eslint/lib/rules/no-extra-strict.js

/**
 * @fileoverview Rule to flag unnecessary strict directives.
 * @author Ian Christian Myers
 */
"use strict";

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

module.exports = function(context) {

    function directives(block) {
        var ds = [], body = block.body, e, i, l;

        for (i = 0, l = body.length; i < l; ++i) {
            e = body[i];

            if (
                e.type === "ExpressionStatement" &&
                e.expression.type === "Literal" &&
                typeof e.expression.value === "string"
            ) {
                ds.push(e.expression);
            } else {
                break;
            }
        }
        return ds;
    }

    function isStrict(directive) {
        return directive.value === "use strict";
    }

    function checkForUnnecessaryUseStrict(node) {
        var useStrictDirectives, scope;
        useStrictDirectives = directives(node).filter(isStrict);

        switch(useStrictDirectives.length) {
            case 0:
                break;

            case 1:
                scope = context.getScope();
                if (scope.upper && scope.upper.isStrict) {
                    context.report(useStrictDirectives[0], "Unnecessary 'use strict'.");
                }
                break;

            default:
                context.report(useStrictDirectives[1], "Multiple 'use strict' directives.");
        }
    }

    return {

        "Program": checkForUnnecessaryUseStrict,

        "FunctionExpression": function(node) {
            checkForUnnecessaryUseStrict(node.body);
        },

        "FunctionDeclaration": function(node) {
            checkForUnnecessaryUseStrict(node.body);
        }
    };

};