Working with Custom Formatters

While ESLint has some built-in formatters available to format the linting results, it's also possible to create and distribute your own custom formatters. You can include custom formatters in your project directly or create an npm package to distribute them separately.

Each formatter is just a function that receives a results object and a context and returns a string. For example, the following is how the json built-in formatter is implemented:

//my-awesome-formatter.js
module.exports = function(results, context) {
    return JSON.stringify(results, null, 2);
};

A formatter can also be an async function (from ESLint v8.4.0), the following shows a simple example:

//my-awesome-formatter.js
module.exports = async function(results) {
    const formatted = await asyncTask();
    return formatted;
};

To run ESLint with this formatter, you can use the -f (or --format) command line flag:

eslint -f ./my-awesome-formatter.js src/

In order to use a local file as a custom formatter, you must begin the filename with a dot (such as ./my-awesome-formatter.js or ../formatters/my-awesome-formatter.js).

Packaging the Custom Formatter

Custom formatters can also be distributed through npm packages. To do so, create an npm package with a name in the format of eslint-formatter-*, where * is the name of your formatter (such as eslint-formatter-awesome). Projects should then install the package and can use the custom formatter with the -f (or --format) flag like this:

eslint -f awesome src/

Because ESLint knows to look for packages beginning with eslint-formatter- when the specified formatter doesn't begin with a dot, there is no need to type eslint-formatter- when using a packaged custom formatter.

Tips for package.json:

• The main entry should be the JavaScript file implementing your custom formatter.
• Add these keywords to help users find your formatter:
  • "eslint"
  • "eslint-formatter"
  • "eslintformatter"

See all formatters on npm;

The results Argument

The results object passed into a formatter is an array of objects containing the lint results for individual files. Here's some example output:

[
    {
        filePath: "/path/to/a/file.js",
        messages: [
            {
                ruleId: "curly",
                severity: 2,
                message: "Expected { after 'if' condition.",
                line: 2,
                column: 1,
                nodeType: "IfStatement"
            },
            {
                ruleId: "no-process-exit",
                severity: 2,
                message: "Don't use process.exit(); throw an error instead.",
                line: 3,
                column: 1,
                nodeType: "CallExpression"
            }
        ],
        errorCount: 2,
        warningCount: 0,
        fixableErrorCount: 0,
        fixableWarningCount: 0,
        source:
            "var err = doStuff();\nif (err) console.log('failed tests: ' + err);\nprocess.exit(1);\n"
    },
    {
        filePath: "/path/to/Gruntfile.js",
        messages: [],
        errorCount: 0,
        warningCount: 0,
        fixableErrorCount: 0,
        fixableWarningCount: 0
    }
]

The result Object

Each object in the results array is a result object. Each result object contains the path of the file that was linted and information about linting issues that were encountered. Here are the properties available on each result object:

• filePath: The absolute path to the file that was linted.
• messages: An array of message objects. See below for more info about messages.
• errorCount: The number of errors for the given file.
• warningCount: The number of warnings for the given file.
• source: The source code for the given file. This property is omitted if this file has no errors/warnings or if the output property is present.
• output: The source code for the given file with as many fixes applied as possible. This property is omitted if no fix is available.

The message Object

Each message object contains information about the ESLint rule that was triggered by some source code. The properties available on each message object are:

• ruleId: the ID of the rule that produced the error or warning.
• severity: the severity of the failure, 1 for warnings and 2 for errors.
• message: the human readable description of the error.
• line: the line where the issue is located.
• column: the column where the issue is located.
• nodeType: the type of the node in the AST

The context Argument

The formatter function receives an object as the second argument. The object has two properties:

• cwd ... The current working directory. This value comes from the cwd constructor option of the ESLint class.
• rulesMeta ... The meta property values of rules. See the Working with Rules page for more information about rules.

For example, here's what the object would look like if one rule, no-extra-semi, had been run:

{
    cwd: "/path/to/cwd",
    rulesMeta: {
        "no-extra-semi": {
            type: "suggestion",
            docs: {
                description: "disallow unnecessary semicolons",
                recommended: true,
                url: "https://eslint.org/docs/rules/no-extra-semi"
            },
            fixable: "code",
            schema: [],
            messages: {
                unexpected: "Unnecessary semicolon."
            }
        }
    }
}

Note: if a linting is executed by deprecated CLIEngine class, the context argument may be a different value because it is up to the API users. Please check whether the context argument is an expected value or not if you want to support legacy environments.

Examples

Summary formatter

A formatter that only cares about the total count of errors and warnings will look like this:

module.exports = function(results, context) {
    // accumulate the errors and warnings
    var summary = results.reduce(
        function(seq, current) {
            seq.errors += current.errorCount;
            seq.warnings += current.warningCount;
            return seq;
        },
        { errors: 0, warnings: 0 }
    );

    if (summary.errors > 0 || summary.warnings > 0) {
        return (
            "Errors: " +
            summary.errors +
            ", Warnings: " +
            summary.warnings +
            "\n"
        );
    }

    return "";
};

Running eslint with the previous custom formatter,

eslint -f ./my-awesome-formatter.js src/

Will produce the following output:

Errors: 2, Warnings: 4

Detailed formatter

A more complex report will look something like this:

module.exports = function(results, context) {
    var results = results || [];

    var summary = results.reduce(
        function(seq, current) {
            current.messages.forEach(function(msg) {
                var logMessage = {
                    filePath: current.filePath,
                    ruleId: msg.ruleId,
                    ruleUrl: context.rulesMeta[msg.ruleId].docs.url,
                    message: msg.message,
                    line: msg.line,
                    column: msg.column
                };

                if (msg.severity === 1) {
                    logMessage.type = "warning";
                    seq.warnings.push(logMessage);
                }
                if (msg.severity === 2) {
                    logMessage.type = "error";
                    seq.errors.push(logMessage);
                }
            });
            return seq;
        },
        {
            errors: [],
            warnings: []
        }
    );

    if (summary.errors.length > 0 || summary.warnings.length > 0) {
        var lines = summary.errors
            .concat(summary.warnings)
            .map(function(msg) {
                return (
                    "\n" +
                    msg.type +
                    " " +
                    msg.ruleId + (msg.ruleUrl ? " (" + msg.ruleUrl + ")" : "") +
                    "\n  " +
                    msg.filePath +
                    ":" +
                    msg.line +
                    ":" +
                    msg.column
                );
            })
            .join("\n");

        return lines + "\n";
    }
};

So running eslint with this custom formatter:

eslint -f ./my-awesome-formatter.js src/

The output will be

error space-infix-ops (https://eslint.org/docs/rules/space-infix-ops)
  src/configs/bundler.js:6:8
error semi (https://eslint.org/docs/rules/semi)
  src/configs/bundler.js:6:10
warning no-unused-vars (https://eslint.org/docs/rules/no-unused-vars)
  src/configs/bundler.js:5:6
warning no-unused-vars (https://eslint.org/docs/rules/no-unused-vars)
  src/configs/bundler.js:6:6
warning no-shadow (https://eslint.org/docs/rules/no-shadow)
  src/configs/bundler.js:65:32
warning no-unused-vars (https://eslint.org/docs/rules/no-unused-vars)
  src/configs/clean.js:3:6

Passing Arguments to Formatters

While formatter functions do not receive arguments in addition to the results object and the context, it is possible to pass additional data into custom formatters using the methods described below.

Using Environment Variables

Custom formatters have access to environment variables and so can change their behavior based on environment variable data. Here's an example that uses a AF_SKIP_WARNINGS environment variable to determine whether or not to show warnings in the results:

module.exports = function(results) {
    var skipWarnings = process.env.AF_SKIP_WARNINGS === "true"; //af stands for awesome-formatter

    var results = results || [];
    var summary = results.reduce(
        function(seq, current) {
            current.messages.forEach(function(msg) {
                var logMessage = {
                    filePath: current.filePath,
                    ruleId: msg.ruleId,
                    message: msg.message,
                    line: msg.line,
                    column: msg.column
                };

                if (msg.severity === 1) {
                    logMessage.type = "warning";
                    seq.warnings.push(logMessage);
                }
                if (msg.severity === 2) {
                    logMessage.type = "error";
                    seq.errors.push(logMessage);
                }
            });
            return seq;
        },
        {
            errors: [],
            warnings: []
        }
    );

    if (summary.errors.length > 0 || summary.warnings.length > 0) {
        var warnings = !skipWarnings ? summary.warnings : []; // skip the warnings in that case

        var lines = summary.errors
            .concat(warnings)
            .map(function(msg) {
                return (
                    "\n" +
                    msg.type +
                    " " +
                    msg.ruleId +
                    "\n  " +
                    msg.filePath +
                    ":" +
                    msg.line +
                    ":" +
                    msg.column
                );
            })
            .join("\n");

        return lines + "\n";
    }
};

You would run ESLint with this custom formatter and an environment variable set like this:

AF_SKIP_WARNINGS=true eslint -f ./my-awesome-formatter.js src/

The output would be:

error space-infix-ops
  src/configs/bundler.js:6:8

error semi
  src/configs/bundler.js:6:10

Complex Argument Passing

If you find the custom formatter pattern doesn't provide enough options for the way you'd like to format ESLint results, the best option is to use ESLint's built-in JSON formatter and pipe the output to a second program. For example:

eslint -f json src/ | your-program-that-reads-JSON --option

In this example, the your-program-that-reads-json program can accept the raw JSON of ESLint results and process it before outputting its own format of the results. You can pass as many command line arguments to that program as are necessary to customize the output.

Note: Formatting for Terminals

Modern terminals like iTerm2 or Guake expect a specific results format to automatically open filenames when they are clicked. Most terminals support this format for that purpose:

file:line:column