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

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";