Node.js API

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.

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.

SourceCode

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 format (including location information, range information, comments, and tokens):

var SourceCode = require("eslint").SourceCode;

var code = new SourceCode("var foo = bar;", ast);

The SourceCode constructor throws an error if the AST is missing any of the required information.

The SourceCode constructor strips Unicode BOM. Please note the AST also should be parsed from stripped text.

var SourceCode = require("eslint").SourceCode;

var code = new SourceCode("\uFEFFvar foo = bar;", ast);

assert(code.hasBOM === true);
assert(code.text === "var foo = bar;");

splitLines()

This is a static function on SourceCode that is used to split the source code text into an array of lines.

var SourceCode = require("eslint").SourceCode;

var code = "var a = 1;\nvar b = 2;"

// split code into an array
var codeLines = SourceCode.splitLines(code);

/*
    Value of codeLines will be
    [
        "var a = 1;",
        "var b = 2;"
    ]
 */

linter

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. You can retrieve linter like this:

var linter = require("eslint").linter;

The most important method on linter is verify(), which initiates linting of the given text. This method accepts four arguments:

You can call verify() like this:

var linter = require("eslint").linter;

var messages = linter.verify("var foo;", {
    rules: {
        semi: 2
    }
}, { filename: "foo.js" });

// or using SourceCode

var linter = require("eslint").linter,
    SourceCode = require("eslint").SourceCode;

var code = new SourceCode("var foo = bar;", ast);

var messages = linter.verify(code, {
    rules: {
        semi: 2
    }
}, { filename: "foo.js" });

The verify() method returns an array of objects containing information about the linting warnings and errors. Here’s an example:

{
    fatal: false,
    ruleId: "semi",
    severity: 2,
    line: 1,
    column: 23,
    message: "Expected a semicolon.",
    fix: {
        range: [1, 15],
        text: ";"
    }
}

The information available for each linting message is:

You can also get an instance of the SourceCode object used inside of linter by using the getSourceCode() method:

var linter = require("eslint").linter;

var messages = linter.verify("var foo = bar;", {
    rules: {
        semi: 2
    }
}, { filename: "foo.js" });

var code = linter.getSourceCode();

console.log(code.text);     // "var foo = bar;"

In this way, you can retrieve the text and AST used for the last run of linter.verify().

CLIEngine

The primary Node.js API is CLIEngine, which is the underlying utility that runs the ESLint command line interface. This object will read the filesystem for configuration and file information but will not output any results. Instead, it allows you direct access to the important information so you can deal with the output yourself.

You can get a reference to the CLIEngine by doing the following:

var CLIEngine = require("eslint").CLIEngine;

The CLIEngine is a constructor, and you can create a new instance by passing in the options you want to use. The available options are:

For example:

var CLIEngine = require("eslint").CLIEngine;

var cli = new CLIEngine({
    envs: ["browser", "mocha"],
    useEslintrc: false,
    rules: {
        semi: 2
    }
});

In this code, a new CLIEngine instance is created that sets two environments, "browser" and "mocha", disables loading of .eslintrc and package.json files, and enables the semi rule as an error. You can then call methods on cli and these options will be used to perform the correct action.

executeOnFiles()

If you want to lint one or more files, use the executeOnFiles() method. This method accepts a single argument, which is an array of files and/or directories to traverse for files. You can pass the same values as you would using the ESLint command line interface, such as "." to search all JavaScript files in the current directory. Here’s an example:

var CLIEngine = require("eslint").CLIEngine;

var cli = new CLIEngine({
    envs: ["browser", "mocha"],
    useEslintrc: false,
    rules: {
        semi: 2
    }
});

// lint myfile.js and all files in lib/
var report = cli.executeOnFiles(["myfile.js", "lib/"]);

The return value is an object containing the results of the linting operation. Here’s an example of a report object:

{
    results: [
        {
            filePath: "./myfile.js",
            output: "foo;",
            messages: [
                {
                    fatal: false,
                    severity: 2,
                    ruleId: "semi",
                    severity: 2,
                    line: 1,
                    column: 23,
                    message: "Expected a semicolon."
                }
            ],
            errorCount: 1,
            warningCount: 0
        }
    ],
    errorCount: 1,
    warningCount: 0
}

The top-level report object has a results array containing all linting results for files that had warnings or errors (any files that did not produce a warning or error are omitted). Each file result includes the filePath, a messages array, errorCount, warningCount, and optionally output. The messages array contains the result of calling linter.verify() on the given file. The errorCount and warningCount give the exact number of errors and warnings respectively on the given file. The output property gives the source code for the file with as many fixes applied as possible, so you can use that to rewrite the files if necessary. The top-level report object also has errorCount and warningCount which give the exact number of errors and warnings respectively on all the files.

Once you get a report object, it’s up to you to determine how to output the results. Fixes will not be automatically applied to the files, even if you set fix: true when constructing the CLIEngine instance. To apply fixes to the files, call outputFixes.

resolveFileGlobPatterns()

You can pass filesystem-style or glob patterns to ESLint and have it function properly. In order to achieve this, ESLint must resolve non-glob patterns into glob patterns before determining which files to execute on. The resolveFileGlobPatterns() methods uses the current settings from CLIEngine to resolve non-glob patterns into glob patterns. Pass an array of patterns that might be passed to the ESLint CLI and it will return an array of glob patterns that mean the same thing. Here’s an example:

var CLIEngine = require("eslint").CLIEngine;

var cli = new CLIEngine({
});

// pass an array of patterns
var globPatterns = cli.resolveFileGlobPatterns(["."]);
console.log(globPatterns[i]);       // ["**/*.js"]

getConfigForFile()

If you want to retrieve a configuration object for a given file, use the getConfigForFile() method. This method accepts one argument, a file path, and returns an object represented the calculated configuration of the file. Here’s an example:

var CLIEngine = require("eslint").CLIEngine;

var cli = new CLIEngine({
    envs: ["browser", "mocha"],
    useEslintrc: false,
    rules: {
        semi: 2
    }
});

var config = cli.getConfigForFile("myfile.js");

Once you have the configuration information, you can pass it into the linter object:

var CLIEngine = require("eslint").CLIEngine,
    linter = require("eslint").linter;

var cli = new CLIEngine({
    envs: ["browser", "mocha"],
    useEslintrc: false,
    rules: {
        semi: 2
    }
});

var config = cli.getConfigForFile("myfile.js");

var messages = linter.verify('var foo;', config);

executeOnText()

If you already have some text to lint, then you can use the executeOnText() method to lint that text. The linter will assume that the text is a file in the current working directory, and so will still obey any .eslintrc and .eslintignore files that may be present. Here’s an example:

var CLIEngine = require("eslint").CLIEngine;

var cli = new CLIEngine({
    envs: ["browser", "mocha"],
    useEslintrc: false,
    rules: {
        semi: 2
    }
});

// lint the supplied text and optionally set
// a filename that is displayed in the report
var report = cli.executeOnText("var foo = 'bar';", "foo.js");

The report returned from executeOnText() is in the same format as from executeOnFiles(), but there is only ever one result in report.results.

If a filename in the optional second parameter matches a file that is configured to be ignored, then this function returns no errors or warnings. To return a warning instead, call the method with true as the optional third parameter.

addPlugin()

Loads a plugin from configuration object with specified name. Name can include plugin prefix (“eslint-plugin-“)

var CLIEngine = require("eslint").CLIEngine;
var cli = new CLIEngine({
    ignore: true
});
cli.addPlugin("eslint-plugin-processor", {
    processors: {
        ".txt": {
            preprocess: function(text) {
                return [text];
            },
            postprocess: function(messages) {
                return messages[0];
            }
        }
    }
});

isPathIgnored()

Checks if a given path is ignored by ESLint.

var CLIEngine = require("eslint").CLIEngine;

var cli = new CLIEngine({
    ignore: true,
    ignorePath: ".customIgnoreFile"
});

var isIgnored = cli.isPathIgnored("foo/bar.js");

getFormatter()

Retrieves a formatter, which you can then use to format a report object. The argument is either the name of a built-in formatter:

or the full path to a JavaScript file containing a custom formatter. You can also omit the argument to retrieve the default formatter.

var CLIEngine = require("eslint").CLIEngine;

var cli = new CLIEngine({
    envs: ["browser", "mocha"],
    useEslintrc: false,
    rules: {
        semi: 2
    }
});

// lint myfile.js and all files in lib/
var report = cli.executeOnFiles(["myfile.js", "lib/"]);

// get the default formatter
var formatter = cli.getFormatter();

// Also could do...
// var formatter = cli.getFormatter("compact");
// var formatter = cli.getFormatter("./my/formatter.js");

// output to console
console.log(formatter(report.results));

Note: Also available as a static function on CLIEngine.

// get the default formatter by calling the static function
var formatter = CLIEngine.getFormatter();

Important: You must pass in the results property of the report. Passing in report directly will result in an error.

getErrorResults()

This is a static function on CLIEngine. It can be used to filter out all the non error messages from the report object.

var CLIEngine = require("eslint").CLIEngine;

var cli = new CLIEngine({
    envs: ["browser", "mocha"],
    useEslintrc: false,
    rules: {
        semi: 2
    }
});

// lint myfile.js and all files in lib/
var report = cli.executeOnFiles(["myfile.js", "lib/"]);

// only get the error messages
var errorReport = CLIEngine.getErrorResults(report.results)

Important: You must pass in the results property of the report. Passing in report directly will result in an error.

outputFixes()

This is a static function on CLIEngine that is used to output fixes from report to disk. It does by looking for files that have an output property in their results. Here’s an example:

var CLIEngine = require("eslint").CLIEngine;

var cli = new CLIEngine({
    envs: ["browser", "mocha"],
    useEslintrc: false,
    rules: {
        semi: 2
    }
});

// lint myfile.js and all files in lib/
var report = cli.executeOnFiles(["myfile.js", "lib/"]);

// output fixes to disk
CLIEngine.outputFixes(report);

Deprecated APIs