What does immutable mean?

Question

If a string is immutable, does that mean that.... (let\'s assume JavaScript)

var str = \'foo\';

alert(str.substr(1)); // oo

alert(str); // foo

Answer 1

From strings to stacks... a simple to understand example taken from Eric Lippert's blog:

Path Finding Using A* in C# 3.0, Part Two...

A mutable stack like System.Collections.Generic.Stack is clearly not suitable. We want to be able to take an existing path and create new paths from it for all of the neighbours of its last element, but pushing a new node onto the standard stack modifies the stack. We’d have to make copies of the stack before pushing it, which is silly because then we’d be duplicating all of its contents unnecessarily.

Immutable stacks do not have this problem. Pushing onto an immutable stack merely creates a brand-new stack which links to the old one as its tail. Since the stack is immutable, there is no danger of some other code coming along and messing with the tail contents. You can keep on using the old stack to your heart’s content.

To go deep on understaning immutability, read Eric's posts starting with this one:

Immutability in C# Part One: Kinds of Immutability