javascript i++ vs ++i
One case all these answers fail to mention is what happens when i++
and ++i
are used in operations with other numbers. While the whole “i++
is before, ++i
is 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.
// 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 after multiplication 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++ * 2
actually calculates i * 2
before it increments i
.
I thought for completeness I would add an answer specific to the first of the OP's question:
One of your example shows the i++ / ++i being used in a for loop :
for (i=1; i<=10; i++) {
alert(i);
}
you will get 1-10 in your alerts no matter which you use. Example:
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.
++variable
increments the variable, returning the new value.
variable++
increments the variable, but returns the old value.
--variable
decrements the variable, returning the new value.
variable--
decrements the variable, but returns the old value.
For example:
a = 5;
b = 5;
c = ++a;
d = b++;
a
is 6, b
is 6, c
is 6 and d
is 5.
If you're not using the result, the prefix operators work equally to the postfix operators.
The difference between i++
and ++i
is the value of the expression.
The value i++
is the value of i
before the increment. The value of ++i
is the value of i
after the increment.
Example:
var i = 42;
alert(i++); // shows 42
alert(i); // shows 43
i = 42;
alert(++i); // shows 43
alert(i); // shows 43
The i--
and --i
operators works the same way.