source: frontend/node_modules/estree-walker/src/index.ts

import { BaseNode } from "estree";

type WalkerContext = {
        skip: () => void;
        remove: () => void;
        replace: (node: BaseNode) => void;
};

type WalkerHandler = (
        this: WalkerContext,
        node: BaseNode,
        parent: BaseNode,
        key: string,
        index: number
) => void

type Walker = {
        enter?: WalkerHandler;
        leave?: WalkerHandler;
}

export function walk(ast: BaseNode, { enter, leave }: Walker) {
        return visit(ast, null, enter, leave);
}

let should_skip = false;
let should_remove = false;
let replacement: BaseNode = null;
const context: WalkerContext = {
        skip: () => should_skip = true,
        remove: () => should_remove = true,
        replace: (node: BaseNode) => replacement = node
};

function replace(parent: any, prop: string, index: number, node: BaseNode) {
        if (parent) {
                if (index !== null) {
                        parent[prop][index] = node;
                } else {
                        parent[prop] = node;
                }
        }
}

function remove(parent: any, prop: string, index: number) {
        if (parent) {
                if (index !== null) {
                        parent[prop].splice(index, 1);
                } else {
                        delete parent[prop];
                }
        }
}

function visit(
        node: BaseNode,
        parent: BaseNode,
        enter: WalkerHandler,
        leave: WalkerHandler,
        prop?: string,
        index?: number
) {
        if (node) {
                if (enter) {
                        const _should_skip = should_skip;
                        const _should_remove = should_remove;
                        const _replacement = replacement;
                        should_skip = false;
                        should_remove = false;
                        replacement = null;

                        enter.call(context, node, parent, prop, index);

                        if (replacement) {
                                node = replacement;
                                replace(parent, prop, index, node);
                        }

                        if (should_remove) {
                                remove(parent, prop, index);
                        }

                        const skipped = should_skip;
                        const removed = should_remove;

                        should_skip = _should_skip;
                        should_remove = _should_remove;
                        replacement = _replacement;

                        if (skipped) return node;
                        if (removed) return null;
                }

                for (const key in node) {
                        const value = (node as any)[key];

                        if (typeof value !== 'object') {
                                continue;
                        }

                        else if (Array.isArray(value)) {
                                for (let j = 0, k = 0; j < value.length; j += 1, k += 1) {
                                        if (value[j] !== null && typeof value[j].type === 'string') {
                                                if (!visit(value[j], node, enter, leave, key, k)) {
                                                        // removed
                                                        j--;
                                                }
                                        }
                                }
                        }

                        else if (value !== null && typeof value.type === 'string') {
                                visit(value, node, enter, leave, key, null);
                        }
                }

                if (leave) {
                        const _replacement = replacement;
                        const _should_remove = should_remove;
                        replacement = null;
                        should_remove = false;

                        leave.call(context, node, parent, prop, index);

                        if (replacement) {
                                node = replacement;
                                replace(parent, prop, index, node);
                        }

                        if (should_remove) {
                                remove(parent, prop, index);
                        }

                        const removed = should_remove;

                        replacement = _replacement;
                        should_remove = _should_remove;

                        if (removed) return null;
                }
        }

        return node;
}