如何使用包含属性名称的变量检查对象属性是否存在？

Question

我正在检查是否存在一个对象属性,其中包含一个保存有问题的属性名称的变量.

var myObj;
myObj.prop = "exists";
var myProp = "p"+"r"+"o"+"p";

if(myObj.myProp){
    alert("yes, i have that property");
};

这是undefined因为它正在寻找,myObj.myProp但我希望它来检查myObj.prop

Answer 1

var myProp = 'prop';
if(myObj.hasOwnProperty(myProp)){
    alert("yes, i have that property");
}

要么

var myProp = 'prop';
if(myProp in myObj){
    alert("yes, i have that property");
}

要么

if('prop' in myObj){
    alert("yes, i have that property");
}

Answer 2

您可以使用hasOwnProperty,但根据引用,您在使用此方法时需要引用:

if (myObj.hasOwnProperty('myProp')) {
    // do something
}

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/hasOwnProperty

另一种方法是在运算符中使用,但这里也需要引号:

if ('myProp' in myObj) {
    // do something
}

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/in

Answer 3

感谢大家的帮助,并推动摆脱eval声明.变量必须在括号中,而不是点符号.这是有效的,干净,正确的代码.

其中每个都是变量:appChoice,underI,underObstr.

if(typeof tData.tonicdata[appChoice][underI][underObstr] !== "undefined"){
    //enter code here
}

Answer 4

有更简单的解决方案，但我没有看到您实际问题的任何答案：

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“它正在寻找 myObj.myProp，但我希望它检查 myObj.prop”

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1. 要从变量获取属性值，请使用方括号表示法。
\n
2. 要测试该属性的真实值，请使用可选\n链接
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3. 要返回布尔值，请使用double-not / bang-bang / (!!)
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4. 如果您确定有一个对象并且只想检查该属性是否存在（即使 prop 值未定义），请使用该in运算符。true或者也许与空值合并运算符结合使用？以避免引发错误。
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var nothing = undefined;\nvar obj = {prop:"hello world"}\nvar myProp = "prop";\n\nconsolelog( 1,()=> obj.myProp); // obj does not have a "myProp"\nconsolelog( 2,()=> obj[myProp]); // brackets works\nconsolelog( 3,()=> nothing[myProp]); // throws if not an object\nconsolelog( 4,()=> obj?.[myProp]); // optional chaining very nice \xe2\xad\x90\xef\xb8\x8f\xe2\xad\x90\xef\xb8\x8f\xe2\xad\x90\xef\xb8\x8f\xe2\xad\x90\xef\xb8\x8f\xe2\xad\x90\xef\xb8\x8f\nconsolelog( 5,()=> nothing?.[myProp]); // optional chaining avoids throwing\nconsolelog( 6,()=> nothing?.[nothing]); // even here it is error-safe\nconsolelog( 7,()=> !!obj?.[myProp]); // double-not yields true\nconsolelog( 8,()=> !!nothing?.[myProp]); // false because undefined\nconsolelog( 9,()=> myProp in obj); // in operator works\nconsolelog(10,()=> myProp in nothing); // throws if not an object\nconsolelog(11,()=> myProp in (nothing ?? {})); // safe from throwing\nconsolelog(12,()=> myProp in {prop:undefined}); // true (prop key exists even though its value undefined)\n\n// beware of \'hasOwnProperty\' pitfalls\n// it can\'t detect inherited properties and \'hasOwnProperty\' is itself inherited\n// also comparatively verbose\nconsolelog(13,()=> obj.hasOwnProperty("hasOwnProperty")); // DANGER: it yields false \nconsolelog(14,()=> nothing.hasOwnProperty("hasOwnProperty")); // throws when undefined\nobj.hasOwnProperty = ()=>true; // someone could overwrite it\nconsolelog(15,()=> obj.hasOwnProperty(nothing)); // DANGER: the foolish overwrite will now always return true\nconsolelog(16,()=> Object.prototype.hasOwnProperty.call(obj,"hasOwnProperty")); // explain?!\nconsolelog(17,()=> Object.hasOwn(obj,"hasOwnProperty")); // explain?!\n\nfunction consolelog(num,myTest){\n    try{\n    console.log(num,myTest());\n    }\n    catch(e){\n    console.log(num,\'throws\',e.message);\n    }\n}

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Answer 5

对于自己的财产:

var loan = { amount: 150 };
if(Object.prototype.hasOwnProperty.call(loan, "amount")) 
{ 
   //will execute
}

注意:使用Object.prototype.hasOwnProperty优于loan.hasOwnProperty(..),以防在原型链中定义自定义hasOwnProperty(这不是这里的情况),如

var foo = {
      hasOwnProperty: function() {
        return false;
      },
      bar: 'Here be dragons'
    };

// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/hasOwnProperty

要在查找中包含继承属性,请使用in运算符:(但是必须将对象放在'in'的右侧,原始值将抛出错误,例如 'home'中的'length'将抛出错误,但'length' in new String('home')不会)

const yoshi = { skulk: true };
const hattori = { sneak: true };
const kuma = { creep: true };
if ("skulk" in yoshi) 
    console.log("Yoshi can skulk");

if (!("sneak" in yoshi)) 
    console.log("Yoshi cannot sneak");

if (!("creep" in yoshi)) 
    console.log("Yoshi cannot creep");

Object.setPrototypeOf(yoshi, hattori);

if ("sneak" in yoshi)
    console.log("Yoshi can now sneak");
if (!("creep" in hattori))
    console.log("Hattori cannot creep");

Object.setPrototypeOf(hattori, kuma);

if ("creep" in hattori)
    console.log("Hattori can now creep");
if ("creep" in yoshi)
    console.log("Yoshi can also creep");

// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/in

注意:有人可能会尝试使用typeof和[]属性访问器,因为以下代码始终不起作用 ...

var loan = { amount: 150 };

loan.installment = undefined;

if("installment" in loan) // correct
{
    // will execute
}

if(typeof loan["installment"] !== "undefined") // incorrect
{
    // will not execute
}

Answer 6

检查对象上是否存在属性的更安全的方法是使用空对象或对象原型来调用 hasOwnProperty()

var foo = {
  hasOwnProperty: function() {
    return false;
  },
  bar: 'Here be dragons'
};

foo.hasOwnProperty('bar'); // always returns false

// Use another Object's hasOwnProperty and call it with 'this' set to foo
({}).hasOwnProperty.call(foo, 'bar'); // true

// It's also possible to use the hasOwnProperty property from the Object
// prototype for this purpose
Object.prototype.hasOwnProperty.call(foo, 'bar'); // true

来自MDN Web Docs的参考- Object.prototype.hasOwnProperty()

Answer 7

您可以使用hasOwnProperty()以及in运算符。

Answer 8

使用新Object.hasOwn方法是另一种选择，其目的是取代该Object.hasOwnProperty方法。

如果指定对象将指定的属性作为其自己的属性，则此静态方法返回 true；如果该属性是继承的或该对象上不存在该属性，则此静态方法返回 false。

请注意，在生产中使用它之前，您必须仔细检查浏览器兼容性表，因为它仍然被认为是一种实验性技术，并且尚未得到所有浏览器的完全支持（但很快就会得到支持）

var myObj = {};
myObj.myProp = "exists";

if (Object.hasOwn(myObj, 'myProp')) {
  alert("yes, i have that property");
}

更多关于Object.hasOwn- https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/hasOwn

Object.hasOwn浏览器兼容性 - https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/hasOwn#browser_compatibility