How to append a string value during assignment in Javascript?

Question

Hey i dont know if that is possible but i want to set a given variable in js by reference.

What i want to do is, that each time i pass a string to the function addstring that the value of the textfield is added like +=

function addstring(string)
{
    document.getElementById("string").value = string; // its a textfield

}

How can i do that?

Do you mean assigning a string reference to the element value so that the value changes when the string changes? — o.k.w
– o.k.w, Commented Mar 11, 2010 at 9:38
It's not clear to me what about your actual question involves passing by reference. — T.J. Crowder
– T.J. Crowder, Commented Mar 11, 2010 at 9:43
This is what happens when a question is unclear, you get a lot of mixed answers that are just guessing at the true meaning of what's being asked. — Andy E
– Andy E, Commented Mar 11, 2010 at 9:45
Hmmm.. the question is more about string concatenation, yea? I was puzzled by the "variable by reference". :P — o.k.w
– o.k.w, Commented Mar 11, 2010 at 12:03
"passing variable by reference" has nothing to do with provided example — Yura Omelchuk
– Yura Omelchuk, Commented Jul 15, 2011 at 14:00

Morten Anderson · Accepted Answer · 2010-03-11 09:46:46Z

9

Javascript does not support passing parameters by reference.

This link offers a good breakdown and some workarounds- Passing by Value or reference

answered Mar 11, 2010 at 9:46

Morten Anderson

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Comments

rahul · Accepted Answer · 2010-03-11 09:47:23Z

2

If its a reference to an input text filed then you can use the value property

function addstring(string)
{
    document.getElementById("string").value += string.value;

}

See value

edited Mar 11, 2010 at 9:47

answered Mar 11, 2010 at 9:41

rahul

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Comments

N 1.1 · Accepted Answer · 2010-03-11 09:46:50Z

1

+= works fine.

var str = "stack";
str += "overflow";

console.log(str); //alert(str); Use firebug!!

stackoverflow

edited Mar 11, 2010 at 9:46

answered Mar 11, 2010 at 9:39

N 1.1

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4 Comments

N 1.1 Over a year ago

@streetparade: show the code where you are adding and the error you are getting. (use firebug(add-on) on firefox or Developer Tools (ctrl+shift+j) if on Chrome)

streetparade Over a year ago

hmm.. this worked for me document.getElementById("string").value +=string; But this didnt work var sending = document.getElementById("string").value; sending+ = string;

T.J. Crowder Over a year ago

@streetparade: Right, because what you've done there is retrieve the value of the text box's content into sending, and then adding your string to that value -- which at that point has nothing to do with the text box.

Douglas Over a year ago

Remember that a += b is shorthand for a = a + b, it isn't a string version of array.push(foo) which does change array itself.

T.J. Crowder · Accepted Answer · 2010-03-11 09:56:44Z

Your code example will work just fine with +=; complete example below. This suggests the problem you're having lies elsewhere.

<!DOCTYPE HTML>
<html>
<head>
<meta http-equiv="Content-type" content="text/html;charset=UTF-8">
<title>Test Page</title>
<style type='text/css'>
body {
    font-family: sans-serif;
}
</style>
<script type='text/javascript'>
function addstring(string)
{
    document.getElementById('string').value += string;
}
</script>
</head>
<body><div>
<input type='text' id='string' value=''>
<br><input type='button' value='One' onClick="addstring('one');">
<input type='button' value='Two'     onClick="addstring('two');">
<input type='button' value='Three'   onClick="addstring('three');">
</div></body>
</html>

Pentium10 · Accepted Answer · 2010-03-11 09:38:40Z

0

document.getElementById("string").value = document.getElementById("string").value + string;

answered Mar 11, 2010 at 9:38

Pentium10

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3 Comments

T.J. Crowder Over a year ago

Why wouldn't you use +=? Why look up the element multiple times?

streetparade Over a year ago

Hy thanks for the fast answer, unfortunatly the += doesnt work, i dont know why

T.J. Crowder Over a year ago

@streetparade: Yes it does (pastie.org/864704). This suggests the problem lies elsewhere.

Matt Brock · Accepted Answer · 2012-05-24 19:44:20Z

0

You can clone the object by first converting it to JSON (watch out for circular references) and then parse it back again. Like so:

function clone(obj) {
  return JSON.parse(JSON.stringify(obj));
}

This uses the internal browser's JSON routines (safer & faster than using an external resource). If you simply must have backwards compatibility you can download the core JSON routines from JSON.org.

answered May 24, 2012 at 19:44

Matt Brock

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Comments

Terje Rosenlund · Accepted Answer · 2011-05-13 14:19:15Z

Javascript does not support passing parameters by reference - Not true

Actually it does. Prototyping makes it possible to create true references to all kinds of javascript objects, including strings.

By true reference I mean when:

Changes made to a variable or object passed by reference is reflected on the actual variable beeing passed
The change is seen by all references to the same variable
Everyone knowing the reference may change the referenced variable

To create a true reference to a variable:

Create a constructor function
Prototype your reference variables on the constructor
DO NOT declare variables with the same name inside the constructor function!
Create a change() function which changes the prototyped variable or do so directly
Optional: Create a change() function-reference inside your constructor which is set to the ChangeRef() function uppon creation

Changes made this way will be seen and may be changed by all other TestRef() objects

function TestRef(s) {
    this.string = 'Own test-string: ' + s;
    this.change = ChangeRef;
}
function ChangeRef(s) {
    TestRef.prototype.refStr = s;
    return TestRef.prototype.refStr;
}
r = 'RefStr';
TestRef.prototype.refStr = r; // PROTOTYPE => 'RefStr', copy of r

s = new TestRef('s'); // Obj.string = Own test-string: s, Obj.refStr = RefStr
o = new TestRef('o'); // Obj.string = Own test-string: o, Obj.refStr = RefStr
ChangeRef('ChangedStr');  // Change referenced string!
TestRef.prototype.refStr; // => ChangedStr, referenced string changed
r; // => RefStr, original string intact
x = new TestRef('x'); // Obj.string = Own test-string: x, Obj.refStr = ChangedStr. New sees changed string
s; // => Obj.string = Own test-string: s, Obj.refStr = ChangedStr. Old sees changed string
o; // => Obj.string = Own test-string: o, Obj.refStr = ChangedStr. Old sees changed string
s.change('Changed by local function');
x; // => Obj.string = Own test-string: o, Obj.refStr = Changed by local function

Using a reference != pass by reference. What we mean by true pass by reference, e.g. in C++, is you can pass a variable into a function (not create some "reference" object; but pass the variable directly), and the function can assign to that variable (not call some method on it; assign) as if the variable was assigned to in the original scope. This is not possible in JavaScript.

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