javascript i++ 与 ++i
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javascript i++ vs ++i
提问by Web_Designer
In javascript I have seen i++used in many cases, and I understand that it adds one to the preceding value:
在 javascript 中，我已经看到i++在许多情况下使用过，并且我知道它会在前面的值上加 1：
for (var i=1; i<=10; i++) {
console.log(i);
}But what happens when I do this:
但是当我这样做时会发生什么：
++i;
And is it any different using the --operator (besides of course that it's subtraction rather than addition)?
使用--运算符有什么不同吗（当然，它是减法而不是加法）？
回答by Guffa
The difference between i++and ++iis the value of the expression.
i++和之间的区别++i是表达式的值。
The value i++is the value of ibefore the increment. The value of ++iis the value of iafter the increment.
该值i++是i增量前的值。的值++i是i增量后的值。
Example:
例子：
var i = 42;
alert(i++); // shows 42
alert(i); // shows 43
i = 42;
alert(++i); // shows 43
alert(i); // shows 43
The i--and --ioperators works the same way.
在i--与--i运营商的工作方式相同。
回答by Delan Azabani
++variableincrements the variable, returning the new value.
++variable增加变量，返回新值。
variable++increments the variable, but returns the old value.
variable++增加变量，但返回旧值。
--variabledecrements the variable, returning the new value.
--variable递减变量，返回新值。
variable--decrements the variable, but returns the old value.
variable--递减变量，但返回旧值。
For example:
例如：
a = 5;
b = 5;
c = ++a;
d = b++;
ais 6, bis 6, cis 6 and dis 5.
a是 6，b是 6，c是 6，d是 5。
If you're not using the result, the prefix operators work equally to the postfix operators.
如果您不使用结果，则前缀运算符与后缀运算符的作用相同。
回答by bladnman
I thought for completeness I would add an answer specific to the first of the OP's question:
我认为为了完整性，我会添加一个特定于 OP 问题的第一个答案：
One of your example shows the i++ / ++i being used in a for loop :
您的示例之一显示了在 for 循环中使用的 i++ / ++i ：
for (i=1; i<=10; i++) {
alert(i);
}
you will get 1-10 in your alerts no matter which you use. Example:
无论您使用哪种警报，您都会在警报中获得 1-10。例子：
console.log("i++");
for (i=1; i<=10; i++) {
console.log(i);
}
console.log("++i");
for (i=1; i<=10; ++i) {
console.log(i);
}
Paste those into a console window and you can see that they both have the same output.
将它们粘贴到控制台窗口中，您可以看到它们都具有相同的输出。
回答by kaush
i++= Use the value of i in statement then increase it by 1 ++i= Increase the value of i by 1 then use in statement.
i++= 在语句中使用 i 的值然后将其++i增加1 = 将 i 的值增加 1 然后在语句中使用。
回答by chharvey
One case all these answers fail to mention is what happens when i++and ++iare used in operations with other numbers. While the whole “i++is before, ++iis after” concept is easy to grasp when the expression is by itself, it gets much more confusing when you start combining statements. See Examples C and D below.
所有这些答案都没有提到的一种情况是，在与其他数字的运算中使用i++和++i使用时会发生什么。虽然当表达式单独使用时i++，++i整个“在之前，在之后”的概念很容易理解，但当您开始组合语句时，它会变得更加混乱。参见下面的示例 C 和 D。
// Example A
var i = 42;
var a = i++; // equivalent to `var a = i; i = i+1;`
console.log(a); // 42
console.log(i); // 43
// Example B
var i = 42;
var b = ++i; // equivalent to `i = i+1; var b = i;`
console.log(b); // 43
console.log(i); // 43
// Example C
var i = 42;
var c = i++ * 2; // equivalent to `var c = i*2; i = i+1;`
console.log(c); // 84
console.log(i); // 43
// Example D
var i = 42;
var d = ++i * 2; // equivalent to `i = i+1; var d = i*2;`
console.log(d); // 86
console.log(i); // 43
Notice that in Example C, the i++is not evaluated until aftermultiplication and the assignment of c. This counters the misconception that “i++should be evaluated first in the order of operations.” So in other words the statement i++ * 2actually calculates i * 2beforeit increments i.
请注意，在示例 C 中，i++直到在乘法和赋值之后才计算c。这反驳了“i++应该按操作顺序首先评估”的误解。所以换句话说，该语句在它递增之前i++ * 2实际计算。i * 2i
回答by jfriend00
It determines whether the increment happens before or after the value of the variable is used.
它确定增量是在使用变量值之前还是之后发生。
var j = 2;
console.log(j++); // 2
console.log(j); // 3
var k = 2;
console.log(++k); // 3
console.log(k); // 3
回答by Joe
var i = 0;
console.log(i++); // 0
console.log(++i); // 2
回答by LukStorms
I know, this 2011 question has long been answered.
我知道，这个 2011 年的问题早就有了答案。
++variable : Increment variable before using variable
variable++ : Increment variable after using variable
++variable : 使用变量前增加变量
++ : 使用变量后增加变量
But I figured it could still be usefull to include an answer with a snippet to confirm how they behave in a for loop.
但是我认为包含一个带有片段的答案以确认它们在 for 循环中的行为仍然很有用。
Just to verify in your browser that there's really no difference when using a ++i versus a i++ in the for loop declaraction.
And throwing --i versus i-- while we're at it.
只是为了在您的浏览器中验证在 for 循环声明中使用 ++i 与 i++ 时确实没有区别。
并在我们进行时抛出--i vs i--。
console.log("-- with looping --");
console.log("using ++i in a for loop");
for (var i=1; i<=3; ++i) {
console.log(i);
}
console.log("using i++ in a for loop");
for (var i=1; i<=3; i++) {
console.log(i);
}
console.log("using --i in a for loop");
for (var i=3; i>=1; --i) {
console.log(i);
}
console.log("using i-- in a for loop");
for (var i=3; i>=1; i--) {
console.log(i);
}
console.log("-- without looping --");
var i = 1;
console.log("i: "+ i);
console.log("i++: "+ i++);
console.log("i: "+ i);
console.log("++i: "+ ++i);
console.log("i: "+ i);
console.log("--i: "+ --i);
console.log("i: "+ i);
console.log("i--: "+ i--);
console.log("i: "+ i);