检查JavaScript中的对象属性是否未定义的最佳方法是什么？

---

Use:

if (typeof something === "undefined") {
    alert("something is undefined");
}

If an object variable which have some properties you can use same thing like this:

if (typeof my_obj.someproperties === "undefined"){
    console.log('the property is not available...'); // print into console
}

---

I believe there are a number of incorrect answers to this topic. Contrary to common belief, "undefined" is not a keyword in JavaScript and can in fact have a value assigned to it.

Correct Code

The most robust way to perform this test is:

if (typeof myVar === "undefined")

This will always return the correct result, and even handles the situation where myVar is not declared.

Degenerate code. DO NOT USE.

var undefined = false;  // Shockingly, this is completely legal!
if (myVar === undefined) {
    alert("You have been misled. Run away!");
}

Additionally, myVar === undefined will raise an error in the situation where myVar is undeclared.

---

In JavaScript there is null and there is undefined . They have different meanings.

undefined means that the variable value has not been defined; it is not known what the value is.

null means that the variable value is defined and set to null (has no value).

Marijn Haverbeke states, in his free, online book "Eloquent JavaScript" (emphasis mine):

There is also a similar value, null, whose meaning is 'this value is defined, but it does not have a value'. The difference in meaning between undefined and null is mostly academic, and usually not very interesting. In practical programs, it is often necessary to check whether something 'has a value'. In these cases, the expression something == undefined may be used, because, even though they are not exactly the same value, null == undefined will produce true.

So, I guess the best way to check if something was undefined would be:

if (something == undefined)

Hope this helps!

Edit: In response to your edit, object properties should work the same way.

var person = {
    name: "John",
    age: 28,
    sex: "male"
};

alert(person.name); // "John"
alert(person.fakeVariable); // undefined

链接地址: http://www.djcxy.com/p/520.html

上一篇: 使用Git版本控制查看文件的更改历史记录

下一篇: 检测未定义的对象属性