JavaScript: Making variable into default?

Question

Javascipt: Making variable into default?

I have code like this :

<script>
document.write(ab);
</script>

ab is a variable but it's not declared yet because I want it will be changing by Users. The default result I want it displays is x+y (for example as). But When Users enter any variable, the result need to be changed. For instance :

<script>
var ab = "The users change code";
</script>

The result will change is The users change code

So, Do you have any idea for my issue. Thank for your help.

Read any basic guide to javascript and you'll figure this out within minutes. — adeneo, Oct 05 '13 at 16:39
Possible duplicate of http://stackoverflow.com/questions/16696642/setting-a-default-variable-value-in-a-function — jonathancardoso, Oct 05 '13 at 16:44

score 2 · Answer 1 · answered Oct 05 '13 at 16:41

2

You can do this in various ways. A ternary is a simple one.

var ab=(typeof ab==='undefined'?) x+y : ab;

This says that if ab is undefined, set it to x+y, otherwise leave it at the current, presumably user set, value.

There may be better ways to handle all of this. Post more code if you'd like.

answered Oct 05 '13 at 16:41

JAL

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why do you complicate things ? – Royi Namir Oct 05 '13 at 16:41
p.s. those kind of solutions are bad because of scoping issues. – Royi Namir Oct 05 '13 at 16:47
@Royi I agree, that's why I solicited more code and said this whole thing could be done more cleanly. The ternary style is seen fairly often for setting defaults in my experience. I like that it looks like an assignment straight off, not an 'if'. – JAL Oct 05 '13 at 16:52

Royi Namir · Accepted Answer · 2013-10-05T17:12:25.563

2

Just to be pedant :

You said :

ab is a variable but it's not declared yet

shom's answer should work fine.

But if I was a robot which analyze your question :

this would pass although you have declared it :

    var ab=undefined;
    if (typeof ab === 'undefined')
        var ab= 'x+y';
    document.write(ab);

the safest way (as answering to your :"not declared yet "):

 if (!('ab' in window))
  var ab = 'x+y';
  document.write(ab);

edited Oct 05 '13 at 17:12

answered Oct 05 '13 at 16:45

Royi Namir

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If you want to use my answer in your answer, perhaps you could've added a comment instead... Should I now write another answer to elaborate on this answer? – Shomz Oct 05 '13 at 16:52
@Shomz I'll remove your code from my answer. – Royi Namir Oct 05 '13 at 16:52
It's still there, but never mind. I said this because it's happening more and more on SO lately, and I can't figure out why are people taking others' answers. – Shomz Oct 05 '13 at 16:55
@Shomz I removed your code. and the second code is a code which the whole world ( including me) would have given . just to make it different. i will write it better and add `===` which is much more precise (and correct). no one is stealing no one's code. – Royi Namir Oct 05 '13 at 16:59
It should be `if (!('ab' in window))`. – nnnnnn Oct 05 '13 at 17:06

score 1 · Answer 3 · answered Oct 05 '13 at 16:40

1

if (typeof ab == 'undefined')
    var ab = 'x+y';
document.write(ab);

answered Oct 05 '13 at 16:40

Shomz

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JavaScript: Making variable into default?

3 Answers3