Working with Rules

Note: This page covers the most recent rule format. There is also a deprecated rule format.

Each rule in ESLint has two files named with its identifier (for example, no-extra-semi).

Important: If you submit a core rule to the ESLint repository, you must follow some conventions explained below.

Here is the basic format of the source file for a rule:

/**
 * @fileoverview Rule to disallow unnecessary semicolons
 * @author Nicholas C. Zakas
 */

"use strict";

//------------------------------------------------------------------------------
// Rule Definition
//------------------------------------------------------------------------------

module.exports = {
    meta: {
        docs: {
            description: "disallow unnecessary semicolons",
            category: "Possible Errors",
            recommended: true
        },
        fixable: "code",
        schema: [] // no options
    },
    create: function(context) {
        return {
            // callback functions
        };
    }
};

Rule Basics

The source file for a rule exports an object with the following properties.

meta (object) contains metadata for the rule:

create (function) returns an object with methods that ESLint calls to “visit” nodes while traversing the abstract syntax tree (AST as defined by ESTree) of JavaScript code:

A rule can use the current node and its surrounding tree to report or fix problems.

Here are methods for the array-callback-return rule:

function checkLastSegment (node) {
    // report problem for function if last code path segment is reachable
}

module.exports = {
    meta: { ... },
    create: function(context) {
        // declare the state of the rule
        return {
            ReturnStatement: function(node) {
                // at a ReturnStatement node while going down
            },
            // at a function expression node while going up:
            "FunctionExpression:exit": checkLastSegment,
            "ArrowFunctionExpression:exit": checkLastSegment,
            onCodePathStart: function (codePath, node) {
                // at the start of analyzing a code path
            },
            onCodePathEnd: function(codePath, node) {
                // at the end of analyzing a code path
            }
        };
    }
};

The Context Object

The context object contains additional functionality that is helpful for rules to do their jobs. As the name implies, the context object contains information that is relevant to the context of the rule. The context object has the following properties:

Additionally, the context object has the following methods:

Note: Earlier versions of ESLint supported additional methods on the context object. Those methods were removed in the new format and should not be relied upon.

context.report()

The main method you’ll use is context.report(), which publishes a warning or error (depending on the configuration being used). This method accepts a single argument, which is an object containing the following properties:

Note that at least one of node or loc is required.

The simplest example is to use just node and message:

context.report({
    node: node,
    message: "Unexpected identifier"
});

The node contains all of the information necessary to figure out the line and column number of the offending text as well the source text representing the node.

You can also use placeholders in the message and provide data:

context.report({
    node: node,
    message: "Unexpected identifier: ",
    data: {
        identifier: node.name
    }
});

Note that leading and trailing whitespace is optional in message parameters.

The node contains all of the information necessary to figure out the line and column number of the offending text as well the source text representing the node.

Applying Fixes

If you’d like ESLint to attempt to fix the problem you’re reporting, you can do so by specifying the fix function when using context.report(). The fix function receives a single argument, a fixer object, that you can use to apply a fix. For example:

context.report({
    node: node,
    message: "Missing semicolon".
    fix: function(fixer) {
        return fixer.insertTextAfter(node, ";");
    }
});

Here, the fix() function is used to insert a semicolon after the node. Note that the fix is not immediately applied and may not be applied at all if there are conflicts with other fixes. If the fix cannot be applied, then the problem message is reported as usual; if the fix can be applied, then the problem message is not reported.

Important: Unless the rule exports the meta.fixable property, ESLint does not apply fixes even if the rule implements fix functions.

The fixer object has the following methods:

Best practices for fixes:

  1. Make fixes that are as small as possible. Anything more than a single character is risky and could prevent other, simpler fixes from being made.
  2. Only make one fix per message. This is enforced because you must return the result of the fixer operation from fix().
  3. Fixes should not introduce clashes with other rules. You can accidentally introduce a new problem that won’t be reported until ESLint is run again. Another good reason to make as small a fix as possible.

context.options

Some rules require options in order to function correctly. These options appear in configuration (.eslintrc, command line, or in comments). For example:

{
    "quotes": ["error", "double"]
}

The quotes rule in this example has one option, "double" (the error is the error level). You can retrieve the options for a rule by using context.options, which is an array containing every configured option for the rule. In this case, context.options[0] would contain "double":

module.exports = {
    create: function(context) {
        var isDouble = (context.options[0] === "double");

        // ...
    }
};

Since context.options is just an array, you can use it to determine how many options have been passed as well as retrieving the actual options themselves. Keep in mind that the error level is not part of context.options, as the error level cannot be known or modified from inside a rule.

When using options, make sure that your rule has some logic defaults in case the options are not provided.

context.getSourceCode()

The SourceCode object is the main object for getting more information about the source code being linted. You can retrieve the SourceCode object at any time by using the getSourceCode() method:

module.exports = {
    create: function(context) {
        var sourceCode = context.getSourceCode();

        // ...
    }
};

Once you have an instance of SourceCode, you can use the methods on it to work with the code:

There are also some properties you can access:

You should use a SourceCode object whenever you need to get more information about the code being linted.

Options Schemas

Rules may export a schema property, which is a JSON schema format description of a rule’s options which will be used by ESLint to validate configuration options and prevent invalid or unexpected inputs before they are passed to the rule in context.options.

There are two formats for a rule’s exported schema. The first is a full JSON Schema object describing all possible options the rule accepts, including the rule’s error level as the first argument and any optional arguments thereafter.

However, to simplify schema creation, rules may also export an array of schemas for each optional positional argument, and ESLint will automatically validate the required error level first. For example, the yoda rule accepts a primary mode argument, as well as an extra options object with named properties.

// "yoda": [2, "never", { "exceptRange": true }]
module.exports = {
    meta: {
        schema: [
            {
                "enum": ["always", "never"]
            },
            {
                "type": "object",
                "properties": {
                    "exceptRange": {
                        "type": "boolean"
                    }
                },
                "additionalProperties": false
            }
        ];
    },
};

In the preceding example, the error level is assumed to be the first argument. It is followed by the first optional argument, a string which may be either "always" or "never". The final optional argument is an object, which may have a Boolean property named exceptRange.

To learn more about JSON Schema, we recommend looking at some examples to start, and also reading Understanding JSON Schema (a free ebook).

Getting the Source

If your rule needs to get the actual JavaScript source to work with, then use the sourceCode.getText() method. This method works as follows:


// get all source
var source = sourceCode.getText();

// get source for just this AST node
var nodeSource = sourceCode.getText(node);

// get source for AST node plus previous two characters
var nodeSourceWithPrev = sourceCode.getText(node, 2);

// get source for AST node plus following two characters
var nodeSourceWithFollowing = sourceCode.getText(node, 0, 2);

In this way, you can look for patterns in the JavaScript text itself when the AST isn’t providing the appropriate data (such as location of commas, semicolons, parentheses, etc.).

Accessing comments

If you need to access comments for a specific node you can use sourceCode.getComments(node):

// the "comments" variable has a "leading" and "trailing" property containing
// its leading and trailing comments, respectively
var comments = sourceCode.getComments(node);

Keep in mind that comments are technically not a part of the AST and are only attached to it on demand, i.e. when you call getComments().

Note: One of the libraries adds AST node properties for comments - do not use these properties. Always use sourceCode.getComments() as this is the only guaranteed API for accessing comments (we will likely change how comments are handled later).

Accessing Code Paths

ESLint analyzes code paths while traversing AST. You can access that code path objects with five events related to code paths.

details here

Rule Unit Tests

Each rule must have a set of unit tests submitted with it to be accepted. The test file is named the same as the source file but lives in tests/lib/. For example, if your rule source file is lib/rules/foo.js then your test file should be tests/lib/rules/foo.js.

For your rule, be sure to test:

  1. All instances that should be flagged as warnings.
  2. At least one pattern that should not be flagged as a warning.

The basic pattern for a rule unit test file is:

/**
 * @fileoverview Tests for no-with rule.
 * @author Nicholas C. Zakas
 */

"use strict";

//------------------------------------------------------------------------------
// Requirements
//------------------------------------------------------------------------------

var rule = require("../../../lib/rules/no-with"),
    RuleTester = require("../../../lib/testers/rule-tester");

//------------------------------------------------------------------------------
// Tests
//------------------------------------------------------------------------------

var ruleTester = new RuleTester();
ruleTester.run("no-with", rule, {
    valid: [
        "foo.bar()"
    ],
    invalid: [
        {
            code: "with(foo) { bar() }",
            errors: [{ message: "Unexpected use of 'with' statement.", type: "WithStatement"}]
        }
    ]
});

Be sure to replace the value of "no-with" with your rule’s ID. There are plenty of examples in the tests/lib/rules/ directory.

Valid Code

Each valid case can be either a string or an object. The object form is used when you need to specify additional global variables or arguments for the rule. For example, the following defines window as a global variable for code that should not trigger the rule being tested:

valid: [
    {
        code: "window.alert()",
        globals: [ "window" ]
    }
]

You can also pass options to the rule (if it accepts them). These arguments are equivalent to how people can configure rules in their .eslintrc file. For example:

valid: [
    {
        code: "var msg = 'Hello';",
        options: [ "single" ]
    }
]

The options property must be an array of options. This gets passed through to context.options in the rule.

Invalid Code

Each invalid case must be an object containing the code to test and at least one message that is produced by the rule. The errors key specifies an array of objects, each containing a message (your rule may trigger multiple messages for the same code). You should also specify the type of AST node you expect to receive back using the type key. The AST node should represent the actual spot in the code where there is a problem. For example:

invalid: [
    {
        code: "function doSomething() { var f; if (true) { var build = true; } f = build; }",
        errors: [
            { message: "build used outside of binding context.", type: "Identifier" }
        ]
    }
]

In this case, the message is specific to the variable being used and the AST node type is Identifier.

You can also check that the rule returns the correct line and column numbers for the message by adding line and column properties as needed (both are optional, but highly recommend):

invalid: [
    {
        code: "function doSomething() { var f; if (true) { var build = true; } f = build; }",
        errors: [
            {
                message: "build used outside of binding context.",
                type: "Identifier",
                line: 1,
                column: 68
            }
        ]
    }
]

The test fails if the line or column reported by the rule doesn’t match the options specified in the test.

Similar to the valid cases, you can also specify options to be passed to the rule:

invalid: [
    {
        code: "function doSomething() { var f; if (true) { var build = true; } f = build; }",
        options: [ "double" ],
        errors: [
            { message: "build used outside of binding context.", type: "Identifier" }
        ]
    }
]

For simpler cases where the only thing that really matters is the error message, you can also specify any errors as strings. You can also have some strings and some objects, if you like.

invalid: [
    {
        code: "'single quotes'",
        options: ["double"],
        errors: ["Strings must use doublequote."]
    }
]

Specifying Globals

If your rule relies on globals to be specified, you can provide global variable declarations by using the globals property. For example:

valid: [
    {
        code: "for (x of a) doSomething();",
        globals: { window: true }
    }
]

The same works on invalid tests:

invalid: [
    {
        code: "'single quotes'",
        globals: { window: true },
        errors: ["Strings must use doublequote."]
    }
]

Specifying Settings

If your rule relies on context.settings to be specified, you can provide those settings by using the settings property. For example:

valid: [
    {
        code: "for (x of a) doSomething();",
        settings: { message: "hi" }
    }
]

The same works on invalid tests:

invalid: [
    {
        code: "'single quotes'",
        settings: { message: "hi" },
        errors: ["Strings must use doublequote."]
    }
]

You can then access context.settings.message inside of the rule.

Specifying Filename

If your rule relies on context.getFilename() to be specified, you can provide the filename by using the filename property. For example:

valid: [
    {
        code: "for (x of a) doSomething();",
        filename: "foo/bar.js"
    }
]

The same works on invalid tests:

invalid: [
    {
        code: "'single quotes'",
        filename: "foo/bar.js",
        errors: ["Strings must use doublequote."]
    }
]

You can then access context.getFilename() inside of the rule.

Specifying Parser and Parser Options

Some tests require that a certain parser configuration must be used. This can be specified in test specifications via the parser and parserOptions properties. While the following examples show usage in valid tests, you can use the same options in invalid tests as well.

For example, to set ecmaVersion to 6 (in order to use constructs like for-of):

valid: [
    {
        code: "for (x of a) doSomething();",
        parserOptions: { ecmaVersion: 6 }
    }
]

If you are working with ES6 modules:

valid: [
    {
        code: "export default function () {};",
        parserOptions: { ecmaVersion: 6, sourceType: "module" }
    }
]

For non-version specific features such as JSX:

valid: [
    {
        code: "var foo = <div>{bar}</div>",
        parserOptions: { ecmaFeatures: { jsx: true } }
    }
]

To use a different parser:

valid: [
    {
        code: "var foo = <div>{bar}</div>",
        parser: "my-custom-parser"
    }
]

The options available and the expected syntax for parserOptions is the same as those used in configuration.

Performance Testing

To keep the linting process efficient and unobtrusive, it is useful to verify the performance impact of new rules or modifications to existing rules.

Overall Performance

The npm run perf command gives a high-level overview of ESLint running time with default rules (eslint:recommended) enabled.

$ git checkout master
Switched to branch 'master'

$ npm run perf
CPU Speed is 2200 with multiplier 7500000
Performance Run #1:  1394.689313ms
Performance Run #2:  1423.295351ms
Performance Run #3:  1385.09515ms
Performance Run #4:  1382.406982ms
Performance Run #5:  1409.68566ms
Performance budget ok:  1394.689313ms (limit: 3409.090909090909ms)

$ git checkout my-rule-branch
Switched to branch 'my-rule-branch'

$ npm run perf
CPU Speed is 2200 with multiplier 7500000
Performance Run #1:  1443.736547ms
Performance Run #2:  1419.193291ms
Performance Run #3:  1436.018228ms
Performance Run #4:  1473.605485ms
Performance Run #5:  1457.455283ms
Performance budget ok:  1443.736547ms (limit: 3409.090909090909ms)

Per-rule Performance

ESLint has a built-in method to track performance of individual rules. Setting the TIMING environment variable will trigger the display, upon linting completion, of the ten longest-running rules, along with their individual running time and relative performance impact as a percentage of total rule processing time.

$ TIMING=1 eslint lib
Rule                    | Time (ms) | Relative
:-----------------------|----------:|--------:
no-multi-spaces         |    52.472 |     6.1%
camelcase               |    48.684 |     5.7%
no-irregular-whitespace |    43.847 |     5.1%
valid-jsdoc             |    40.346 |     4.7%
handle-callback-err     |    39.153 |     4.6%
space-infix-ops         |    35.444 |     4.1%
no-undefined            |    25.693 |     3.0%
no-shadow               |    22.759 |     2.7%
no-empty-class          |    21.976 |     2.6%
semi                    |    19.359 |     2.3%

To test one rule explicitly, combine the --no-eslintrc, and --rule options:

$ TIMING=1 eslint --no-eslintrc --rule "quotes: [2, 'double']" lib
Rule   | Time (ms) | Relative
:------|----------:|--------:
quotes |    18.066 |   100.0%

Rule Naming Conventions

The rule naming conventions for ESLint are fairly simple:

Runtime Rules

The thing that makes ESLint different from other linters is the ability to define custom rules at runtime. This is perfect for rules that are specific to your project or company and wouldn’t make sense for ESLint to ship with. With runtime rules, you don’t have to wait for the next version of ESLint or be disappointed that your rule isn’t general enough to apply to the larger JavaScript community, just write your rules and include them at runtime.

Runtime rules are written in the same format as all other rules. Create your rule as you would any other and then follow these steps:

  1. Place all of your runtime rules in the same directory (i.e., eslint_rules).
  2. Create a configuration file and specify your rule ID error level under the rules key. Your rule will not run unless it has a value of 1 or 2 in the configuration file.
  3. Run the command line interface using the --rulesdir option to specify the location of your runtime rules.