Can a for Loop Increment/Decrement by More Than One?

A for loop:

for(INIT; TEST; ADVANCE) {
    BODY
}

Means the following:

INIT;
while (true) {
    if (!TEST)
        break;
    BODY;
    ADVANCE;
}

You can write almost any expression for INIT, TEST, ADVANCE, and BODY.

Do note that the ++ operators and variants are operators with side-effects (one should try to avoid them if you are not using them like i+=1 and the like):

• ++i means i+=1; return i
• i++ means oldI=i; i+=1; return oldI

Example:

> i=0
> [i++, i, ++i, i, i--, i, --i, i]
[0, 1, 2, 2, 2, 1, 0, 0]

for (var i = 0; i < 10; i = i + 2) {
    // code here
}​

Andrew Whitaker's answer is true, but you can use any expression for any part.
Just remember the second (middle) expression should evaluate so it can be compared to a boolean true or false.

When I use a for loop, I think of it as

for (var i = 0; i < 10; ++i) {
    /* expression */
}

as being

var i = 0;
while( i < 10 ) {
    /* expression */
    ++i;
}

for (var i = 0; i < myVar.length; i+=3) {
   //every three
}

additional

Operator   Example    Same As
  ++       X ++        x = x + 1
  --       X --        x = x - 1
  +=       x += y      x = x + y
  -=       x -= y      x = x - y
  *=       x *= y      x = x * y
  /=       x /= y      x = x / y
  %=       x %= y      x = x % y

You certainly can. Others have pointed out correctly that you need to do i += 3. You can't do what you have posted because all you are doing here is adding i + 3 but never assigning the result back to i. i++ is just a shorthand for i = i + 1, similarly i +=3 is a shorthand for i = i + 3.

The last part of the ternary operator allows you to specify the increment step size. For instance, i++ means increment by 1. i+=2 is same as i=i+2,... etc. Example:

let val= [];

for (let i = 0; i < 9; i+=2) {
  val = val + i+",";
}


console.log(val);

Expected results: "2,4,6,8"

'i' can be any floating point or whole number depending on the desired step size.

There is an operator just for this. For example, if I wanted to change a variable i by 3 then:

var someValue = 9;
var Increment  = 3;
for(var i=0;i<someValue;i+=Increment){
//do whatever
}

var someValue = 3;
var Increment  = 3;
for(var i=9;i>someValue;i+=Increment){
//do whatever
}

For those who are looking to increment pair of numbers (like 1-2 to 3-4):

Solution one:

//initial values
var n_left = 1;
var n_right = 2;
    
for (i = 1; i <= 5; i++) {
        
    console.log(n_left + "-" + n_right);   
        
    n_left =+ n_left+2;
    n_right =+ n_right+2; 
}
//result: 1-2 3-4 5-6 7-8 9-10

Solution two:

for (x = 0; x <= 9; x+=2) {
    
    console.log((x+1) + "-" + (x+2));  

}
//result: 1-2 3-4 5-6 7-8 9-10