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Pass a string by reference in Javascript

Asked 2009-08-20T20:28:24.103
59

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.
javascript string pass-by-reference
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2 Answers

75

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";
answered 2009-08-20T20:36:28.377
10

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.

answered 2009-08-20T20:32:38.993

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