Javascript variable incrementing in evaluation?

If I have

a = 0;
if(a++ < 1){
    console.log(a);
}

I get the value 1 in the console. If a became 1 with the incrementation, then why did the expression evaluate true?

If I do

a = 0;
if(++a < 1){
    console.log(a);
}

Then I don't get anything in the console, meaning the expression evaluated to be false.

I have always used variable++ to increment variables in for loops and the like. I have seen the ++variable, but I assumed it was another way to write the same thing. Can someone explain what happens and why? What's the difference between the two?

Does ++variable increment the variable at the time of evaluation, while variable++ increments after?

I have seen the ++variable, but I assumed it was another way to write the same thing.

No, they're not the same at all.

• ++variable is pre-increment. It increments variable and evaluates to the new value.

• variable++ is post-increment. It increments variable and evaluates to the old value.

This is common to most C-style languages, including C itself, C++, PHP, Java and Javascript.

i.e.:

Does ++variable increment the variable at the time of evaluation, while variable++ increments after?

Yes, exactly. :)

There's a very important difference here. a++ increments a after evaluation, where ++a increments before evaluation. Conveniently the position of the ++ is either before or after as well, so that's how you can remember which is which.

In other words, this is what you're effectively doing:

a = 0;
if (a < 1)
{
   a = a + 1;
   console.log(a);
}
else
{
   a = a + 1;
}

It is post incrementing, so for the comparison it is 0, then in the block statement it is 1.

a = 0;

// v-----0 < 1
if(a++ < 1){
      // now a has been incremented to 1
    console.log(a);
}

The pre incrementing version looks like:

a = 0;

//   v-----1 < 1
if(++a < 1){
      // evaluate to false, so this doesn't run
    console.log(a);
}

++var adds one, then does the comparison.

var++ does the comparison, then adds one.

The trick is to remember it by the ++ coming before the variable (increment happens before), or after the variable (increment happens after).