Language Options

Specifying Environments

An environment provides predefined global variables. The available environments are:

These environments are not mutually exclusive, so you can define more than one at a time.

Environments can be specified inside of a file, in configuration files or using the --env command line flag.

Using configuration comments

To specify environments using a comment inside of your JavaScript file, use the following format:

/* eslint-env node, mocha */

This enables Node.js and Mocha environments.

Using configuration files

To specify environments in a configuration file, use the env key and specify which environments you want to enable by setting each to true. For example, the following enables the browser and Node.js environments:

{
    "env": {
        "browser": true,
        "node": true
    }
}

Or in a package.json file

{
    "name": "mypackage",
    "version": "0.0.1",
    "eslintConfig": {
        "env": {
            "browser": true,
            "node": true
        }
    }
}

And in YAML:

---
  env:
    browser: true
    node: true

Using a plugin

If you want to use an environment from a plugin, be sure to specify the plugin name in the plugins array and then use the unprefixed plugin name, followed by a slash, followed by the environment name. For example:

{
    "plugins": ["example"],
    "env": {
        "example/custom": true
    }
}

Or in a package.json file

{
    "name": "mypackage",
    "version": "0.0.1",
    "eslintConfig": {
        "plugins": ["example"],
        "env": {
            "example/custom": true
        }
    }
}

Specifying Globals

Some of ESLint's core rules rely on knowledge of the global variables available to your code at runtime. Since these can vary greatly between different environments as well as be modified at runtime, ESLint makes no assumptions about what global variables exist in your execution environment. If you would like to use rules that require knowledge of what global variables are available, you can define global variables in your configuration file or by using configuration comments in your source code.

Using configuration comments

To specify globals using a comment inside of your JavaScript file, use the following format:

/* global var1, var2 */

This defines two global variables, var1 and var2. If you want to optionally specify that these global variables can be written to (rather than only being read), then you can set each with a "writable" flag:

/* global var1:writable, var2:writable */

Using configuration files

To configure global variables inside of a configuration file, set the globals configuration property to an object containing keys named for each of the global variables you want to use. For each global variable key, set the corresponding value equal to "writable" to allow the variable to be overwritten or "readonly" to disallow overwriting. For example:

{
    "globals": {
        "var1": "writable",
        "var2": "readonly"
    }
}

And in YAML:

---
  globals:
    var1: writable
    var2: readonly

These examples allow var1 to be overwritten in your code, but disallow it for var2.

Globals can be disabled with the string "off". For example, in an environment where most ES2015 globals are available but Promise is unavailable, you might use this config:

{
    "env": {
        "es6": true
    },
    "globals": {
        "Promise": "off"
    }
}

For historical reasons, the boolean value false and the string value "readable" are equivalent to "readonly". Similarly, the boolean value true and the string value "writeable" are equivalent to "writable". However, the use of older values is deprecated.

Specifying Parser Options

ESLint allows you to specify the JavaScript language options you want to support. By default, ESLint expects ECMAScript 5 syntax. You can override that setting to enable support for other ECMAScript versions as well as JSX by using parser options.

Please note that supporting JSX syntax is not the same as supporting React. React applies specific semantics to JSX syntax that ESLint doesn't recognize. We recommend using eslint-plugin-react if you are using React and want React semantics. By the same token, supporting ES6 syntax is not the same as supporting new ES6 globals (e.g., new types such as Set). For ES6 syntax, use { "parserOptions": { "ecmaVersion": 6 } }; for new ES6 global variables, use { "env": { "es6": true } }. { "env": { "es6": true } } enables ES6 syntax automatically, but { "parserOptions": { "ecmaVersion": 6 } } does not enable ES6 globals automatically.

Parser options are set in your .eslintrc.* file by using the parserOptions property. The available options are:

Here's an example .eslintrc.json file:

{
    "parserOptions": {
        "ecmaVersion": 6,
        "sourceType": "module",
        "ecmaFeatures": {
            "jsx": true
        }
    },
    "rules": {
        "semi": "error"
    }
}

Setting parser options helps ESLint determine what is a parsing error. All language options are false by default.