编写一个javascript元循环求值器的方法

在上一篇文章中，我们通过AST完成了微信小程序组件的多端编译，在这篇文章中，让我们更深入一点，通过AST完成一个javascript元循环求值器

结构

一个元循环求值器，完整的应该包含以下内容：

• tokenizer：对代码文本进行词法和语法分析，将代码分割成若干个token
• parser：根据token，生成AST树
• evaluate：根据AST树节点的type，执行对应的apply方法
• apply：根据环境，执行实际的求值计算
• scope：当前代码执行的环境

代码目录

根据结构看，我将代码目录大致拆分为以下几个文件

• parser
• eval
• scope

tokenizer和parser这两个过程不是本文的重点，我统一放在了parser中，交由 @babel/parser 来处理。
evaluate和apply这两个过程我统一放在了eval文件中处理，一会我们重点看下这部分。
scope则放入scope文件。

evaluate-apply

这其实是一个递归计算的过程。

首先，evaluate 接收两个参数，node 当前遍历的AST树节点和 scope 当前环境。然后，evaluate去根据 node 的 type 属性，判断该节点是什么类型。判断出类型后，执行 apply 去求值这个节点所代表的表达式。apply 中会再次递归的执行 evaluate 去计算当前节点的子节点。最终，执行完整颗AST树。

我们来看下具体代码吧

const evaluate = (node: t.Node, scope) => {
 const evalFunc = evaluateMap[node.type];
 if (!evalFunc) {
 throw `${node.loc} ${node.type} 还未实现`;
 }
 return evalFunc(node, scope);
}

以上就是evaluate具体做的事。

其中，evaluateMap 是目前实现的内容集合，我们来看下具体的代码

const evaluateMap: EvaluateMap = {
 File(node: t.File, scope) {
 evaluate(node.program, scope);
 },

 Program(node: t.Program, scope) {
 for (const n of node.body) {
  evaluate(n, scope);
 }
 },

 Identifier(node: t.Identifier, scope) {
 const $var = scope.$find(node.name);
 if (!$var) {
  throw `[Error] ${node.loc}, '${node.name}' 未定义`;
 }
 return $var.$get();
 },

 StringLiteral(node: t.StringLiteral, scope) {
 return node.value;
 },

 NumericLiteral(node: t.NumericLiteral, scope) {
 return node.value;
 },

 BooleanLiteral(node: t.BooleanLiteral, scope) {
 return node.value;
 },

 NullLiteral(node: t.NullLiteral, scope) {
 return null;
 },

 BlockStatement(block: t.BlockStatement, scope) {
 const blockScope = scope.shared ? scope : new Scope('block', scope);
 for (const node of block.body) {
  const res = evaluate(node, blockScope);
  if (res === BREAK || res === CONTINUE || res === RETURN) {
  return res;
  }
 }
 },

 DebuggerStatement(node: t.DebuggerStatement, scope) {
 debugger;
 },

 ExpressionStatement(node: t.ExpressionStatement, scope) {
 evaluate(node.expression, scope);
 },

 ReturnStatement(node: t.ReturnStatement, scope) {
 RETURN.result = (node.argument ? evaluate(node.argument, scope) : void 0);
 return RETURN;
 },

 BreakStatement(node: t.BreakStatement, scope) {
 return BREAK;
 },

 ContinueStatement(node: t.ContinueStatement, scope) {
 return CONTINUE;
 },

 IfStatement(node: t.IfStatement, scope) {
 if (evaluate(node.test, scope)) {
  return evaluate(node.consequent, scope);
 }

 if (node.alternate) {
  const ifScope = new Scope('block', scope, true);
  return evaluate(node.alternate, ifScope)
 }
 },

 SwitchStatement(node: t.SwitchStatement, scope) {
 const discriminant = evaluate(node.discriminant, scope);
 const switchScope = new Scope('switch', scope);
 for (const ca of node.cases){
  if (ca.test === null || evaluate(ca.test, switchScope) === discriminant) {
  const res = evaluate(ca, switchScope);
  if (res === BREAK) {
   break;
  } else if (res === RETURN) {
   return res;
  }
  }
 }
 },

 SwitchCase(node: t.SwitchCase, scope) {
 for (const item of node.consequent) {
  const res = evaluate(item, scope);
  if (res === BREAK || res === RETURN) {
  return res;
  }
 }
 },

 ThrowStatement(node: t.ThrowStatement, scope) {
 throw evaluate(node.argument, scope);
 },

 TryStatement(node: t.TryStatement, scope) {
 try {
  return evaluate(node.block, scope);
 } catch (error) {
  if (node.handler) {
  const catchScope = new Scope('block', scope, true);
  catchScope.$let((node.handler.param).name, error);
  return evaluate(node.handler, catchScope);
  } else {
  throw error;
  }
 } finally {
  if (node.finalizer) {
  return evaluate(node.finalizer, scope);
  }
 }
 },

 CatchClause(node: t.CatchClause, scope) {
 return evaluate(node.body, scope);
 },

 WhileStatement(node: t.WhileStatement, scope) {
 while (evaluate(node.test, scope)) {
  const whileScope = new Scope('loop', scope, true);
  const res = evaluate(node.body, whileScope);
  if (res === CONTINUE) continue;
  if (res === BREAK) break;
  if (res === RETURN) return res;
 }
 },

 ForStatement(node: t.ForStatement, scope) {
 for (
  const forScope = new Scope('loop', scope),
  initVal = evaluate(node.init, forScope);
  evaluate(node.test, forScope);
  evaluate(node.update, forScope)
 ) {
  const res = evaluate(node.body, forScope);
  if (res === CONTINUE) continue;
  if (res === BREAK) break;
  if (res === RETURN) return res;
 }
 },

 ForInStatement(node: t.ForInStatement, scope) {
 const kind = (node.left).kind;
 const decl = (node.left).declarations[0];
 const name = (decl.id).name;

 for (const value in evaluate(node.right, scope)) {
  const forScope = new Scope('loop', scope, true);
  scope.$define(kind, name, value);
  const res = evaluate(node.body, forScope);
  if (res === CONTINUE) continue;
  if (res === BREAK) break;
  if (res === RETURN) return res;
 }
 },

 ForOfStatement(node: t.ForOfStatement, scope) {
 const kind = (node.left).kind;
 const decl = (node.left).declarations[0];
 const name = (decl.id).name;

 for (const value of evaluate(node.right, scope)) {
  const forScope = new Scope('loop', scope, true);
  scope.$define(kind, name, value);
  const res = evaluate(node.body, forScope);
  if (res === CONTINUE) continue;
  if (res === BREAK) break;
  if (res === RETURN) return res;
 }
 },

 FunctionDeclaration(node: t.FunctionDeclaration, scope) {
 const func = evaluateMap.FunctionExpression(node, scope);
 scope.$var(node.id.name, func);
 },

 VariableDeclaration(node: t.VariableDeclaration, scope) {
 const { kind, declarations } = node;
 for (const decl of declarations) {
  const varName = (decl.id).name;
  const value = decl.init ? evaluate(decl.init, scope) : void 0;
  if (!scope.$define(kind, varName, value)) {
  throw `[Error] ${name} 重复定义`
  }
 }
 },

 ThisExpression(node: t.ThisExpression, scope) {
 const _this = scope.$find('this');
 return _this ? _this.$get() : null;
 },

 ArrayExpression(node: t.ArrayExpression, scope) {
 return node.elements.map(item => evaluate(item, scope));
 },

 ObjectExpression(node: t.ObjectExpression, scope) {
 let res = Object.create(null);
 node.properties.forEach((prop) => {
  let key;
  let value;
  if(prop.type === 'ObjectProperty'){
  key = prop.key.name;
  value = evaluate(prop.value, scope);
  res[key] = value;
  }else if (prop.type === 'ObjectMethod'){
  const kind = prop.kind;
  key = prop.key.name;
  value = evaluate(prop.body, scope);
  if(kind === 'method') {
   res[key] = value;
  }else if(kind === 'get') {
   Object.defineProperty(res, key, { get: value });
  }else if(kind === 'set') {
   Object.defineProperty(res, key, { set: value });
  }
  }else if(prop.type === 'SpreadElement'){
  const arg = evaluate(prop.argument, scope);
  res = Object.assign(res, arg);
  }
 });
 return res;
 },

 FunctionExpression(node: t.FunctionExpression, scope) {
 return function (...args: any) {
  const funcScope = new Scope('function', scope, true);
  node.params.forEach((param: t.Identifier, idx) => {
  const { name: paramName } = param;
  funcScope.$let(paramName, args[idx]);
  });
  funcScope.$const('this', this);
  funcScope.$const('arguments', arguments);
  const res = evaluate(node.body, funcScope);
  if (res === RETURN) {
  return res.result;
  }
 }
 },

 ArrowFunctionExpression(node: t.ArrowFunctionExpression, scope) {
 return (...args) => {
  const funcScope = new Scope('function', scope, true);
  node.params.forEach((param: t.Identifier, idx) => {
  const { name: paramName } = param;
  funcScope.$let(paramName, args[idx]);
  });
  const _this = funcScope.$find('this');
  funcScope.$const('this', _this ? _this.$get() : null);
  funcScope.$const('arguments', args);
  const res = evaluate(node.body, funcScope);
  if (res === RETURN) {
  return res.result;
  }
 }
 },

 UnaryExpression(node: t.UnaryExpression, scope) {
 const expressionMap = {
  '~': () => ~evaluate(node.argument, scope),
  '+': () => +evaluate(node.argument, scope),
  '-': () => -evaluate(node.argument, scope),
  '!': () => !evaluate(node.argument, scope),
  'void': () => void evaluate(node.argument, scope),
  'typeof': () => {
  if (node.argument.type === 'Identifier') {
   const $var = scope.$find(node.argument.name);
   const value = $var ? $var.$get() : void 0;
   return typeof value;
  }
  return typeof evaluate(node.argument, scope);
  },
  'delete': () => {
  if (node.argument.type === 'MemberExpression') {
   const { object, property, computed } = node.argument;
   const obj = evaluate(object, scope);
   let prop;
   if (computed) {
   prop = evaluate(property, scope);
   } else {
   prop = property.name;
   }
   return delete obj[prop];
  } else {
   throw '[Error] 出现错误'
  }
  },
 }
 return expressionMap[node.operator]();
 },

 UpdateExpression(node: t.UpdateExpression, scope) {
 const { prefix, argument, operator } = node;
 let $var: IVariable;
 if (argument.type === 'Identifier') {
  $var = scope.$find(argument.name);
  if (!$var) throw `${argument.name} 未定义`;
 } else if (argument.type === 'MemberExpression') {
  const obj = evaluate(argument.object, scope);
  let prop;
  if (argument.computed) {
  prop = evaluate(argument.property, scope);
  } else {
  prop = argument.property.name;
  }
  $var = {
  $set(value: any) {
   obj[prop] = value;
   return true;
  },
  $get() {
   return obj[prop];
  }
  }
 } else {
  throw '[Error] 出现错误'
 }

 const expressionMap = {
  '++': v => {
  $var.$set(v + 1);
  return prefix ? ++v : v++
  },
  '--': v => {
  $var.$set(v - 1);
  return prefix ? --v : v--
  },
 }

 return expressionMap[operator]($var.$get());
 },

 BinaryExpression(node: t.BinaryExpression, scope) {
 const { left, operator, right } = node;
 const expressionMap = {
  '==': (a, b) => a == b,
  '===': (a, b) => a === b,
  '>': (a, b) => a > b,
  '<': (a, b) => a < b,
  '!=': (a, b) => a != b,
  '!==': (a, b) => a !== b,
  '>=': (a, b) => a >= b,
  '<=': (a, b) => a <= b,
  '<<': (a, b) => a << b,
  '>>': (a, b) => a >> b,
  '>>>': (a, b) => a >>> b,
  '+': (a, b) => a + b,
  '-': (a, b) => a - b,
  '*': (a, b) => a * b,
  '/': (a, b) => a / b,
  '&': (a, b) => a & b,
  '%': (a, b) => a % b,
  '|': (a, b) => a | b,
  '^': (a, b) => a ^ b,
  'in': (a, b) => a in b,
  'instanceof': (a, b) => a instanceof b,
 }
 return expressionMap[operator](evaluate(left, scope), evaluate(right, scope));
 },

 AssignmentExpression(node: t.AssignmentExpression, scope) {
 const { left, right, operator } = node;
 let $var: IVariable;

 if (left.type === 'Identifier') {
  $var = scope.$find(left.name);
  if(!$var) throw `${left.name} 未定义`;
 } else if (left.type === 'MemberExpression') {
  const obj = evaluate(left.object, scope);
  let prop;
  if (left.computed) {
  prop = evaluate(left.property, scope);
  } else {
  prop = left.property.name;
  }
  $var = {
  $set(value: any) {
   obj[prop] = value;
   return true;
  },
  $get() {
   return obj[prop];
  }
  }
 } else {
  throw '[Error] 出现错误'
 }

 const expressionMap = {
  '=': v => { $var.$set(v); return $var.$get() },
  '+=': v => { $var.$set($var.$get() + v); return $var.$get() },
  '-=': v => { $var.$set($var.$get() - v); return $var.$get() },
  '*=': v => { $var.$set($var.$get() * v); return $var.$get() },
  '/=': v => { $var.$set($var.$get() / v); return $var.$get() },
  '%=': v => { $var.$set($var.$get() % v); return $var.$get() },
  '<<=': v => { $var.$set($var.$get() << v); return $var.$get() },
  '>>=': v => { $var.$set($var.$get() >> v); return $var.$get() },
  '>>>=': v => { $var.$set($var.$get() >>> v); return $var.$get() },
  '|=': v => { $var.$set($var.$get() | v); return $var.$get() },
  '&=': v => { $var.$set($var.$get() & v); return $var.$get() },
  '^=': v => { $var.$set($var.$get() ^ v); return $var.$get() },
 }

 return expressionMap[operator](evaluate(right, scope));
 },

 LogicalExpression(node: t.LogicalExpression, scope) {
 const { left, right, operator } = node;
 const expressionMap = {
  '&&': () => evaluate(left, scope) && evaluate(right, scope),
  '||': () => evaluate(left, scope) || evaluate(right, scope),
 }
 return expressionMap[operator]();
 },

 MemberExpression(node: t.MemberExpression, scope) {
 const { object, property, computed } = node;
 const obj = evaluate(object, scope);
 let prop;
 if (computed) {
  prop = evaluate(property, scope);
 } else {
  prop = property.name;
 }
 return obj[prop];
 },

 ConditionalExpression(node: t.ConditionalExpression, scope) {
 const { test, consequent, alternate } = node;
 return evaluate(test, scope) ? evaluate(consequent, scope) : evaluate(alternate, scope);
 },

 CallExpression(node: t.CallExpression, scope) {
 const func = evaluate(node.callee, scope);
 const args = node.arguments.map(arg => evaluate(arg, scope));
 let _this;
 if (node.callee.type === 'MemberExpression') {
  _this = evaluate(node.callee.object, scope);
 } else {
  const $var = scope.$find('this');
  _this = $var ? $var.$get() : null;
 }
 return func.apply(_this, args);
 },

 NewExpression(node: t.NewExpression, scope) {
 const func = evaluate(node.callee, scope);
 const args = node.arguments.map(arg => evaluate(arg, scope));
 return new (func.bind(func, ...args));
 },

 SequenceExpression(node: t.SequenceExpression, scope) {
 let last;
 node.expressions.forEach(expr => {
  last = evaluate(expr, scope);
 })
 return last;
 },
}

以上，evaluate-apply 这个过程就完了。

scope

我们再来看下 scope 该如何实现。

class Scope implements IScope {
 public readonly variables: EmptyObj = Object.create(null);

 constructor(
 private readonly scopeType: ScopeType,
 private parent: Scope = null,
 public readonly shared = false,
 ) { }
}

我们构造一个类来模拟 scope。可以看到，Scope 类包含了以下4个属性：

• variables：当前环境下存在的变量
• scopeType：当前环境的type
• parent：当前环境的父环境
• shared：有些时候不需要重复构造子环境，故用此标识

接下来我们看下该如何在环境中声明变量

首先构造一个类来模拟变量

class Variable implements IVariable {
 constructor(
 private kind: Kind,
 private value: any
 ){ }

 $get() {
 return this.value
 }

 $set(value: any) {
 if (this.kind === 'const') {
  return false
 }
 this.value = value;
 return true;
 }
}

这个类中有两个属性和两个方法

• kind 用于标识该变量是通过 var、let 还是 const 声明
• value 表示该变量的值
• $get 和 $set 分别用于获取和设置该变量的值

有了 Variable 类之后，我们就可以编写 Scope 类中的声明变量的方法了。

let 和 const 的声明方式基本一样

$const(varName: string, value: any) {
 const variable = this.variables[varName];
 if (!variable) {
 this.variables[varName] = new Variable('const', value);
 return true;
 }
 return false;
}

$let(varName: string, value: any) {
 const variable = this.variables[varName];
 if (!variable) {
 this.variables[varName] = new Variable('let', value);
 return true;
 }
 return false;
}

var 的声明方式稍微有一点差异，因为js中，除了在 function 中，用var 声明的变量是会被声明到父级作用域的（js的历史遗留坑）。我们看下代码

$var(varName: string, value: any) {
 let scope: Scope = this;
 while (!!scope.parent && scope.scopeType !== 'function') {
 scope = scope.parent;
 }
 const variable = scope.variables[varName];
 if (!variable) {
 scope.variables[varName] = new Variable('var', value);
 } else {
 scope.variables[varName] = variable.$set(value);
 }
 return true
}

除了声明，我们还需要一个寻找变量的方法，该方法会从当前环境开始，一直沿着作用域链，找到最外层的环境为止。因此，代码实现如下

$find(varName: string): null | IVariable {
 if (Reflect.has(this.variables, varName)) {
 return Reflect.get(this.variables, varName);
 }
 if (this.parent) {
 return this.parent.$find(varName);
 }
 return null;
}

以上，一个基本的javascript元循环求值器就完成了

最后

大家可以在 codesandbox 在线体验一下。
完整的项目地址是：nvwajs，欢迎鞭策，欢迎star。

参考

《SICP》
微信小程序也要强行热更代码，鹅厂不服你来肛我呀

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