index.js

var postcss = require('postcss');

module.exports = postcss.plugin('postcss-pseudo-content-insert',
function (opts) {
    opts = opts || {};

    // Removes an element from an array.
    // String value: the value to search and remove.
    // return: an array with the removed element; false otherwise.
    function removeFromArray(arr, selector) {
        var idx = arr.indexOf(selector);
        if (idx !== -1) {
            return arr.splice(idx, 1);
        }
        return false;
    }

    // Checks to see if a value exists in an array
    function arrayHasValue(arr, value) {
        var idx = arr.indexOf(value);
        if (idx !== -1) {
            return true;
        }
        return false;
    }

    // Adds an element to an array
    // Checks to see if it exists
    function addToArray(arr, selector) {
        if(!arrayHasValue(arr, selector))
            arr.push(selector);
    }

    // Define array for seelectors with missing content & present
    var contentAwaiting = [];
    var contentPresent = [];


    return function (css) {
        var customPseudoExp = /(.*::?)(after|before)$/;

        // Step over each rule
        css.walkRules(function (rule) {
            // If there is more than one selector, loop over each
            // and if there is content missing, add to array
            if(rule.selectors.length > 1) {
                rule.selectors.forEach(function (selector) {
                    var hasContent = false;
                    if(customPseudoExp.test(selector)) {
                        rule.walkDecls(function transformDecl(decl) {
                            // If the property = content, assign flag to true
                            if(decl.prop === 'content')
                                hasContent = true;
                        });
                        if(!hasContent)
                            addToArray(contentAwaiting, selector);
                        else
                            addToArray(contentPresent, selector);
                    }
                });
            }
        });

        // Step again
        css.walkRules(function (rule) {
            // Loop again, this time only rules that have 1 selector
            // only. If content is missing, add it.
            // Then remove the selector from the contentAwaiting array
            if(rule.selectors.length === 1) {
                var hasContent = false;
                if(customPseudoExp.test(rule.selector)) {
                    rule.walkDecls(function transformDecl(decl) {
                        // If the property = content, assign flag to true
                        if(decl.prop === 'content')
                            hasContent = true;
                    });

                    if(!hasContent)
                        rule.append({ prop: 'content', value: '\'\'' });

                    // Remove the selector from contentAwaiting array
                    removeFromArray(contentAwaiting, rule.selector);
                }

            }
        });

        for (var i = 0; i < contentAwaiting.length; i++) {
            css.append(postcss.rule({
                selector: contentAwaiting[i]
            }).append({ prop: 'content', value: '\'\'' }));
        }


    };
});