site/node_modules/css-tree/lib/walker/create.js

const { hasOwnProperty } = Object.prototype;
const noop = function() {};

function ensureFunction(value) {
    return typeof value === 'function' ? value : noop;
}

function invokeForType(fn, type) {
    return function(node, item, list) {
        if (node.type === type) {
            fn.call(this, node, item, list);
        }
    };
}

function getWalkersFromStructure(name, nodeType) {
    const structure = nodeType.structure;
    const walkers = [];

    for (const key in structure) {
        if (hasOwnProperty.call(structure, key) === false) {
            continue;
        }

        let fieldTypes = structure[key];
        const walker = {
            name: key,
            type: false,
            nullable: false
        };

        if (!Array.isArray(fieldTypes)) {
            fieldTypes = [fieldTypes];
        }

        for (const fieldType of fieldTypes) {
            if (fieldType === null) {
                walker.nullable = true;
            } else if (typeof fieldType === 'string') {
                walker.type = 'node';
            } else if (Array.isArray(fieldType)) {
                walker.type = 'list';
            }
        }

        if (walker.type) {
            walkers.push(walker);
        }
    }

    if (walkers.length) {
        return {
            context: nodeType.walkContext,
            fields: walkers
        };
    }

    return null;
}

function getTypesFromConfig(config) {
    const types = {};

    for (const name in config.node) {
        if (hasOwnProperty.call(config.node, name)) {
            const nodeType = config.node[name];

            if (!nodeType.structure) {
                throw new Error('Missed `structure` field in `' + name + '` node type definition');
            }

            types[name] = getWalkersFromStructure(name, nodeType);
        }
    }

    return types;
}

function createTypeIterator(config, reverse) {
    const fields = config.fields.slice();
    const contextName = config.context;
    const useContext = typeof contextName === 'string';

    if (reverse) {
        fields.reverse();
    }

    return function(node, context, walk, walkReducer) {
        let prevContextValue;

        if (useContext) {
            prevContextValue = context[contextName];
            context[contextName] = node;
        }

        for (const field of fields) {
            const ref = node[field.name];

            if (!field.nullable || ref) {
                if (field.type === 'list') {
                    const breakWalk = reverse
                        ? ref.reduceRight(walkReducer, false)
                        : ref.reduce(walkReducer, false);

                    if (breakWalk) {
                        return true;
                    }
                } else if (walk(ref)) {
                    return true;
                }
            }
        }

        if (useContext) {
            context[contextName] = prevContextValue;
        }
    };
}

function createFastTraveralMap({
    StyleSheet,
    Atrule,
    Rule,
    Block,
    DeclarationList
}) {
    return {
        Atrule: {
            StyleSheet,
            Atrule,
            Rule,
            Block
        },
        Rule: {
            StyleSheet,
            Atrule,
            Rule,
            Block
        },
        Declaration: {
            StyleSheet,
            Atrule,
            Rule,
            Block,
            DeclarationList
        }
    };
}

export function createWalker(config) {
    const types = getTypesFromConfig(config);
    const iteratorsNatural = {};
    const iteratorsReverse = {};
    const breakWalk = Symbol('break-walk');
    const skipNode = Symbol('skip-node');

    for (const name in types) {
        if (hasOwnProperty.call(types, name) && types[name] !== null) {
            iteratorsNatural[name] = createTypeIterator(types[name], false);
            iteratorsReverse[name] = createTypeIterator(types[name], true);
        }
    }

    const fastTraversalIteratorsNatural = createFastTraveralMap(iteratorsNatural);
    const fastTraversalIteratorsReverse = createFastTraveralMap(iteratorsReverse);

    const walk = function(root, options) {
        function walkNode(node, item, list) {
            const enterRet = enter.call(context, node, item, list);

            if (enterRet === breakWalk) {
                return true;
            }

            if (enterRet === skipNode) {
                return false;
            }

            if (iterators.hasOwnProperty(node.type)) {
                if (iterators[node.type](node, context, walkNode, walkReducer)) {
                    return true;
                }
            }

            if (leave.call(context, node, item, list) === breakWalk) {
                return true;
            }

            return false;
        }

        let enter = noop;
        let leave = noop;
        let iterators = iteratorsNatural;
        let walkReducer = (ret, data, item, list) => ret || walkNode(data, item, list);
        const context = {
            break: breakWalk,
            skip: skipNode,

            root,
            stylesheet: null,
            atrule: null,
            atrulePrelude: null,
            rule: null,
            selector: null,
            block: null,
            declaration: null,
            function: null
        };

        if (typeof options === 'function') {
            enter = options;
        } else if (options) {
            enter = ensureFunction(options.enter);
            leave = ensureFunction(options.leave);

            if (options.reverse) {
                iterators = iteratorsReverse;
            }

            if (options.visit) {
                if (fastTraversalIteratorsNatural.hasOwnProperty(options.visit)) {
                    iterators = options.reverse
                        ? fastTraversalIteratorsReverse[options.visit]
                        : fastTraversalIteratorsNatural[options.visit];
                } else if (!types.hasOwnProperty(options.visit)) {
                    throw new Error('Bad value `' + options.visit + '` for `visit` option (should be: ' + Object.keys(types).sort().join(', ') + ')');
                }

                enter = invokeForType(enter, options.visit);
                leave = invokeForType(leave, options.visit);
            }
        }

        if (enter === noop && leave === noop) {
            throw new Error('Neither `enter` nor `leave` walker handler is set or both aren\'t a function');
        }

        walkNode(root);
    };

    walk.break = breakWalk;
    walk.skip = skipNode;

    walk.find = function(ast, fn) {
        let found = null;

        walk(ast, function(node, item, list) {
            if (fn.call(this, node, item, list)) {
                found = node;
                return breakWalk;
            }
        });

        return found;
    };

    walk.findLast = function(ast, fn) {
        let found = null;

        walk(ast, {
            reverse: true,
            enter(node, item, list) {
                if (fn.call(this, node, item, list)) {
                    found = node;
                    return breakWalk;
                }
            }
        });

        return found;
    };

    walk.findAll = function(ast, fn) {
        const found = [];

        walk(ast, function(node, item, list) {
            if (fn.call(this, node, item, list)) {
                found.push(node);
            }
        });

        return found;
    };

    return walk;
};