点和方括号表示法 [英] Dot and Square Bracket Notation

问题描述

我正在尝试理解Dot和Square Bracket符号之间的区别。在SO和其他一些网站上浏览各种示例时，我遇到了这两个简单的例子：

I'm trying to understand the difference between the Dot and the Square Bracket Notation. While going through various examples here on SO and on some other sites, I came across these two simple examples:

var obj = { "abc" : "hello" };
var x = "abc";
var y = obj[x];
console.log(y); //output - hello

var user = {
  name: "John Doe",
  age: 30
};
var key = prompt("Enter the property to modify","name or age");
var value = prompt("Enter new value for " + key);
user[key] = value;
alert("New " + key + ": " + user[key]);

第一个例子返回y是未定义的，如果在第三行我替换 obj [x] ， obj.x 。为什么不你好

The first example returns y to be undefined if in third line I replace the obj[x] with obj.x. Why not "hello"

但在第二个例子中表达式 user [key] 可以简单地替换为 user.key 而不会出现任何异常行为（至少对我而言） 。
现在这让我感到困惑，因为我最近了解到，如果我们想要按存储在变量中的名称访问属性，我们使用[]方括号表示法。

But in the second example the expression user[key] can simply be replaced with user.key without any anomalous behavior (at-least to me). Now this confuses me as I recently learned that if we want to access properties by name stored in a variable, we use the [ ] Square bracket Notation.

推荐答案

在点表示法中，点后面的名称是被引用属性的名称。所以：

In dot notation, the name after the dot is the name of the property being referenced. So:

var foo = "bar";
var obj = { foo: 1, bar: 2 };

console.log(obj.foo) // = 1, since the "foo" property of obj is 1,
                     //      independent of the variable foo

但是，在方括号表示法中，被引用的属性的名称是值，无论是什么在方括号中：

However, in square-bracket notation, the name of the property being referenced is the value of whatever is in the square brackets:

var foo = "bar";
var obj = { foo: 1, bar: 2 };

console.log(obj[foo])   // = 2, since the value of the variable foo is "bar" and
                        //      the "bar" property of obj is 2

console.log(obj["foo"]) // = 1, since the value of the literal "foo" is "foo" and
                        //      the "foo" property of obj is 1

换句话说，点符号 obj.foo 总是相当于 obj [foo] ，而 obj [foo] 取决于值变量 foo 。

In other words, dot-notation obj.foo is always equivalent to obj["foo"], while obj[foo] depends on the value of the variable foo.

在你的特定情况下问题，请注意点表示法和方括号表示法之间的区别：

In the specific case of your question, note the differences between dot notation and square-bracket notation:

// with dot notation
var obj = { name: "John Doe", age: 30 };
var key = "age";
var value = 60;

obj.key = value; // referencing the literal property "key"
console.log(obj) // = { name: "John Doe", age: 30, key: 60 }


// with square bracket notation
var obj = { name: "John Doe", age: 30 };
var key = "age";
var value = 60;

obj[key] = value; // referencing property by the value of the key variable ("age")
console.log(obj)  // = { name: "John Doe", age: 60 }

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