4 - Model

We saw Snippets of Patches, Turtles and Links. Now we want to use those to create our first Model, our Hello World model.

There will be three parts:

  • Creating the HelloModel.js, and
  • Creating the hello.html file which uses HelloModel.js and
  • Using a browser to run hello.html


Our HelloModel.js is simple, yet uses all the elements of more complex models.

Lets look at HelloModel.js

import Model from 'https://code.agentscript.org/src/Model.js'
import * as util from 'https://code.agentscript.org/src/utils.js'

class HelloModel extends Model {
    population = 10 // number of turtles
    speed = 0.1 // step size in patch units
    wiggleAngle = 10 // wiggle angle in degrees
    linksToo = true // handy to show just turtles if false

    // default worldOptions: 33 x 33 patches with 0,0 origin at the center.
    constructor(worldOptions = undefined) {

    setup() {
        // Have turtles "bounce" at the Patches edges. Default is to wrap
        this.turtles.setDefault('atEdge', 'bounce')

        // create "population" turtles placed on random patches
        this.turtles.create(this.population, t => {
            const patch = this.patches.oneOf()
            t.setxy(patch.x, patch.y)

        // If links.too is true, create a link from every turtle to another turtle
        if (this.linksToo) {
            this.turtles.ask(t => {
                this.links.create(t, this.turtles.otherOneOf(t))

    step() {
        // change heading randomly, moving forward by "speed"
        this.turtles.ask(t => {
            t.heading += util.randomCentered(this.wiggleAngle)

export default HelloModel

We first import two modules from our src/ folder

  • Model.js: This is used by all models and creates the Patches, Turtles and Links.
  • utils.js: A collection of useful utility functions. We'll use util.randomCentered

Then we create a Class which inherits from the Model.js import (which manages the Patches, Turtles, Links). All Models start out inheriting from Model.js.

The class has 4 variables, a constructor, and two methods.

  • The constructor just calls the Model.js super class using the default "world".
    • The default world is 33 x 33 patches with 0,0 origin at the center.
  • The 4 variables are used in the 2 methods to create the model's behavior
  • setup:
    • creates "population" turtles
    • If links.too is true, create a link from every turtle to another turtle
  • step:
    • use "wiggleAngle" to ask each turtle to turn randomly from -(wiggleAngle/2) to +(wiggleAngle/2)
    • use "speed" to move each turtle forward by speed patches
  • Finally we export HelloModel to be used elsewhere. For example, in the HTML below.


The HTML used by the browser is quite simple. It provides a title, and a script which imports the HelloModel.js above, calls model.step 500 times, and it prints a start message and a random sample of the model's data.

        <script type="module">
            import * as util from 'https://code.agentscript.org/src/utils.js'
            import Model from 'https://code.agentscript.org/models/HelloModel.js'

            util.printToPage('Running for 500 steps.')

            const model = new Model()
            await model.startup()

            util.repeat(500, model.step)

            const sample = util.sampleModel(model)

You can run it here. Click the browser's reload button to get a new random sampling. They look like:


But wait! Where's the pretty View? This is just text!

Well, it is MVC, and we haven't discussed Views yet. That's next.