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问检查对象是否是构造函数- IsConstructor
EN

Stack Overflow用户

提问于 2016-09-05 16:09:03

回答 1查看 1.9K关注 0票数 3

我想检查一个JavaScript值是否是一个构造函数，也就是说，它是否有一个[ constructor ]内部方法。

ECMAScript定义了IsConstructor，它正是这样做的，但它是一个内部操作。

所以我想模仿它。我考虑过尝试在try语句中实例化或子类化，但这两种方法都不能可靠地适用于所有情况。

function isConstructor(value) {
  try {
    return new value(), true;
  } catch(err) {
    return false;
  }
}

function isConstructor(value) {
  try {
    return new value(), true;
  } catch(err) {
    return false;
  }
}
var tests = 0,
    failed = 0;  
function test(value, expected, msg) {
  ++tests;
  try {
    var result = isConstructor(window.eval(value));
  } catch(err) {
    result = err;
  }
  if(result !== expected) {
    ++failed;
    console.log('Testing: ' + value + '\nMessage: ' + msg + '\nResult: ' + result + '\nExpected: ' + expected);
  }
}
function testEnd() {
  console.log(failed + ' out of ' + tests + ' tests failed.');
}
test('undefined', false, 'undefined is not a constructor');
test('null', false, 'null is not a constructor');
test('true', false, 'booleans are not constructors');
test('0', false, 'numbers are not constructors');
test('"abc"', false, 'strings are not constructors');
test('Symbol()', false, 'symbols are not constructors');
test('({})', false, '{} is not a constructor');
test('[]', false, 'arrays are not constructors');
test('(function(){})', true, 'normal functions are constructors');
test('(function(){throw TypeError()})', true, 'normal functions are constructors');
test('(function(){}.bind())', true, 'bounded normal functions are constructors');
test('() => {}', false, 'arrow functions are not constructors');
test('((() => {}).bind())', false, 'bounded arrow functions are not constructors');
test('(function*(){})', false, 'generator functions are not constructors');
test('(function*(){}.bind())', false, 'bounded generator functions are not constructors');
test('(class{})', true, 'classes are constructors');
test('(class extends function(){}{})', true, 'classes are constructors');
test('new Proxy([],{})', false, 'proxies whose target is not constructor are not constructors');
test('new Proxy(function(){},{})', true, 'proxies whose target is a constructor are constructors');
test('new Proxy(function(){},{get:()=>{throw TypeError()}})', true, 'proxies whose target is a constructor are constructors');
test('new Proxy(function(){},{construct:()=>{throw TypeError()}})', true, 'proxies whose target is a constructor are constructors');
test('var r1 = Proxy.revocable([],{}); r1.proxy', false, 'revocable proxies whose target is not a constructor are notconstructors');
test('r1.revoke(); r1.proxy', false, 'revoked proxies whose target was not a constructor are not constructors');
test('var r2 = Proxy.revocable(function(){},{}); r2.proxy', true, 'revocable proxies whose target is a constructor are constructors');
test('r2.revoke(); r2.proxy', true, 'revoked proxies whose target was a constructor are constructors');
testEnd();

function isConstructor(value) {
  if(value === null) return false;
  try {
    return class extends value {}, true;
  } catch(err) {
    return false;
  }
}

function isConstructor(value) {
  if(value === null) return false;
  try {
    return class extends value {}, true;
  } catch(err) {
    return false;
  }
}
var tests = 0,
    failed = 0;  
function test(value, expected, msg) {
  ++tests;
  try {
    var result = isConstructor(window.eval(value));
  } catch(err) {
    result = err;
  }
  if(result !== expected) {
    ++failed;
    console.log('Testing: ' + value + '\nMessage: ' + msg + '\nResult: ' + result + '\nExpected: ' + expected);
  }
}
function testEnd() {
  console.log(failed + ' out of ' + tests + ' tests failed.');
}
test('undefined', false, 'undefined is not a constructor');
test('null', false, 'null is not a constructor');
test('true', false, 'booleans are not constructors');
test('0', false, 'numbers are not constructors');
test('"abc"', false, 'strings are not constructors');
test('Symbol()', false, 'symbols are not constructors');
test('({})', false, '{} is not a constructor');
test('[]', false, 'arrays are not constructors');
test('(function(){})', true, 'normal functions are constructors');
test('(function(){throw TypeError()})', true, 'normal functions are constructors');
test('(function(){}.bind())', true, 'bounded normal functions are constructors');
test('() => {}', false, 'arrow functions are not constructors');
test('((() => {}).bind())', false, 'bounded arrow functions are not constructors');
test('(function*(){})', false, 'generator functions are not constructors');
test('(function*(){}.bind())', false, 'bounded generator functions are not constructors');
test('(class{})', true, 'classes are constructors');
test('(class extends function(){}{})', true, 'classes are constructors');
test('new Proxy([],{})', false, 'proxies whose target is not constructor are not constructors');
test('new Proxy(function(){},{})', true, 'proxies whose target is a constructor are constructors');
test('new Proxy(function(){},{get:()=>{throw TypeError()}})', true, 'proxies whose target is a constructor are constructors');
test('new Proxy(function(){},{construct:()=>{throw TypeError()}})', true, 'proxies whose target is a constructor are constructors');
test('var r1 = Proxy.revocable([],{}); r1.proxy', false, 'revocable proxies whose target is not a constructor are notconstructors');
test('r1.revoke(); r1.proxy', false, 'revoked proxies whose target was not a constructor are not constructors');
test('var r2 = Proxy.revocable(function(){},{}); r2.proxy', true, 'revocable proxies whose target is a constructor are constructors');
test('r2.revoke(); r2.proxy', true, 'revoked proxies whose target was a constructor are constructors');
testEnd();

有什么方法可以可靠地测试吗？如果不在ES6或ES7中，可能在某些草稿或建议的特性中？

javascript

constructor

ecma262

回答 1

Stack Overflow用户

回答已采纳

发布于 2016-09-05 18:50:35

这是基于贾森·奥伦多夫在小题大做上发布的代码。

function isConstructor(value) {
  try {
    new new Proxy(value, {construct() { return {}; }});
    return true;
  } catch (err) {
    return false;
  }
}

function isConstructor(value) {
  try {
    new new Proxy(value, {construct() { return {}; }});
    return true;
  } catch (err) {
    return false;
  }
}
var tests = 0,
    failed = 0;
function test(value, expected, msg) {
  ++tests;
  try {
    var result = isConstructor(window.eval(value));
  } catch(err) {
    result = err;
  }
  if(result !== expected) {
    ++failed;
    console.log('Testing: ' + value + '\nMessage: ' + msg + '\nResult: ' + result + '\nExpected: ' + expected);
  }
}
function testEnd() {
  console.log(failed + ' out of ' + tests + ' tests failed.');
}
test('undefined', false, 'undefined is not a constructor');
test('null', false, 'null is not a constructor');
test('true', false, 'booleans are not constructors');
test('0', false, 'numbers are not constructors');
test('"abc"', false, 'strings are not constructors');
test('Symbol()', false, 'symbols are not constructors');
test('({})', false, '{} is not a constructor');
test('[]', false, 'arrays are not constructors');
test('(function(){})', true, 'normal functions are constructors');
test('(function(){throw TypeError()})', true, 'normal functions are constructors');
test('(function(){}.bind())', true, 'bounded normal functions are constructors');
test('() => {}', false, 'arrow functions are not constructors');
test('((() => {}).bind())', false, 'bounded arrow functions are not constructors');
test('(function*(){})', false, 'generator functions are not constructors');
test('(function*(){}.bind())', false, 'bounded generator functions are not constructors');
test('(class{})', true, 'classes are constructors');
test('(class extends function(){}{})', true, 'classes are constructors');
test('new Proxy([],{})', false, 'proxies whose target is not constructor are not constructors');
test('new Proxy(function(){},{})', true, 'proxies whose target is a constructor are constructors');
test('new Proxy(function(){},{get:()=>{throw TypeError()}})', true, 'proxies whose target is a constructor are constructors');
test('new Proxy(function(){},{construct:()=>{throw TypeError()}})', true, 'proxies whose target is a constructor are constructors');
test('var r1 = Proxy.revocable([],{}); r1.proxy', false, 'revocable proxies whose target is not a constructor are notconstructors');
test('r1.revoke(); r1.proxy', false, 'revoked proxies whose target was not a constructor are not constructors');
test('var r2 = Proxy.revocable(function(){},{}); r2.proxy', true, 'revocable proxies whose target is a constructor are constructors');
test('r2.revoke(); r2.proxy', true, 'revoked proxies whose target was a constructor are constructors');
testEnd();

只有当代理对象的初始目标是构造函数时，它们才是构造函数。来自ProxyCreate，

如果目标具有[构造]内部方法，则

设置9.5.14中指定的P的[Construct]内部方法。

因此，代码创建一个代理对象，其目标是我们要检查的值，其处理程序具有不抛出的构造陷阱。

这样，如果代理是构造函数(即测试的值是构造函数)，实例化时它将在陷阱中运行代码，而不是将操作重定向到目标，并且不会抛出。如果代理不是构造函数(即测试值两者都不是)，实例化时它将抛出一个错误。

不过有个小问题。在创建代理时，目标必须是非代理对象，或者是未撤销的代理对象。否则，它会抛出，因此上面的代码认为它是一个非构造函数。

对于原语来说，这是可以的，因为它们不能是构造函数。但是，被撤销的代理可以是构造函数，我们不能正确地测试它们。

为了以不同的方式处理这种情况，您可能需要检测该值是否为已撤销的代理。。

票数 6

页面原文内容由Stack Overflow提供。腾讯云小微IT领域专用引擎提供翻译支持

原文链接：

https://stackoverflow.com/questions/39334278

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