There is a common practice in JavaScript where i++ is used to increment the value of a variable by one. This is commonly seen in loops like the following:
for (var i=1; i<=10; i++) {
console.log(i);
}However, what happens when we use ++i; instead? How does it differ from using the -- operator for subtraction?
When it comes to the expressions i++ and ++i, the key difference lies in the resulting value.
For i++, the value is that of i before it undergoes an increment. On the other hand, for ++i, the value represents i after being incremented.
Here's a simple illustration:
var i = 42;
alert(i++); // displays 42
alert(i); // displays 43
i = 42;
alert(++i); // displays 43
alert(i); // displays 43
The operators i-- and --i follow the same principles.
++variable adds one to the variable, giving back the new value.
variable++ increments the variable by one, but returns the previous value.
--variable subtracts one from the variable, providing the new value.
variable-- reduces the variable by one, while returning the previous value.
For instance:
a = 5;
b = 5;
c = ++a;
d = b++;
a equals 6, b is 6, c becomes 6 and d turns out to be 5.
If you do not need the output, both prefix and postfix operators operate in a similar manner.
For the sake of thoroughness, I wanted to provide a specific answer addressing the first part of the original poster's question:
In one of your examples, you demonstrated the use of i++ / ++i in a for loop :
for (i=1; i<=10; i++) {
alert(i);
}
Regardless of whether you use i++ or ++i, you will receive alerts displaying numbers 1 through 10. For instance:
console.log("i++");
for (i=1; i<=10; i++) {
console.log(i);
}
console.log("++i");
for (i=1; i<=10; ++i) {
console.log(i);
}If you paste these code snippets into a console window, you'll notice that they produce identical outputs.
One scenario overlooked by all these responses is the impact of using i++ and ++i in conjunction with other numbers. While it's easy to understand the concept of "i++ happens before, ++i happens after" when dealing with single expressions, things get more complex when combining statements. Check out 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
It's important to note that in Example C, i++ is only evaluated after the multiplication and assignment of c. This goes against the common belief that "i++ should always be assessed first in the sequence of operations." Essentially, the statement i++ * 2 actually computes i * 2 before incrementing i.
i++ means to utilize the value of i in a statement and then increment it by 1.
++i signifies increasing the value of i by 1 and then using it in a statement.
It determines the sequence in which the increment operation is performed in relation to when the variable's value is accessed.
var x = 2;
console.log(x++); // 2
console.log(x); // 3
var y = 2;
console.log(++y); // 3
console.log(y); // 3
let num = 10;
console.log(num++); // 10
console.log(++num); // 12
++variable : Increment variable before using variable
variable++ : Increment variable after using variable
I thought it would be beneficial to provide an explanation along with a code snippet to demonstrate how they operate within a for loop.
To confirm in your browser that there is truly no distinction between using ++i and i++ in the for loop initialization.
Let's also explore the difference between --i and 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--);
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