Javascript tutorial confusion about variable

Question

Im reading the tutorials here.

I am getting confused trying to understand some of this example, why is the variable declared as nothing and what does the ,i indicate

var x="",i;

and also why do you use

x=x

at the beginning of the line?

<!DOCTYPE html>
<html>
<body>

<p>Click the button to loop from 1 to 6, to make HTML headings.</p>
<button onclick="myFunction()">Try it</button>
<div id="demo"></div>

<script>
function myFunction()
{
var x="",i;
for (i=1; i<=6; i++)
{
 x=x + "<h" + i + ">Heading " + i + "</h" + i + ">";
}
document.getElementById("demo").innerHTML=x;
}
</script>

</body>
</html>

x = x + i; means take the current value of x, append i to it, then assign that back to x. It's building the string up during the loop. — Evan Trimboli
– Evan Trimboli, Commented Dec 19, 2013 at 2:39
Oh that's a bumber they seem to have a large collection of knowledge that I was excited to go through. Do you have any recommendations on simulare sites with more creditable information that I can learn from? — nick
– nick, Commented Dec 19, 2013 at 2:40
I always recommend the book Object-Oriented JavaScript by Stoyan Stefanov, it does a great job of teaching modern JavaScript. Eloquent Javascript is a pretty good, free book. MDN is also a great resource. — Useless Code
– Useless Code, Commented Dec 19, 2013 at 4:32

Phil · Accepted Answer · 2013-12-19 02:40:18Z

1

var x="",i;

This translates as

var x = "";
var i;

which simply declares those variables within the current scope.

x=x + ...

This means replace the value of x with the value of the expression to the right of the = sign. In this case, you are concatenating a string to the end of the current value of x.

answered Dec 19, 2013 at 2:40

Phil

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Comments

gee · Accepted Answer · 2013-12-19 02:38:54Z

0

var x="",i;

is the same as

var x = "";
var i;

Declare variables in the form of

var a=1,
    b=2,
    c=3;

is common and make the code style looking clear and easy to read.

answered Dec 19, 2013 at 2:38

gee

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4 Comments

nick Over a year ago

So whats the differance between var x; and var x="";

gee Over a year ago

var x = "" make x an empty string, then x is now really containing somthing, while var x will just declare the existence of x and the x now is undefined and it is ready to be set value for x later

gee Over a year ago

It's always a good habit in JS to use var to declare an variable first no matter by var x or var x = 'blabla', otherwise, x will become global and that is not a good pattern in JS

gee Over a year ago

If you want to start learning javascript, the book javascript definitive guide is something you can start with. This is the best javascript book to start.

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