In my work with QML, I needed a way to mutate an immutable array. It's surprisingly easy to do this in Javascript. In this article, I give an outline of the technique followed by working Javascript code.

[toc align_right=“yes” depth=“3”]

The Outline

Variables

Let S, A, and I be zero-based arrays.
Let A be an immutable array
Let I be empty
Let N be an integer with initial value 0

Let S[i]=i+1 for all i in {0..||A||}

Let a positive value in S indicate an element of A:
S[i] indicates A[S[i] - 1].

Let a negative value in S indicate an element of I:
S[i] indicates I[~~S[i]~~ 1]

S is the mutable "face" of A.

Let N indicate the next unused element of I.

Pseudocode

To insert an object X into S at index i:
S.splice(i, 0, I[N]=X N=N+1
To remove an object from S at index i:
I[i]=undefined S.splice(i, 1)
To change an the object at S[i] to X:
if S[i] > 0: S[i]=(N+1) I[N]=X N=N+1 else: I[S[i] 1]=X

The value of S[i] is
A[S[i] - 1] if S[i] > 0 I[S[i] 1] otherwise

Bugs?

It may be possible for I to grow without bound.

The Code

This code assumes

<br />// A is the unmutable array<br />var A

// S is the mutable face of A.<br />// You must initialize S as described in the outline<br />var S=new Array()

// I contains the values you insert<br />// or change in A and its mutable face.<br />var I=new Array()

// N is the index of the next element of S.<br />var N=0<br />

function get(i) { return (S[i] > 0) ? A[S[i] - 1] : I[S[i] 1] }
function insert(i, j) { S.splice(i, 0, ) I[N]=j N++ }
function remove(i) { I[i]=undefined S.splice(i, 1) }
function set(i,j) { if (S[i] > 0) { S[i]=(N+1) I[N]=j N++ } else { I[S[i] 1]=j } }