A JavaScript language problem

Question

In the following example

<!DOCTYPE html>
<meta charset="utf-8">
<title>Untitled Document</title>
<script>
    var str = '11';
    str = str++;
    alert(str); // 11
</script>

why is the result 11 and not 12?

In this example the result is 12:

<!DOCTYPE html>
<meta charset="utf-8">
<title>Untitled Document</title>
<script>
    var str = '11';
    str++;
    alert(str); // 12
</script>

Why is this so?

Thank you!

var str = 10;
var re = str++;
alert(re); // 10
alert(str); // 11

str will return 10 to re first, and then increments str itself to 11.

But

var str = 10;
var str = str++;
alert(str); // 10

In this case, str return 10 to str first, and then str should increments str itself to 11.

But it doesn't. Can anyone explain this?

Thank you!

Jacob Relkin · Accepted Answer · 2011-01-25 08:55:40Z

10

The reason why this happens is because the return value of a post increment (str++) is the value before incrementing.

Example:

var x = 10;
alert(x++); //10, because return value is value before increment
alert(x); //11 because the variable was incremented on the previous line

The way to solve this issue is to use the pre-increment operator, which is simply the ++ operator before its operand (++str). This will add 1 to the operand and return the new value - as opposed to the post-increment operator (str++) which will add 1 to operand and return the old value.

Example:

var x = 10;
alert(++x); //11

By the way, the statement str = str++ is a real gotcha. The reason why:

The expression str++ returns the original value, so when you reassign it to str, you are essentially re-assigning the OLD VALUE back into str. Hence zero mutation to the variable and you are back at square one.

Here's how your code should look like:

<!DOCTYPE html>
<meta charset="utf-8">
<title>Untitled Document</title>
<script>
    var str = '11';
    alert(++str); // 12
</script>

edited Jan 25, 2011 at 8:55

answered Jan 25, 2011 at 5:12

Jacob Relkin

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1 Comment

weilou Over a year ago

Thank you for your answer! And can you explain this (stackoverflow.com/questions/4790200/…)?

Marcelo · Accepted Answer · 2011-01-25 05:11:44Z

1

The expression with the ++ operator first evaluates to 11 and then increments the value of str.

Try ++str. It will return 12.

answered Jan 25, 2011 at 5:11

Marcelo

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1 Comment

Ken Franqueiro Over a year ago

And then - don't forget - assigns the pre-increment value, after incrementing, so you end up right back where you started! :)

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