Javascript 如何使用保存属性名称的变量检查对象属性是否存在？

How to check if object property exists with a variable holding the property name?

提问by Slopeside Creative

I am checking for the existence of an object property with a variable holding the property name in question.

我正在检查对象属性是否存在，其中的变量包含相关属性名称。

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

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

This is undefinedbecause it's looking for myObj.myPropbut I want it to check for myObj.prop

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

回答by Rocket Hazmat

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

Or

或者

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

Or

或者

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

Note that hasOwnPropertydoesn't check for inherited properties, whereas indoes. For example 'constructor' in myObjis true, but myObj.hasOwnProperty('constructor')is not.

请注意，hasOwnProperty它不会检查继承的属性，而in会检查。例如'constructor' in myObj是真的，但myObj.hasOwnProperty('constructor')不是。

回答by Adorjan Princz

You can use hasOwnProperty, but based on the reference you need quoteswhen using this method:

您可以使用hasOwnProperty，但根据参考，使用此方法时需要引号：

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

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

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

Another way is to use inoperator, but you need quoteshere as well:

另一种方法是使用in运算符，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

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

回答by Slopeside Creative

Thank you for everyone's assistance and pushing to get rid of the eval statement. Variables needed to be in brackets, not dot notation. This works and is clean, proper code.

感谢大家的帮助和推动摆脱 eval 语句。变量需要在括号中，而不是点符号。这有效并且是干净，正确的代码。

Each of these are variables: appChoice, underI, underObstr.

每一个都是变量：appChoice、underI、underObstr。

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

回答by adnan2nd

For own property :

对于自有财产：

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

Note: using Object.prototype.hasOwnPropertyis better than loan.hasOwnProperty(..), in case a custom hasOwnProperty is defined in the prototype chain (which is not the case here), like

注意：使用Object.prototype.hasOwnProperty比 Loan.hasOwnProperty(..) 好，以防在原型链中定义了自定义 hasOwnProperty（这里不是这种情况），例如

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

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

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

To include inherited properties in the finding use the inoperator: (but you must place an object at the right side of 'in', primitive values will throw error, e.g. 'length' in 'home'will throw error, but 'length' in new String('home')won't)

要在发现中包含继承的属性，请使用in运算符：（但您必须在 'in' 的右侧放置一个对象，原始值将抛出错误，例如 'home' 中的 'length'将抛出错误，但'length'在 new String('home') 中不会)

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

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

Note: One may be tempted to use typeof and [ ] property accessor as the following code which doesn't work always...

注意：人们可能会尝试使用 typeof 和 [ ] 属性访问器作为以下代码，这并不总是有效......

var loan = { amount: 150 };

loan.installment = undefined;

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

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

回答by skmasq

A much more secure way to check if property exists on the object is to use empty object or object prototype to call hasOwnProperty()

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

Reference from MDN Web Docs - Object.prototype.hasOwnProperty()

来自MDN 网络文档的参考- Object.prototype.hasOwnProperty()

回答by Simran Kaur

You can use hasOwnProperty()as well as inoperator.

您可以使用hasOwnProperty()以及in运算符。