Disallow declarations in the global scope (no-implicit-globals)

It is the best practice to avoid 'polluting' the global scope with variables that are intended to be local to the script.

Global variables created from a script can produce name collisions with global variables created from another script, which will usually lead to runtime errors or unexpected behavior.

This rule disallows the following:

• Declarations that create one or more variables in the global scope.
• Global variable leaks.
• Redeclarations of read-only global variables and assignments to read-only global variables.

There is an explicit way to create a global variable when needed, by assigning to a property of the global object.

This rule is mostly useful for browser scripts. Top-level declarations in ES modules and CommonJS modules create module-scoped variables. ES modules also have implicit strict mode, which prevents global variable leaks.

By default, this rule does not check const, let and class declarations.

This rule has an object option with one option:

• Set "lexicalBindings" to true if you want this rule to check const, let and class declarations as well.

Rule Details

var and function declarations

When working with browser scripts, developers often forget that variable and function declarations at the top-level scope become global variables on the window object. As opposed to modules which have their own scope. Globals should be explicitly assigned to window or self if that is the intent. Otherwise variables intended to be local to the script should be wrapped in an IIFE.

This rule disallows var and function declarations at the top-level script scope. This does not apply to ES and CommonJS modules since they have a module scope.

Examples of incorrect code for this rule:

/*eslint no-implicit-globals: "error"*/

var foo = 1;

function bar() {}

Examples of correct code for this rule:

/*eslint no-implicit-globals: "error"*/

// explicitly set on window
window.foo = 1;
window.bar = function() {};

// intended to be scope to this file
(function() {
  var foo = 1;

  function bar() {}
})();

Examples of correct code for this rule with "parserOptions": { "sourceType": "module" } in the ESLint configuration:

/*eslint no-implicit-globals: "error"*/

// foo and bar are local to module
var foo = 1;
function bar() {}

Global variable leaks

When the code is not in strict mode, an assignment to an undeclared variable creates a new global variable. This will happen even is the code is in a function.

This does not apply to ES modules since the module code is implicitly in strict mode.

Examples of incorrect code for this rule:

/*eslint no-implicit-globals: "error"*/

foo = 1;

Bar.prototype.baz = function () {
    a = 1; // Intended to be this.a = 1;
};

Read-only global variables

This rule also disallows redeclarations of read-only global variables and assigments to read-only global variables.

A read-only global variable can be a built-in ES global (e.g. Array), an environment specific global (e.g. window in the browser environment), or a global variable defined as readonly in the configuration file or in a /*global */ comment.

• Specifying Environments
• Specifying Globals

Examples of incorrect code for this rule:

/*eslint no-implicit-globals: "error"*/

/*global foo:readonly*/

foo = 1;

Array = [];
var Object;

const, let and class declarations

Lexical declarations const and let, as well as class declarations, create variables that are block-scoped.

However, when declared in the top-level of a browser script these variables are not 'script-scoped'. They are actually created in the global scope and could produce name collisions with var, const and let variables and function and class declarations from other scripts. This does not apply to ES and CommonJS modules.

If the variable is intended to be local to the script, wrap the code with a block or with an immediately-invoked function expression (IIFE).

Examples of correct code for this rule with "lexicalBindings" option set to false (default):

/*eslint no-implicit-globals: ["error", {"lexicalBindings": false}]*/

const foo = 1;

let baz;

class Bar {}

Examples of incorrect code for this rule with "lexicalBindings" option set to true:

/*eslint no-implicit-globals: ["error", {"lexicalBindings": true}]*/

const foo = 1;

let baz;

class Bar {}

Examples of correct code for this rule with "lexicalBindings" option set to true:

/*eslint no-implicit-globals: ["error", {"lexicalBindings": true}]*/

{
    const foo = 1;
    let baz;
    class Bar {}
}

(function() {
    const foo = 1;
    let baz;
    class Bar {}
}());

If you intend to create a global const or let variable or a global class declaration, to be used from other scripts, be aware that there are certain differences when compared to the traditional methods, which are var declarations and assigning to a property of the global window object:

• Lexically declared variables cannot be conditionally created. A script cannot check for the existence of a variable and then create a new one. var variables are also always created, but redeclarations do not cause runtime exceptions.
• Lexically declared variables do not create properties on the global object, which is what a consuming script might expect.
• Lexically declared variables are shadowing properties of the global object, which might produce errors if a consuming script is using both the variable and the property.
• Lexically declared variables can produce a permanent Temporal Dead Zone (TDZ) if the initialization throws an exception. Even the typeof check is not safe from TDZ reference exceptions.

Examples of incorrect code for this rule with "lexicalBindings" option set to true:

/*eslint no-implicit-globals: ["error", {"lexicalBindings": true}]*/

const MyGobalFunction = (function() {
    const a = 1;
    let b = 2;
    return function() {
        return a + b;
    }
}());

Examples of correct code for this rule with "lexicalBindings" option set to true:

/*eslint no-implicit-globals: ["error", {"lexicalBindings": true}]*/

window.MyGobalFunction = (function() {
    const a = 1;
    let b = 2;
    return function() {
        return a + b;
    }
}());

When Not To Use It

In the case of a browser script, if you want to be able to explicitly declare variables and functions in the global scope, and your code is in strict mode or you don't want this rule to warn you about undeclared variables, and you also don't want this rule to warn you about read-only globals, you can disable this rule.

In the case of a CommonJS module, if your code is in strict mode or you don't want this rule to warn you about undeclared variables, and you also don't want this rule to warn you about the read-only globals, you can disable this rule.

In the case of an ES module, if you don't want this rule to warn you about the read-only globals you can disable this rule.

Further Reading

• Immediately-Invoked Function Expression (IIFE)
• ReferenceError: assignment to undeclared variable
• Temporal Dead Zone

Related Rules

• no-undef
• no-global-assign

Version

This rule was introduced in ESLint 2.0.0-alpha-1.

Resources

• Rule source
• Documentation source