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BOOK: Beginning JavaScript 5th Edition
This is the forum to discuss the Wrox book Beginning JavaScript, 5th Edition by Jeremy McPeak; 978-1-118-90333-9
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Old June 24th, 2015, 05:51 AM
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Exclamation Error found ?

Hey..
Correct me if i'm wrong but in chapter 2 in section Increment and Decrement Operators there is something like this:

myNumber = 1;
myVar = (myNumber++ * 10 + 1);

" What value will myVar contain? Well, because the ++ is postfixed (it’s after the myNumber variable), it will be incremented afterward. So the equation reads: Multiply myNumber by 10 plus 1 and then increment myNumber by one.
myVar = 1 * 10 + 1 = 11
Then add 1 to myNumber to get 12, but do this after the value 11 has been assigned to myVar. "

In which moment we get myNumber = 12 ?

Shouldn't myNumber be just '2' instead of '12' ?
In my calculations the result is:

var myNumber = 1;
var myVar = (myNumber++ *10 +1);
console.log(myNumber); // = 2
console.log(myVar); // = 11

Greetings
 
Old June 27th, 2015, 02:24 AM
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myNumber is equal to 2 and will only be 2 in this expression. However, from what you are suggesting here it is asking what is the value of "myVar" which is different than that of "myNumber". By order of operation on the expression "myVar" is 11, multiplication must happen before an addition, and since post fix increment means "evaluate then increment", we must evaluate the entire expression (multiplication first then addition) then increment the value of myNumber, thus the 2 following set of statements are equivalent

myNumber = 1;
myVar = (myNumber++ * 10 + 1);

is the same as

myNumber = 1;
myVar = (((myNumber * 10)+1)); myNumber += 1;


where we first evaluate the multiplication and addition expression then the increment expression.

all though the 2 set of statement are equivalent 1 is better than the other, the first set of statement is open to sight interpretation error and is less self documenting than the second.

in the second set we can verbosely see the statement evaluation order and is less apt to need comments to explain the coarse of action (assuming we have more to go one above and below the set of statements).

in short the shorter statement is not always best.




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