什么是javascript中的[] + []

问题描述

[] + []，[] + {}，{} + []，{} + {}的值是多少？怎么样？

What are the values of []+[], []+{}, {}+[], {}+{}? And How?

我的控制台日志

>{} + {}
>NaN
>[] + []
>''
>{} + []
>0
>[] + {}
>[Object object]

此外

>var a = [] + {}
>a
>[Object object]
>var b = {} + []
>b
>[Object object]

分配值如何变化？

SRC：

推荐答案

{的区别} + [] 就是当你执行赋值时，它会在表达式上下文中解析，但是当你自己键入它时它会被解析为一个语句

The difference for the {} + [] is that when you do an assignment it is parsed in an expression context but when you type it by itself it is parsed as a statement

var b = /*an expression*/

使用匿名函数时可以看到这种区别

This distinction is visible when using anonymous functions

//expression
var f = function(){}; //allowed

//statement
function(){}; //not allowed. function statements need a name.

---

所以在表达式上下文中，我们添加 {} （空对象）和 [] （空列表），其中包含。由于， {} 转换为字符串和 [] 转换为，其串联也是[object Object]。

---

So in the expression context, we add {} (empty object) and [] (empty list) with the binary + operator. Due to the type conversion rules, {} is converted to the string "[object Object]" and [] is converted to the empty string, the concatenation of which is "[object Object]" too.

//parenthesis force an expression
({} + "hello")  //"[object Object]hello"
({} + [])       //"[object Object]"
({} + [1])      //"[object Object]1"
({} + [1,2])    //"[object Object]1,2"

同时，当我们删除括号时， {} 被解析为空块语句。出于实际目的，它就好像我们已完全承诺它一样，留下一个在列表上行动。这会将其转换为数字。

Meanwhile, when we remove the parenthesis, the {} is parsed as an empty block statement. For pratical purposes it works as if we had commited it entirely, leaving an unary + operator to act on the list. This converts it to a number.

+[]     // 0
{} + [] // 0
+{}     // NaN
{} + {} // NaN
{} +"17"// 17

虽然能够从语句中获取值（而不仅仅是表达式）似乎很奇怪，但是Javascript标准，确实 语句的值。在我们的例子中，我们的程序是两个源元素的列表，其 ，。

While it might seem weird to be able to get a value from a statement (instead of only from expressions), the Javascript standard, does specify a value for a statement. In our case, our program is a list of two source elements, whose value is given by the last statement, an expression statement.

查看操作中的语句完成值的方法是通过eval：

A way to see the statement completion value in action is via eval:

 eval("{}")  //undefined
 eval("1")   //1
 eval("1;2") //2

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