Concatenating strings in Javascript?

Question

With the code below, looking at the 2nd paragraph in particular...if var str = "sting equality test..." + strA, why do the 2nd, 3rd lines etc not output that same output plus their own line?

EDIT sorry for not explaining properly - I am actually just wondering why this code (once I have cleaned it up) DOESN'T produce the first line (of the 2nd paragraph) repeated, plus whatever I state in the 2nd and 3rd lines etc. I don't need it to, it's just an exercise, I just don't understand. It seems as though it should

function init()
{
    var strA = "Javascript" === "JAVASCRIPT" ;
    var strB = "Javascript" === "Javascript" ;
    var flt = 7.5 === 7.5 ;
    var intA = 8 !== 8 ;
    var intB = 24 > 12 ;
    var intC = 24 < 12 ;
    var intD = 24 <= 24 ;

    var str = "String equality test: " + strA ;
    str += "<br>String equality test 2: " + strB ;
    str += "<br>Float equality test: " ; + strC ;
    str += "<br>Integer inequality test: " + intA ;
    str += "<br>Greater than test: " + intB ;
    str += "<br>Less than test: " + intC ;
    str += "<br>Less than/Equal to test: " + intD ;

    document.getElementById( "panel" ).innerHTML = str ;
}
document.addEventListener("DOMContentLoaded" , init , false) ;

So the output I get is as follows;

String equality test: false
String equality test 2: true
Float equality test: true
Integer inequality test: false
Greater than test: true
Less than test: false
Less than/Equal to test: true

This is obviously correct, I just don't understand how something like the following isn't outputted because surely I am adding each line to the var str, which is "String equakity test: false

 String equality test: false
String equality test: false String equality test 2: true
String equality test: false Float equality test: true
String equality test: false Integer inequality test: false
String equality test: false Greater than test: true
String equality test: false Less than test: false
String equality test: false Less than/Equal to test: true

Why have you got a mysterious floating ; in the third line? — David Thomas
– David Thomas, Commented Oct 6, 2013 at 14:32
@Jeffman: was that a response to me, or the OP? DJC: if you'd looked at your Web Developer tools (F12 in most browsers) they would have told you this. Also, pressing the 'JS Hint' button, if you'd put together a JS Fiddle. — David Thomas
– David Thomas, Commented Oct 6, 2013 at 14:35
@David Both, I guess. To OP, the undefined variable might be a problem. To you, the seemingly stray ; might be a misguided attempt at short-circuiting the concatenation of a value that doesn't exist. — user2625787
– user2625787, Commented Oct 6, 2013 at 14:38

Patrick Kostjens · Accepted Answer · 2013-10-06 14:32:37Z

1

There is a ; on the third line in front of the +. Removing that should solve the problem. The variable strC which you try to add after that strange + doesn't exist.

answered Oct 6, 2013 at 14:32

Patrick Kostjens

5,1006 gold badges31 silver badges46 bronze badges

Sign up to request clarification or add additional context in comments.

5 Comments

DJC Over a year ago

thanks, have changed that now, but please see my edit, sorry for not explaining properly first time around

Patrick Kostjens Over a year ago

@DJC, you mean your code still doesn't output all lines after solving the problems I mentioned?

DJC Over a year ago

This is an exercise from a book and I am trying to understand why the var str, when used in the 2nd, 3rd line etc doesn't include the "string equality test" bit as isn't that what I have defined it as. The other lines look as though they should produce that, then the added values they contain?

Patrick Kostjens Over a year ago

Can you add your exact output and your expected output to the question?

DJC Over a year ago

sorry for delay, will edit question with expected and actual output. thanks forthe help, much appreciated

nnnnnn · Accepted Answer · 2013-10-07 22:02:36Z

0

I just don't understand how something like the following isnt outputted because surely I am adding each line to the var str, which is "String equakity test: false"

No, the only time that str is equal to "String equality test: false" is immediately after the first line. Each line modifies the str variable by adding something to whatever it became after the line before. So after the first line:

var str = "String equality test: " + strA ;

...the variable str is now equal to

"String equality test: false"

Then after the second line:

str += "<br>String equality test 2: " + strB ;

...the variable str is now equal to

"String equality test: false<br>String equality test 2:"

The third line has an error, but assuming you remove the extra semicolon and fix the variable name:

str += "<br>Float equality test: " ; + strC ;
// should be
str += "<br>Float equality test: " + flt;

...then the variable str will be equal to

"String equality test: false<br>String equality test 2: true<br>Float equality test: true"

...and so forth.

You can see this clearly if you add a console.log(str); statement in between each line and open your browser's console before running the code. As shown here: http://jsfiddle.net/mfKy9/

edited Oct 7, 2013 at 22:02

answered Oct 7, 2013 at 21:56

nnnnnn

150k30 gold badges210 silver badges247 bronze badges

1 Comment

DJC Over a year ago

thanks for that, very clear and helpful. i was just getting confused with the += , thinking it was keeping the line then adding the next, whereas like you say, it is actually replacing the previous value of var str. thanks!

Collectives™ on Stack Overflow

Concatenating strings in Javascript?

2 Answers 2

5 Comments

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

5 Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related