Let's increment !

Posted on April 8, 2022  •  2 minutes  • 270 words  • Other languages:  Français

In computer development, a basic notion present in all or almost all languages is the increment and its opposite the decrement. For the uninitiated, this means adding 1 or removing 1 from a variable by using a specific operator which is usually :

• ++ : for incrementing
• -- : for decrementing

For example, in JS :

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let x = 1;
x++; //x++ is the same as x + 1

But although this concept and its operator are known to everyone, fewer people have identified that there are actually two concepts:

• The post-increment
• The pre-increment

So what is the difference ? Well, the difference will be the return value ! In the case of pre-incrementing, the returned value will be the one before incrementing. In concrete terms this gives :

• Post-increment : y = x++ is the same as y = x ; x = x + 1.
• Pre-increment : y = ++x is the same as x = x + 1 ; y = x

Check this with JS…

• Post-increment :
• Pré-increment :

A more concrete example ? Let’s take this piece of code that displays a counter, if you do a post-increment on the variable seconds you get an infinite loop… with a pre-increment, your program will work :

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function timer(secondes) {
    if (secondes > 0) {
        console.log(secondes);
        timer(--secondes);
    }
}

timer(10);

• Post-increment generating an infinite loop:

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function timer(secondes) {
    if (secondes > 0) {
        console.log(secondes);
        timer(secondes--);
    }
}

timer(10);

If you are interested, these notations come directly from mathematics: Wikipedia