.
330 This is a static method.
334 * `results` (`LintResult[]`)<br>
335 The [LintResult] objects to filter.
339 * (`LintResult[]`)<br>
340 The filtered [LintResult] objects.
342 ### ◆ LintResult type
344 The `LintResult` value is the information of the linting result of each file. The [`eslint.lintFiles()`][eslint-lintfiles] and [`eslint.lintText()`][eslint-linttext] methods return it. It has the following properties:
346 * `filePath` (`string`)<br>
347 The absolute path to the file of this result. This is the string `"<text>"` if the file path is unknown (when you didn't pass the `options.filePath` option to the [`eslint.lintText()`][eslint-linttext] method).
348 * `messages` (`LintMessage[]`)<br>
349 The array of [LintMessage] objects.
350 * `fixableErrorCount` (`number`)<br>
351 The number of errors that can be fixed automatically by the `fix` constructor option.
352 * `fixableWarningCount` (`number`)<br>
353 The number of warnings that can be fixed automatically by the `fix` constructor option.
354 * `errorCount` (`number`)<br>
355 The number of errors. This includes fixable errors and fatal errors.
356 * `fatalErrorCount` (`number`)<br>
357 The number of fatal errors.
358 * `warningCount` (`number`)<br>
359 The number of warnings. This includes fixable warnings.
360 * `output` (`string | undefined`)<br>
361 The modified source code text. This property is undefined if any fixable messages didn't exist.
362 * `source` (`string | undefined`)<br>
363 The original source code text. This property is undefined if any messages didn't exist or the `output` property exists.
364 * `usedDeprecatedRules` (`{ ruleId: string; replacedBy: string[] }[]`)<br>
365 The information about the deprecated rules that were used to check this file.
367 ### ◆ LintMessage type
369 The `LintMessage` value is the information of each linting error. The `messages` property of the [LintResult] type contains it. It has the following properties:
371 * `ruleId` (`string` | `null`)<br>
372 The rule name that generates this lint message. If this message is generated by the ESLint core rather than rules, this is `null`.
373 * `severity` (`1 | 2`)<br>
374 The severity of this message. `1` means warning and `2` means error.
375 * `fatal` (`boolean | undefined`)<br>
376 `true` if this is a fatal error unrelated to a rule, like a parsing error.
377 * `message` (`string`)<br>
379 * `line` (`number | undefined`)<br>
380 The 1-based line number of the begin point of this message.
381 * `column` (`number | undefined`)<br>
382 The 1-based column number of the begin point of this message.
383 * `endLine` (`number | undefined`)<br>
384 The 1-based line number of the end point of this message. This property is undefined if this message is not a range.
385 * `endColumn` (`number | undefined`)<br>
386 The 1-based column number of the end point of this message. This property is undefined if this message is not a range.
387 * `fix` (`EditInfo | undefined`)<br>
388 The [EditInfo] object of autofix. This property is undefined if this message is not fixable.
389 * `suggestions` (`{ desc: string; fix: EditInfo }[] | undefined`)<br>
390 The list of suggestions. Each suggestion is the pair of a description and an [EditInfo] object to fix code. API users such as editor integrations can choose one of them to fix the problem of this message. This property is undefined if this message doesn't have any suggestions.
394 The `EditInfo` value is information to edit text. The `fix` and `suggestions` properties of [LintMessage] type contain it. It has following properties:
396 * `range` (`[number, number]`)<br>
397 The pair of 0-based indices in source code text to remove.
398 * `text` (`string`)<br>
401 This edit information means replacing the range of the `range` property by the `text` property value. It's like `sourceCodeText.slice(0, edit.range[0]) + edit.text + sourceCodeText.slice(edit.range[1])`. Therefore, it's an add if the `range[0]` and `range[1]` property values are the same value, and it's removal if the `text` property value is empty string.
405 The `Formatter` value is the object to convert the [LintResult] objects to text. The [eslint.loadFormatter()][eslint-loadformatter] method returns it. It has the following method:
407 * `format` (`(results: LintResult[]) => string | Promise<string>`)<br>
408 The method to convert the [LintResult] objects to text.
414 The `SourceCode` type represents the parsed source code that ESLint executes on. It's used internally in ESLint and is also available so that already-parsed code can be used. You can create a new instance of `SourceCode` by passing in the text string representing the code and an abstract syntax tree (AST) in [ESTree](https://github.com/estree/estree) format (including location information, range information, comments, and tokens):
417 const SourceCode = require("eslint").SourceCode;
419 const code = new SourceCode("var foo = bar;", ast);
422 The `SourceCode` constructor throws an error if the AST is missing any of the required information.
424 The `SourceCode` constructor strips Unicode BOM.
425 Please note the AST also should be parsed from stripped text.
428 const SourceCode = require("eslint").SourceCode;
430 const code = new SourceCode("\uFEFFvar foo = bar;", ast);
432 assert(code.hasBOM === true);
433 assert(code.text === "var foo = bar;");
436 ### SourceCode#splitLines()
438 This is a static function on `SourceCode` that is used to split the source code text into an array of lines.
441 const SourceCode = require("eslint").SourceCode;
443 const code = "var a = 1;\nvar b = 2;"
445 // split code into an array
446 const codeLines = SourceCode.splitLines(code);
449 Value of codeLines will be
461 The `Linter` object does the actual evaluation of the JavaScript code. It doesn't do any filesystem operations, it simply parses and reports on the code. In particular, the `Linter` object does not process configuration objects or files. Unless you are working in the browser, you probably want to use the [ESLint class](#eslint-class) class instead.
463 The `Linter` is a constructor, and you can create a new instance by passing in the options you want to use. The available options are:
465 * `cwd` - Path to a directory that should be considered as the current working directory. It is accessible to rules by calling `context.getCwd()` (see [The Context Object](./working-with-rules.md#the-context-object)). If `cwd` is `undefined`, it will be normalized to `process.cwd()` if the global `process` object is defined (for example, in the Node.js runtime) , or `undefined` otherwise.
470 const Linter = require("eslint").Linter;
471 const linter1 = new Linter({ cwd: 'path/to/project' });
472 const linter2 = new Linter();
475 In this example, rules run on `linter1` will get `path/to/project` when calling `context.getCwd()`.
476 Those run on `linter2` will get `process.cwd()` if the global `process` object is defined or `undefined` otherwise (e.g. on the browser <https://eslint.org/demo>).
480 The most important method on `Linter` is `verify()`, which initiates linting of the given text. This method accepts three arguments:
482 * `code` - the source code to lint (a string or instance of `SourceCode`).
483 * `config` - a configuration object that has been processed and normalized by `ESLint` using eslintrc files and/or other configuration arguments.
484 * **Note**: If you want to lint text and have your configuration be read and processed, use [`ESLint#lintFiles()`][eslint-lintfiles] or [`ESLint#lintText()`][eslint-linttext] instead.
485 * `options` - (optional) Additional options for this run.
486 * `filename` - (optional) the filename to associate with the source code.
487 * `preprocess` - (optional) A function that [Processors in Plugins](/docs/developer-guide/working-with-plugins.md#processors-in-plugins) documentation describes as the `preprocess` method.
488 * `postprocess` - (optional) A function that [Processors in Plugins](/docs/developer-guide/working-with-plugins.md#processors-in-plugins) documentation describes as the `postprocess` method.
489 * `filterCodeBlock` - (optional) A function that decides which code blocks the linter should adopt. The function receives two arguments. The first argument is the virtual filename of a code block. The second argument is the text of the code block. If the function returned `true` then the linter adopts the code block. If the function was omitted, the linter adopts only `*.js` code blocks. If you provided a `filterCodeBlock` function, it overrides this default behavior, so the linter doesn't adopt `*.js` code blocks automatically.
490 * `disableFixes` - (optional) when set to `true`, the linter doesn't make either the `fix` or `suggestions` property of the lint result.
491 * `allowInlineConfig` - (optional) set to `false` to disable inline comments from changing ESLint rules.
492 * `reportUnusedDisableDirectives` - (optional) when set to `true`, adds reported errors for unused `eslint-disable` directives when no problems would be reported in the disabled area anyway.
494 If the third argument is a string, it is interpreted as the `filename`.
496 You can call `verify()` like this:
499 const Linter = require("eslint").Linter;
500 const linter = new Linter();
502 const messages = linter.verify("var foo;", {
506 }, { filename: "foo.js" });
508 // or using SourceCode
510 const Linter = require("eslint").Linter,
511 linter = new Linter(),
512 SourceCode = require("eslint").SourceCode;
514 const code = new SourceCode("var foo = bar;", ast);
516 const messages = linter.verify(code, {
520 }, { filename: "foo.js" });
523 The `verify()` method returns an array of objects containing information about the linting warnings and errors. Here's an example:
532 message: "Expected a semicolon.",
540 The information available for each linting message is:
542 * `column` - the column on which the error occurred.
543 * `fatal` - usually omitted, but will be set to true if there's a parsing error (not related to a rule).
544 * `line` - the line on which the error occurred.
545 * `message` - the message that should be output.
546 * `nodeType` - the node or token type that was reported with the problem.
547 * `ruleId` - the ID of the rule that triggered the messages (or null if `fatal` is true).
548 * `severity` - either 1 or 2, depending on your configuration.
549 * `endColumn` - the end column of the range on which the error occurred (this property is omitted if it's not range).
550 * `endLine` - the end line of the range on which the error occurred (this property is omitted if it's not range).
551 * `fix` - an object describing the fix for the problem (this property is omitted if no fix is available).
552 * `suggestions` - an array of objects describing possible lint fixes for editors to programmatically enable (see details in the [Working with Rules docs](./working-with-rules.md#providing-suggestions)).
554 Linting message objects have a deprecated `source` property. This property **will be removed** from linting messages in an upcoming breaking release. If you depend on this property, you should now use the `SourceCode` instance provided by the linter.
556 You can also get an instance of the `SourceCode` object used inside of `linter` by using the `getSourceCode()` method:
559 const Linter = require("eslint").Linter;
560 const linter = new Linter();
562 const messages = linter.verify("var foo = bar;", {
566 }, { filename: "foo.js" });
568 const code = linter.getSourceCode();
570 console.log(code.text); // "var foo = bar;"
573 In this way, you can retrieve the text and AST used for the last run of `linter.verify()`.
575 ### Linter#verifyAndFix()
577 This method is similar to verify except that it also runs autofixing logic, similar to the `--fix` flag on the command line. The result object will contain the autofixed code, along with any remaining linting messages for the code that were not autofixed.
580 const Linter = require("eslint").Linter;
581 const linter = new Linter();
583 const messages = linter.verifyAndFix("var foo", {
590 Output object from this method:
600 The information available is:
602 * `fixed` - True, if the code was fixed.
603 * `output` - Fixed code text (might be the same as input if no fixes were applied).
604 * `messages` - Collection of all messages for the given code (It has the same information as explained above under `verify` block).
606 ### Linter#defineRule
608 Each `Linter` instance holds a map of rule names to loaded rule objects. By default, all ESLint core rules are loaded. If you want to use `Linter` with custom rules, you should use the `defineRule` method to register your rules by ID.
611 const Linter = require("eslint").Linter;
612 const linter = new Linter();
614 linter.defineRule("my-custom-rule", {
622 const results = linter.verify("// some source text", { rules: { "my-custom-rule": "error" } });
625 ### Linter#defineRules
627 This is a convenience method similar to `Linter#defineRule`, except that it allows you to define many rules at once using an object.
630 const Linter = require("eslint").Linter;
631 const linter = new Linter();
634 "my-custom-rule": { /* an ESLint rule */ create() {} },
635 "another-custom-rule": { /* an ESLint rule */ create() {} }
638 const results = linter.verify("// some source text", {
640 "my-custom-rule": "error",
641 "another-custom-rule": "warn"
648 This method returns a map of all loaded rules.
651 const Linter = require("eslint").Linter;
652 const linter = new Linter();
658 'accessor-pairs' => { meta: { docs: [Object], schema: [Array] }, create: [Function: create] },
659 'array-bracket-newline' => { meta: { docs: [Object], schema: [Array] }, create: [Function: create] },
665 ### Linter#defineParser
667 Each instance of `Linter` holds a map of custom parsers. If you want to define a parser programmatically, you can add this function
668 with the name of the parser as first argument and the [parser object](/docs/developer-guide/working-with-custom-parsers.md) as second argument. The default `"espree"` parser will already be loaded for every `Linter` instance.
671 const Linter = require("eslint").Linter;
672 const linter = new Linter();
674 linter.defineParser("my-custom-parser", {
675 parse(code, options) {
680 const results = linter.verify("// some source text", { parser: "my-custom-parser" });
683 ### Linter#version/Linter.version
685 Each instance of `Linter` has a `version` property containing the semantic version number of ESLint that the `Linter` instance is from.
688 const Linter = require("eslint").Linter;
689 const linter = new Linter();
691 linter.version; // => '4.5.0'
694 There is also a `Linter.version` property that you can read without instantiating `Linter`:
697 const Linter = require("eslint").Linter;
699 Linter.version; // => '4.5.0'
706 `eslint.RuleTester` is a utility to write tests for ESLint rules. It is used internally for the bundled rules that come with ESLint, and it can also be used by plugins.
713 const rule = require("../../../lib/rules/my-rule"),
714 RuleTester = require("eslint").RuleTester;
716 const ruleTester = new RuleTester();
718 ruleTester.run("my-rule", rule, {
721 code: "var foo = true",
722 options: [{ allowFoo: true }]
728 code: "var invalidVariable = true",
729 errors: [{ message: "Unexpected invalid variable." }]
732 code: "var invalidVariable = true",
733 errors: [{ message: /^Unexpected.+variable/ }]
739 The `RuleTester` constructor accepts an optional object argument, which can be used to specify defaults for your test cases. For example, if all of your test cases use ES2015, you can set it as a default:
742 const ruleTester = new RuleTester({ parserOptions: { ecmaVersion: 2015 } });
745 The `RuleTester#run()` method is used to run the tests. It should be passed the following arguments:
747 * The name of the rule (string)
748 * The rule object itself (see ["working with rules"](./working-with-rules))
749 * An object containing `valid` and `invalid` properties, each of which is an array containing test cases.
751 A test case is an object with the following properties:
753 * `name` (string, optional): The name to use for the test case, to make it easier to find
754 * `code` (string, required): The source code that the rule should be run on
755 * `options` (array, optional): The options passed to the rule. The rule severity should not be included in this list.
756 * `filename` (string, optional): The filename for the given case (useful for rules that make assertions about filenames).
757 * `only` (boolean, optional): Run this case exclusively for debugging in supported test frameworks.
759 In addition to the properties above, invalid test cases can also have the following properties:
761 * `errors` (number or array, required): Asserts some properties of the errors that the rule is expected to produce when run on this code. If this is a number, asserts the number of errors produced. Otherwise, this should be a list of objects, each containing information about a single reported error. The following properties can be used for an error (all are optional):
762 * `message` (string/regexp): The message for the error
763 * `messageId` (string): The Id for the error. See [testing errors with messageId](#testing-errors-with-messageid) for details
764 * `data` (object): Placeholder data which can be used in combination with `messageId`
765 * `type` (string): The type of the reported AST node
766 * `line` (number): The 1-based line number of the reported location
767 * `column` (number): The 1-based column number of the reported location
768 * `endLine` (number): The 1-based line number of the end of the reported location
769 * `endColumn` (number): The 1-based column number of the end of the reported location
770 * `suggestions` (array): An array of objects with suggestion details to check. See [Testing Suggestions](#testing-suggestions) for details
772 If a string is provided as an error instead of an object, the string is used to assert the `message` of the error.
773 * `output` (string, required if the rule fixes code): Asserts the output that will be produced when using this rule for a single pass of autofixing (e.g. with the `--fix` command line flag). If this is `null`, asserts that none of the reported problems suggest autofixes.
775 Any additional properties of a test case will be passed directly to the linter as config options. For example, a test case can have a `parserOptions` property to configure parser behavior:
780 parserOptions: { ecmaVersion: 2015 }
784 If a valid test case only uses the `code` property, it can optionally be provided as a string containing the code, rather than an object with a `code` key.
786 ### Testing errors with `messageId`
788 If the rule under test uses `messageId`s, you can use `messageId` property in a test case to assert reported error's `messageId` instead of its `message`.
793 errors: [{ messageId: "unexpected" }]
797 For messages with placeholders, a test case can also use `data` property to additionally assert reported error's `message`.
802 errors: [{ messageId: "unexpected", data: { name: "foo" } }]
806 Please note that `data` in a test case does not assert `data` passed to `context.report`. Instead, it is used to form the expected message text which is then compared with the received `message`.
808 ### Testing Suggestions
810 Suggestions can be tested by defining a `suggestions` key on an errors object. The options to check for the suggestions are the following (all are optional):
812 * `desc` (string): The suggestion `desc` value
813 * `messageId` (string): The suggestion `messageId` value for suggestions that use `messageId`s
814 * `data` (object): Placeholder data which can be used in combination with `messageId`
815 * `output` (string): A code string representing the result of applying the suggestion fix to the input code
820 ruleTester.run("my-rule-for-no-foo", rule, {
826 desc: "Rename identifier 'foo' to 'bar'",
834 `messageId` and `data` properties in suggestion test objects work the same way as in error test objects. See [testing errors with messageId](#testing-errors-with-messageid) for details.
837 ruleTester.run("my-rule-for-no-foo", rule, {
843 messageId: "renameFoo",
844 data: { newName: "bar" },
852 ### Customizing RuleTester
854 `RuleTester` depends on two functions to run tests: `describe` and `it`. These functions can come from various places:
856 1. If `RuleTester.describe` and `RuleTester.it` have been set to function values, `RuleTester` will use `RuleTester.describe` and `RuleTester.it` to run tests. You can use this to customize the behavior of `RuleTester` to match a test framework that you're using.
858 If `RuleTester.itOnly` has been set to a function value, `RuleTester` will call `RuleTester.itOnly` instead of `RuleTester.it` to run cases with `only: true`. If `RuleTester.itOnly` is not set but `RuleTester.it` has an `only` function property, `RuleTester` will fall back to `RuleTester.it.only`.
860 2. Otherwise, if `describe` and `it` are present as globals, `RuleTester` will use `global.describe` and `global.it` to run tests and `global.it.only` to run cases with `only: true`. This allows `RuleTester` to work when using frameworks like [Mocha](https://mochajs.org/) without any additional configuration.
861 3. Otherwise, `RuleTester#run` will simply execute all of the tests in sequence, and will throw an error if one of them fails. This means you can simply execute a test file that calls `RuleTester.run` using `Node.js`, without needing a testing framework.
863 `RuleTester#run` calls the `describe` function with two arguments: a string describing the rule, and a callback function. The callback calls the `it` function with a string describing the test case, and a test function. The test function will return successfully if the test passes, and throw an error if the test fails. The signature for `only` is the same as `it`. `RuleTester` calls either `it` or `only` for every case even when some cases have `only: true`, and the test framework is responsible for implementing test case exclusivity. (Note that this is the standard behavior for test suites when using frameworks like [Mocha](https://mochajs.org/); this information is only relevant if you plan to customize `RuleTester.describe`, `RuleTester.it`, or `RuleTester.itOnly`.)
865 Example of customizing `RuleTester`:
870 const RuleTester = require("eslint").RuleTester,
871 test = require("my-test-runner"),
872 myRule = require("../../../lib/rules/my-rule");
874 RuleTester.describe = function(text, method) {
875 RuleTester.it.title = text;
876 return method.call(this);
879 RuleTester.it = function(text, method) {
880 test(RuleTester.it.title + ": " + text, method);
883 // then use RuleTester as documented
885 const ruleTester = new RuleTester();
887 ruleTester.run("my-rule", myRule, {
892 // invalid test cases
899 [configuration object]: ../user-guide/configuring
900 [builtin-formatters]: https://eslint.org/docs/user-guide/formatters/
901 [thirdparty-formatters]: https://www.npmjs.com/search?q=eslintformatter
902 [eslint]: #eslint-class
903 [eslint-constructor]: #-new-eslintoptions
904 [eslint-lintfiles]: #-eslintlintfilespatterns
905 [eslint-linttext]: #-eslintlinttextcode-options
906 [eslint-getrulesmetaforresults]: #-eslintgetrulesmetaforresultsresults
907 [eslint-calculateconfigforfile]: #-eslintcalculateconfigforfilefilepath
908 [eslint-ispathignored]: #-eslintispathignoredfilepath
909 [eslint-loadformatter]: #-eslintloadformatternameorpath
910 [eslint-version]: #-eslintversion
911 [eslint-outputfixes]: #-eslintoutputfixesresults
912 [eslint-geterrorresults]: #-eslintgeterrorresultsresults
913 [lintresult]: #-lintresult-type
914 [lintmessage]: #-lintmessage-type
915 [editinfo]: #-editinfo-type
916 [formatter]: #-formatter-type