Pass a string by reference in Javascript

Question

I want to create a string and pass it by reference such that I can change a single variable and have that propagate to any other object that references it.

Take this example:

function Report(a, b) {
    this.ShowMe = function() { alert(a + " of " + b); }
}

var metric = new String("count");
var a = new Report(metric, "a"); 
var b = new Report(metric, "b"); 
var c = new Report(metric, "c"); 
a.ShowMe();  // outputs:  "count of a";
b.ShowMe();  // outputs:  "count of b";
c.ShowMe();  // outputs:  "count of c";

I want to be able to have this happen:

var metric = new String("count");
var a = new Report(metric, "a"); 
var b = new Report(metric, "b"); 
var c = new Report(metric, "c"); 
a.ShowMe();  // outputs:  "count of a";
metric = new String("avg");
b.ShowMe();  // outputs:  "avg of b";
c.ShowMe();  // outputs:  "avg of c";

Why doesn't this work?

The MDC reference on strings says metric is an object.

I've tried this, which is not what I want, but is very close:

var metric = {toString:function(){ return "count";}};
var a = new Report(metric, "a"); 
var b = new Report(metric, "b"); 
var c = new Report(metric, "c"); 
a.ShowMe();  // outputs:  "count of a";
metric.toString = function(){ return "avg";}; // notice I had to change the function
b.ShowMe();  // outputs:  "avg of b";
c.ShowMe();  // outputs:  "avg of c";

alert(String(metric).charAt(1)); // notice I had to use the String constructor
// I want to be able to call this: 
// metric.charAt(1)

The important points here:

1. I want to be able to use metric like it's a normal string object
2. I want each report to reference the same object.

Answer 1

Closure?

var metric = new function() {
    var _value = "count";

    this.setValue = function(s) { _value = s; };
    this.toString = function() { return _value; };
};

// snip ...
a.ShowMe();

metric.setValue("avg");
b.ShowMe();
c.ShowMe();

or making it a little more generic and performant:

function RefString(s) {
    this.value = s;
}

RefString.prototype.toString = function() { return this.value; }
RefString.prototype.charAt = String.prototype.charAt;

var metric = new RefString("count");

// snip ...

a.ShowMe();

metric.value = "avg";
b.ShowMe();
c.ShowMe();

If you don't close on the desired string variable, then I suppose the only other way would be to modify the ShowMe function, as in @John Millikin's answer or re-architect the codebase.

Answer 2

You can wrap the string in an object and modify the field the string is stored in. This is similar to what you are doing in the last example only without needing to change the functions.

var metric = { str : "count" } 
metric.str = "avg";

Now metric.str will contain "avg"

Answer 3

In JavaScript, strings are immutable. You can't change the string itself one the Report instances have a handle to it.

your solution works, but this may be simpler:

function Report(a, b) {
  this.showMe = function() { alert(a.str + " of " + b); }
}

var metric = {};
metric.str = "count";

a.Showme();
metric.str = "avg";
b.ShowMe();

Answer 4

If you pass the variable as an object it will work, since objects are passed by reference in Javascript.

http://sirdarckcat.blogspot.com/2007/07/passing-reference-to-javascript.html

function modifyVar(obj,newVal){
obj.value=newVal;
}
var m={value: 1};
alert(x);
modifyVar("x",321);
alert(x);

Answer 5

Strings in Javascript are already passed "by reference" -- calling a procedure with a string does not involve copying the string's contents. There are two issues at hand:

• Strings are immutable. In contrast to C++ strings, once a JavaScript string has been created it cannot be modified.
• In JavaScript, variables are not statically assigned slots like in C++. In your code, metric is a label which applies to two entirely separate string variables.

Here's one way to achieve what you want, using closures to implement dynamic scoping of metric:

function Report(a, b) {
    this.ShowMe = function() { alert(a() + " of " + b); }
}

var metric = "count";
var metric_fnc = function() { return metric; }
var a = new Report(metric_fnc, "a"); 
var b = new Report(metric_fnc, "b"); 
a.ShowMe();  // outputs:  "count of a";
metric = "avg";
b.ShowMe();  // outputs:  "avg of b";