JavaScript 中 == 和 === 有什么区别？

发布于 2024-12-15 21:33:24 字数 587 浏览 2 评论 0原文

可能的重复：
Javascript === 与 == ：哪个重要我使用“等于”运算符？
什么时候 JavaScript == 比 === 更有意义？ < /p>

是将字符串与未定义值进行比较时以下方法之间的区别。

 var x; 
 if(x==undefined) 
 { 
  alert(x); 
 }

为什么

if(x===undefined)
{ 
  alert(x); 
}

在这种情况下我应该更喜欢第二种方法..请让我知道优点..

原文

Possible Duplicate:
Javascript === vs == : Does it matter which “equal” operator I use?
When would JavaScript == make more sense than ===?

What is the difference between below methods in comparing a string with undefined value.

 var x; 
 if(x==undefined) 
 { 
  alert(x); 
 }

and

if(x===undefined)
{ 
  alert(x); 
}

Why should i prefer second method in this case.. Please let me know advantages..

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不疑不惑不回忆 2024-12-22 21:33:24

== 尝试将值转换为相同类型，然后测试它们是否相同。 "5" == 5
=== 不会这样做；它要求对象具有相同类型才相等。 "5" !== 5

在这种情况下，结果是：

如果 x 是 undefined，x == undefined 将为 true 或 null。
仅当 x 为 undefined 时，x === undefined 才会为 true。

如果您希望同等对待 undefined 和 null，您应该更喜欢第一种方法。其常见用途之一是可选函数参数。

function greet(name, greeting) {
    if (name == undefined) name = 'World';
    if (greeting == undefined) greeting = 'Hello';
    alert(greeting + ' ' + name);
}

greet(); // alerts "Hello World"
greet("Bob"); // alerts "Hello Bob"
greet(null, "Goodbye"); // alerts "Goodbye World"

== attempts to convert the values to the same type before testing if they're the same. "5" == 5
=== does not do this; it requires objects to be of the same type to be equal. "5" !== 5

In this case, the result is:

x == undefined will be true if x is undefined or null.
x === undefined will only be true if x is undefined.

You should prefer the first method if you'd like undefined and null to be treated equivalently. One common use of this is optional function arguments.

function greet(name, greeting) {
    if (name == undefined) name = 'World';
    if (greeting == undefined) greeting = 'Hello';
    alert(greeting + ' ' + name);
}

greet(); // alerts "Hello World"
greet("Bob"); // alerts "Hello Bob"
greet(null, "Goodbye"); // alerts "Goodbye World"

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你是年少的欢喜 2024-12-22 21:33:24

假设我们有 x=5，

== 等于

x==8 为 false
x==5 为 true

=== 完全等于（值和类型）

x===5 为 true
x==="5" 是 false

希望你理解这个概念

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疑心病 2024-12-22 21:33:24

=== 也会检查相同的类型。通过几个例子您就会明白：

(1 == '1') //Returns true

由于 == 不关心类型，因此返回 true。但是，如果您想要严格的类型检查，则可以使用 === 因为只有在类型相同且值相同时才返回 true。

(1 === '1') //Returns false
(1 === 1) //Returns true

当两个字符串具有相同的字符序列、相同的长度以及对应的字符相同时，它们是严格相等的
职位。
当两个数字在数值上相等（具有相同的数值）时，它们严格相等。 NaN 不等于任何内容，
包括 NaN。正零和负零彼此相等。
如果两个布尔操作数都为 true 或都为 false，则它们严格相等。
如果两个对象引用同一个对象，那么它们是严格相等的。
Null 和未定义类型是 ==（但不是 ===）。

参考

=== checks for the same type as well. You'll understand with a few examples:

(1 == '1') //Returns true

Since == doesn't bother with types, that returns true. However, if you want strict type checking, you'd use === because that returns true only if the it's of the same type, and is the same value.

(1 === '1') //Returns false
(1 === 1) //Returns true

Two strings are strictly equal when they have the same sequence of characters, same length, and same characters in corresponding
positions.
Two numbers are strictly equal when they are numerically equal (have the same number value). NaN is not equal to anything,
including NaN. Positive and negative zeros are equal to one another.
Two Boolean operands are strictly equal if both are true or both are false.
Two objects are strictly equal if they refer to the same Object.
Null and Undefined types are == (but not ===).

Reference

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