Failed JS Mandelbrot Set generator outputs odd structure

I made this simple Mandelbrot Set generator in Javascript last night, but it outputs a really strange structure. I think it looks similar to the mandelbrot set, yet oddly deformed. I have no idea why it distorts like this, and I've been trying to find out all day. Does anyone know what causes this or how to fix this?

c = document.getElementById("canvas");
ctx = c.getContext("2d");
c.width = 4000;
c.height = 4000;

declareVariables();
calculateShape();
drawShape();

function declareVariables() {
    Re = -2;
    Im = -2;
    input = [Re,Im];
    precision = prompt("input precision (higher is better)");
    precision = 1/(precision - precision%4);
    segmentAmt = 4/precision;
    segmentSize = c.width/segmentAmt;
    iterate = prompt("input test amount (higher is better)");
    set = [];
    for (i=0; i<segmentAmt; i++) {
        set[i] = [];
    }
    numberGrid = [];
    for (i=0; i<segmentAmt; i++) {
        numberGrid[i] = [];
        for (j=0; j<segmentAmt; j++) {

        }
    }
}

function calculateShape() {
    for (i=0; i<segmentAmt; i++) {
        input[1] = -2;
        input[0] += precision;
        for (j=0; j<segmentAmt; j++) {
            input[1] += precision;
            set[i][j] = 0;
            z = [0,0];
            for (k=1; k<=iterate; k++) {
                store = z;
                z[0] = store[0]**2 - store[1]**2 + input[0];
                z[1] = 2 * store[0] * store[1] + input[1];
                if (z[0]**2 + z[1]**2 > 4) {
                    set[i][j] = k;
                    k = iterate+1;
                }
            }
        }
    }
}

function drawShape() {
    ctx.fillStyle = "white";
    ctx.fillRect(0,0,c.width,c.height);
    for (i=0; i<segmentAmt; i++) {
        for (j=0; j<segmentAmt; j++) {
            if (set[i][j] == 0) {
                ctx.fillStyle = "black";
            } else if (set[i][j] >= 1) {
                ctx.fillStyle = 'hsl(' + (25*(set[i][j]-1))**0.75 + ', 100%, 50%)';
            } 
            convertCoords(i,j);
            ctx.fillRect(xCoord,yCoord,segmentSize,segmentSize);
        }
    }
}

function convertCoords(var1,var2) {
    xCoord = var1 * segmentSize;
    yCoord = var2 * segmentSize;
}

Output image:

The error appears to be on this line in calculateShape():

                store = z;

It seems you want store to be a copy of z, but this just ends up with store and z referring to the same array. The next line calculates z[0], but as store and z refer to the same array, store[0] has the new value of z[0] rather than the previous. Hence the calculation of z[1] in the line after that is incorrect.

Replace the above line with either

                store = [z[0], z[1]];

or

                store = z.slice();

Both of these lines ensure that store refers to a different array to z, so when you recalculate z[0], store[0] is unaffected.