Javascript for loop?

Question

I'm currently studying javascript on my own and would like to see if anyone could help me clarify this questions I have. I'm currently learning loops and came across this code in the book:

var scores = [34, 45, 66, 1023, 1030, 'Done!'];
var arrayLength = scores.length;
var roundNumber = 0;
var msg = '';
var i;

for (i = 0; i < arrayLength; i++) {

  roundNumber = (i + 1);

  msg += 'Round ' + roundNumber + ': ';

  msg += scores[i] + '<br />';

}

document.getElementById('answer').innerHTML = msg;

<div id="answer"></div>

Now that loops through the array and returns all numbers in the array. But If i were to change:

var msg = ''; to var msg;

msg = 'Round ' + roundNumber + ': ';

it only returns the last item in the array. Why does that affect it? How does making the msg variable as null change everything?

You removed the + sign before the = sign. Therefore you're creating a new string everytime instead of appening more content to it. — Romain Braun
– Romain Braun, Commented Sep 17, 2014 at 22:47
@RomainBraun—strictly, a new string is created every time regardless of = or +=. ;-) — RobG
– RobG, Commented Sep 17, 2014 at 22:57

LcSalazar · Accepted Answer · 2014-09-17 23:00:36Z

2

There are two things here you must understand:

=
It is an assignment operator. The operation var x = 'something' means that any value inside x will be forgotten, and replaced by the new value 'something'.
+=
It is a binary operator. It increments the value of a variable, so x += 'something' will add the value 'something' to whatever value was already inside the x variable. It's the same as x = x + 'something'

So, don't initializing a value for the variable, would just make it not having anything to add to:

var msg;

msg += 'Round' //This would give you 'undefinedRound'

Now, in your case, you removed the + sign, so you used a simple assign operator =. Every time the for loops, it will reset the variable's value.

answered Sep 17, 2014 at 23:00

LcSalazar

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smakateer · Accepted Answer · 2014-09-17 22:54:34Z

2

Making the following declaration

var msg;

DOES make msg == undefined initially, but the problem is that second line you changed:

msg = 'Round ' + roundNumber + ': ';

This causes msg to be equal to ONLY THAT value on the right side of the value assignment. By using += rather than just =, you are continuously concatenating onto previous values of msg.

answered Sep 17, 2014 at 22:54

smakateer

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bl2b · Accepted Answer · 2014-09-17 22:59:06Z

1

msg += 'Round ' + roundNumber + ': '; //update the msg by adding previus value plus current value
msg = 'Round ' + roundNumber + ': ';  //update the msg by adding only the current value. Just like assigning new value in the msg variable.

answered Sep 17, 2014 at 22:59

bl2b

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mdewitt · Accepted Answer · 2014-09-17 23:33:07Z

0

The issue is at the following line:

msg = 'Round ' + roundNumber + ': ';

The line should actually read:

msg += 'Round ' + roundNumber + ': ';

edited Sep 17, 2014 at 23:33

mdewitt

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answered Sep 17, 2014 at 23:00

Gopi

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