JavaScript 中的 ++i 和 i++ 有什么区别
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What's the difference between ++i and i++ in JavaScript
提问by Mark Sandman
on my quest to learn and improve my JavaScript I came across a script that has a switch / case statement and I noticed some variables are incremented using ++ with the variable before the ++ and then some variables have the ++ after the variable. What's the difference between these? Here's an example of what I'm trying to explain notice the m and y variables.
在我寻求学习和改进我的 JavaScript 的过程中，我遇到了一个带有 switch / case 语句的脚本，我注意到一些变量使用 ++ 递增，并且在 ++ 之前使用变量，然后一些变量在变量之后使用 ++。这些有什么区别？这是我试图解释的一个例子，注意 m 和 y 变量。
switch(f){
case 0:{
++m;
if(m==12){
m=0;
y++;
}
break;
}
case 1:{
--m;
if(m==-1){
m=11;
y--;
}
break;
}
case 2:{
++y;
break;
}
case 3:{
--y;
break;
}
case 4:{
break;
}
}
回答by Mike Samuel
++ireturns the value of iafter it has been incremented. i++returns the value of ibefore incrementing.
++ii增加后返回的值。 i++返回i递增前的值。
When the ++comes before its operand it is called the "pre-increment" operator, and when it comes after it is called the "post-increment" operator.
当它++出现在其操作数之前时，它被称为“前增量”运算符，当它出现在它之后时，它被称为“后增量”运算符。
This distinction is only important if you do something with the result.
只有当你对结果做一些事情时，这种区别才重要。
var i = 0, j = 0;
alert(++i); // alerts 1
alert(j++); // alerts 0
One thing to note though is that even though i++returns the value before incrementing, it still returns the value after it has been converted to a number.
但需要注意的一件事是，即使i++在递增之前返回值，它仍然返回转换为数字后的值。
So
所以
var s = "1";
alert(typeof s++); // alerts "number"
alert(s); // alerts 2, not "11" as if by ("1" + 1)
回答by zzzzBov
The same difference as any other c-style ++incrementor.
与任何其他 c 样式++增量器的区别相同。
foo = ++iis the same as:
foo = ++i是相同的：
i = i + 1;
foo = i;
foo = i++is the same as;
foo = i++是相同的;
foo = i;
i = i + 1;
回答by Connor Smith
var i = 0;
alert('i is ' + (++i)); // i is 1
// i is now 1
var i = 0;
alert('i is ' + (i++)); // i is 0
// i is now 1
回答by Flimzy
In JS (as well as C, Perl, and probably a dozen other languages), the ++i operator increments i beforeit evaluates the statement, and i++ increments it after. Same with --.
在 JS（以及 C、Perl 和其他十几种语言）中，++i 运算符在评估语句之前增加 i ，而 i++在评估语句之后增加它。同--。
Example:
例子：
var i=1;
alert(i++);
Will show "1", but:
将显示“1”，但是：
var i=1;
alert(++i);
Will show "2".
将显示“2”。
回答by Rob Raisch
To illustrate, assuming:
为了说明，假设：
var a = 1;
then
然后
var b = ++a;
results in
结果是
true === (b === 2 && a === 2)
while
尽管
var b = a++;
results in
结果是
true === (b === 1 && a === 2)
回答by erlando
++i is pre-increment, i++ is post-increment.
++i 是前增量，i++ 是后增量。
If you're only incrementing the variable and not using the result at the same time they are equivalent.
如果您只是增加变量而不是同时使用结果，则它们是等效的。
In which case you really should use i += 1 for readability (says Douglas Crockford..)
在这种情况下，您真的应该使用 i += 1 来提高可读性（Douglas Crockford 说..）
