怎样实现一个基于JavaScript的简单虚拟机或解释器？

function tokenize(input) {
  const tokens = [];
  const regex = /\s*(\d+|[a-zA-Z_]\w*|=.|\+|\(|\)|;|\n)\s*/g;
  let match;
  while ((match = regex.exec(input))) {
    const value = match[1];
    if (value === '\n') {
      tokens.push({ type: 'newline' });
    } else if (/^[a-zA-Z_]\w*$/.test(value)) {
      tokens.push({ type: 'identifier', value });
    } else if (/^\d+$/.test(value)) {
      tokens.push({ type: 'number', value: parseInt(value) });
    } else if (value === '=' || value === '+' || value === '(' || value === ')') {
      tokens.push({ type: 'operator', value });
    } else if (value === 'print') {
      tokens.push({ type: 'keyword', value });
    }
  }
  return tokens;
}

{
  type: "Program",
  body: [
    { type: "Assignment", name: "x", value: { type: "Number", value: 10 } },
    { type: "Assignment", name: "y", value: { type: "BinaryExpression", left: { type: "Identifier", name: "x" }, operator: "+", right: { type: "Number", value: 5 } } },
    { type: "Print", argument: { type: "Identifier", name: "y" } }
  ]
}

function parse(tokens) {
  let current = 0;
<p>function walk() {
let token = tokens[current];</p><pre class="brush:php;toolbar:false;"><pre class="brush:php;toolbar:false;">if (token.type === 'identifier') {
  // 可能是赋值或表达式
  const name = token.value;
  token = tokens[++current];
  if (token && token.type === 'operator' && token.value === '=') {
    current++;
    const value = walk();
    return {
      type: 'Assignment',
      name,
      value
    };
  }
  return { type: 'Identifier', name };
}

if (token.type === 'number') {
  current++;
  return { type: 'Number', value: token.value };
}

if (token.type === 'operator' && token.value === '+') {
  current++;
  return {
    type: 'BinaryExpression',
    left: { type: 'Identifier', name: tokens[current - 2].value },
    operator: '+',
    right: walk()
  };
}

if (token.type === 'keyword' && token.value === 'print') {
  current++;
  const arg = walk();
  return { type: 'Print', argument: arg };
}

throw new Error(`Unknown token: ${token.type}`);

function evaluate(ast) {
  const env = {};
<p>function exec(node) {
switch (node.type) {
case 'Program':
node.body.forEach(exec);
break;</p><pre class="brush:php;toolbar:false;"><pre class="brush:php;toolbar:false;">  case 'Assignment':
    env[node.name] = exec(node.value);
    break;

  case 'Number':
    return node.value;

  case 'Identifier':
    if (!(node.name in env)) throw new Error(`Undefined variable: ${node.name}`);
    return env[node.name];

  case 'BinaryExpression':
    const left = exec(node.left);
    const right = exec(node.right);
    if (node.operator === '+') return left + right;
    break;

  case 'Print':
    console.log(exec(node.argument));
    break;

  default:
    throw new Error(`Unknown node type: ${node.type}`);
}

const input = `
x = 10
y = x + 5
print(y)
`;
<p>const tokens = tokenize(input);
const ast = parse(tokens);
evaluate(ast); // 输出 15