When do I get this error?

The "Unnecessary 'use strict'" error (and the alternative "Unnecessary directive '{a}'" error) is thrown when JSLint, JSHint or ESLint encounters a "use strict" directive in code that is already running in strict mode. The following example features a factory function that runs in strict mode and returns another function that has its own strict mode directive:

function factory() {
    "use strict";
    return function () {
        "use strict";
        return true;
    };
}

Why do I get this error?

This error is raised to highlight a completely pointless piece of code. The "use strict" directive applies to the scope in which it appears, and any descendant execution contexts. Here's what the ECMAScript 5 specification tells us about strict mode and functions (ES5 §10.1.1):

Function code that is part of a FunctionDeclaration, FunctionExpression, or accessor PropertyAssignment is strict function code if its FunctionDeclaration, FunctionExpression, or PropertyAssignment is contained in strict mode code or if the function code begins with a Directive Prologue that contains a Use Strict Directive.

If you're receiving this error you can safely remove the highlighted instances of the "use strict" directive and be sure that the function in question will still run in strict mode:

function factory() {
    "use strict";
    return function () {
        return true;
    };
}

In JSHint 1.0.0 and above you have the ability to ignore any warning with a special option syntax. The identifier of this warning is W034. This means you can tell JSHint to not issue this warning with the /*jshint -W034 */ directive.

In ESLint the rule that generates this warning is named no-extra-strict. You can disable it by setting it to 0, or enable it by setting it to 1.