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JavaScript系列（66）--类型系统实现详解

news 2025/2/9 5:11:30

JavaScript类型系统实现详解 🎯

今天，让我们深入探讨JavaScript类型系统的实现。类型系统是确保代码正确性和可维护性的重要工具，理解其工作原理对于构建大型JavaScript应用至关重要。

类型系统基础概念 🌟

💡 小知识：类型系统是一种语言特性，它为程序中的每个表达式关联一个类型，并确保这些类型在程序执行过程中保持一致。JavaScript虽然是动态类型语言，但我们可以实现静态类型检查来提高代码质量。

类型检查器实现 📊

// 1. 类型定义
class Type {constructor(name) {this.name = name;}toString() {return this.name;}equals(other) {return this.name === other.name;}
}class UnionType extends Type {constructor(types) {super('union');this.types = types;}toString() {return this.types.map(t => t.toString()).join(' | ');}equals(other) {if (!(other instanceof UnionType)) return false;return this.types.every(t => other.types.some(ot => t.equals(ot)));}
}class FunctionType extends Type {constructor(params, returnType) {super('function');this.params = params;this.returnType = returnType;}toString() {const params = this.params.map(p => p.toString()).join(', ');return `(${params}) => ${this.returnType}`;}equals(other) {if (!(other instanceof FunctionType)) return false;return this.params.length === other.params.length &&this.params.every((p, i) => p.equals(other.params[i])) &&this.returnType.equals(other.returnType);}
}// 2. 类型检查器
class TypeChecker {constructor() {this.errors = [];this.scope = new Map();}check(ast) {this.visit(ast);return this.errors;}visit(node) {const method = `visit${node.type}`;if (this[method]) {return this[method](node);}throw new Error(`Unknown node type: ${node.type}`);}visitProgram(node) {node.body.forEach(stmt => this.visit(stmt));}visitVariableDeclaration(node) {node.declarations.forEach(decl => {const valueType = decl.init ? this.visit(decl.init) : null;const declaredType = decl.typeAnnotation ? this.resolveType(decl.typeAnnotation) : null;if (declaredType && valueType && !this.isAssignable(valueType, declaredType)) {this.addError(decl,`Type '${valueType}' is not assignable to type '${declaredType}'`);}this.scope.set(decl.id.name, declaredType || valueType);});}visitBinaryExpression(node) {const leftType = this.visit(node.left);const rightType = this.visit(node.right);switch (node.operator) {case '+':if (leftType.name === 'string' || rightType.name === 'string') {return new Type('string');}return this.checkNumberOperands(node, leftType, rightType);case '-':case '*':case '/':return this.checkNumberOperands(node, leftType, rightType);case '===':case '!==':case '<':case '>':case '<=':case '>=':this.checkComparable(node, leftType, rightType);return new Type('boolean');default:throw new Error(`Unknown operator: ${node.operator}`);}}visitIdentifier(node) {const type = this.scope.get(node.name);if (!type) {this.addError(node, `Cannot find name '${node.name}'`);return new Type('any');}return type;}// 辅助方法isAssignable(source, target) {if (target.name === 'any') return true;if (source.name === 'null' && target instanceof UnionType) {return target.types.some(t => t.name === 'null');}return source.equals(target);}checkNumberOperands(node, left, right) {if (left.name !== 'number' || right.name !== 'number') {this.addError(node,`Operator '${node.operator}' cannot be applied to types '${left}' and '${right}'`);}return new Type('number');}checkComparable(node, left, right) {if (!this.isComparable(left) || !this.isComparable(right)) {this.addError(node,`Operator '${node.operator}' cannot be applied to types '${left}' and '${right}'`);}}isComparable(type) {return ['number', 'string', 'boolean'].includes(type.name);}addError(node, message) {this.errors.push({message,line: node.line,column: node.column});}
}

类型推导系统 🚀

// 1. 类型推导器
class TypeInferer {constructor() {this.constraints = new Map();this.substitutions = new Map();}infer(ast) {this.visit(ast);this.solve();return this.applySubstitutions(ast);}visit(node) {const method = `visit${node.type}`;if (this[method]) {return this[method](node);}throw new Error(`Unknown node type: ${node.type}`);}visitFunctionDeclaration(node) {const paramTypes = node.params.map(() => this.freshTypeVar());const returnType = this.freshTypeVar();// 创建函数类型变量const functionType = new FunctionType(paramTypes, returnType);// 在新作用域中推导函数体const bodyType = this.withScope(() => {node.params.forEach((param, i) => {this.addConstraint(this.getTypeVar(param),paramTypes[i]);});return this.visit(node.body);});// 添加返回类型约束this.addConstraint(returnType, bodyType);return functionType;}visitCallExpression(node) {const funcType = this.visit(node.callee);const argTypes = node.arguments.map(arg => this.visit(arg));const returnType = this.freshTypeVar();this.addConstraint(funcType,new FunctionType(argTypes, returnType));return returnType;}// 约束求解solve() {let changed = true;while (changed) {changed = false;for (const [typeVar, type] of this.constraints) {const newType = this.substitute(type);if (!newType.equals(type)) {this.substitutions.set(typeVar, newType);changed = true;}}}}// 类型替换substitute(type) {if (type instanceof TypeVar) {const subst = this.substitutions.get(type);return subst ? this.substitute(subst) : type;}if (type instanceof FunctionType) {return new FunctionType(type.params.map(t => this.substitute(t)),this.substitute(type.returnType));}return type;}// 辅助方法freshTypeVar() {return new TypeVar();}addConstraint(typeVar, type) {this.constraints.set(typeVar, type);}
}// 2. 类型变量
class TypeVar extends Type {static nextId = 0;constructor() {super(`T${TypeVar.nextId++}`);}
}// 3. 类型统一器
class TypeUnifier {unify(type1, type2) {if (type1 instanceof TypeVar) {return this.unifyVar(type1, type2);}if (type2 instanceof TypeVar) {return this.unifyVar(type2, type1);}if (type1 instanceof FunctionType && type2 instanceof FunctionType) {return this.unifyFunction(type1, type2);}if (type1.equals(type2)) {return type1;}throw new Error(`Cannot unify types ${type1} and ${type2}`);}unifyVar(typeVar, type) {if (this.occursCheck(typeVar, type)) {throw new Error('Recursive type detected');}return type;}unifyFunction(func1, func2) {if (func1.params.length !== func2.params.length) {throw new Error('Function parameter count mismatch');}const params = func1.params.map((p, i) => this.unify(p, func2.params[i]));const returnType = this.unify(func1.returnType,func2.returnType);return new FunctionType(params, returnType);}occursCheck(typeVar, type) {if (type instanceof TypeVar) {return typeVar.equals(type);}if (type instanceof FunctionType) {return type.params.some(p => this.occursCheck(typeVar, p)) ||this.occursCheck(typeVar, type.returnType);}return false;}
}

泛型系统实现 ⚡

// 1. 泛型类型定义
class GenericType extends Type {constructor(name, typeParams) {super(name);this.typeParams = typeParams;}instantiate(typeArgs) {if (typeArgs.length !== this.typeParams.length) {throw new Error('Type argument count mismatch');}const substitutions = new Map();this.typeParams.forEach((param, i) => {substitutions.set(param, typeArgs[i]);});return this.substitute(substitutions);}substitute(substitutions) {return new GenericType(this.name,this.typeParams.map(p => {const subst = substitutions.get(p);return subst || p;}));}
}// 2. 泛型函数处理
class GenericFunction extends FunctionType {constructor(typeParams, params, returnType) {super(params, returnType);this.typeParams = typeParams;}instantiate(typeArgs) {const substitutions = new Map();this.typeParams.forEach((param, i) => {substitutions.set(param, typeArgs[i]);});return new FunctionType(this.params.map(p => this.substituteType(p, substitutions)),this.substituteType(this.returnType, substitutions));}substituteType(type, substitutions) {if (type instanceof TypeVar) {return substitutions.get(type) || type;}if (type instanceof FunctionType) {return new FunctionType(type.params.map(p => this.substituteType(p, substitutions)),this.substituteType(type.returnType, substitutions));}return type;}
}// 3. 泛型约束处理
class ConstrainedType extends Type {constructor(typeVar, constraints) {super(typeVar.name);this.typeVar = typeVar;this.constraints = constraints;}satisfies(type) {return this.constraints.every(constraint => {if (constraint instanceof Type) {return this.isSubtypeOf(type, constraint);}if (typeof constraint === 'function') {return constraint(type);}return false;});}isSubtypeOf(sub, sup) {if (sub.equals(sup)) return true;if (sup instanceof UnionType) {return sup.types.some(t => this.isSubtypeOf(sub, t));}// 添加更多子类型关系检查...return false;}
}

最佳实践建议 💡

类型系统扩展

// 1. 交叉类型支持
class IntersectionType extends Type {constructor(types) {super('intersection');this.types = types;}toString() {return this.types.map(t => t.toString()).join(' & ');}equals(other) {if (!(other instanceof IntersectionType)) return false;return this.types.every(t => other.types.some(ot => t.equals(ot)));}
}// 2. 条件类型支持
class ConditionalType extends Type {constructor(checkType, extendsType, trueType, falseType) {super('conditional');this.checkType = checkType;this.extendsType = extendsType;this.trueType = trueType;this.falseType = falseType;}resolve(type) {return this.isExtends(type, this.extendsType) ?this.trueType :this.falseType;}isExtends(sub, sup) {// 实现类型关系检查return false;}
}// 3. 映射类型支持
class MappedType extends Type {constructor(sourceType, transform) {super('mapped');this.sourceType = sourceType;this.transform = transform;}resolve() {const result = {};for (const key in this.sourceType.properties) {result[key] = this.transform(this.sourceType.properties[key],key);}return new ObjectType(result);}
}