Disallow Unused Variables (no-unused-vars)

Variables that are declared and not used anywhere in the code are most likely an error due to incomplete refactoring. Such variables take up space in the code and can lead to confusion by readers.

Rule Details

This rule is aimed at eliminating unused variables, functions and variables in parameters of functions, as such, warns when one is found.

The following patterns are considered warnings:

var x = 10;

var y = 10;
y = 5;

// By default, unused arguments cause warnings.
(function(foo) {
    return 5;
})();

// Unused recursive functions also cause warnings.
function fact(n) {
    if (n < 2) return 1;
    return n * fact(n - 1);
}

The following patterns are not considered warnings:

var x = 10;
alert(x);

// foo is considered used here
myFunc(function foo() {
    // ...
}.bind(this));

(function(foo) {
    return foo;
})();

Options

By default this rule is enabled with all option for variables and after-used for arguments.

{
    "rules": {
        "no-unused-vars": [2, {"vars": "all", "args": "after-used"}]
    }
}

vars

This option has two settings:

all checks all variables for usage, including those in the global scope. This is the default setting.
local checks only that locally-declared variables are used but will allow global variables to be unused.

args

This option has three settings:

all - all named arguments must be used.
after-used - only the last argument must be used. This allows you, for instance, to have two named parameters to a function and as long as you use the second argument, ESLint will not warn you about the first. This is the default setting.
none - do not check arguments.

The following code:

will throw baz is defined but never used when args: after-used
will throw foo is defined but never used and baz is defined but never used when args: all
will throw nothing when args: none

(function(foo, bar, baz) {
    return bar;
})();

When Not to Use It

If you don’t want to be notified about unused variables or function arguments, you can safely turn this rule off.

Version

This rule was introduced in ESLint 0.0.9.