k Nearest Neighbours Classifier (kNN) with Javascript Implementation for Beginners

k Nearest Neighbours Classifier (kNN) is a powerful but computational costly nonlinear classifier. Basically, kNN works based on distance calculation and finding of closest neighbour vectors according to a distance function. Distance function can be l1 (Manhattan distance), l2 (Euclidean distance) or a different available appropriate distance function. kNN uses a hyper parameter (k) that implies count of closest neighbours. 

Algorithm find closest k vector to current vector, and returns classification result as class label of the largest matched number of class between these k closest vectors.

In this example; classes 1,2,3 corresponds to red, green and purple respectively. Algorithm takes k, x and y as input and return class label as color and draw last input point as a orange point. 

->k

->X

->Y

Javascript Codes:

document.addEventListener('DOMContentLoaded', (e) => {

            drawPoints();
        });

        var point = { x: 0, y: 0 };
        var k = 3;

        var datas = [

            { c: 1, x: 10, y: 100 },
            { c: 1, x: -10, y: 50 },
            { c: 1, x: 50, y: 150 },
            { c: 2, x: 100, y: 80 },
            { c: 1, x: 30, y: 200 },
            { c: 2, x: 200, y: 50 },
            { c: 2, x: 120, y: 30 },
            { c: 2, x: 100, y: 150 },
            { c: 3, x: 300, y: 50 },
            { c: 1, x: -150, y: 200 },
            { c: 3, x: -300, y: 400 },
            { c: 3, x: 233, y: 333 },
            { c: 2, x: 127, y: 190 },
            { c: 3, x: 100, y: -32 },
            { c: 1, x: -50, y: -12 },
            { c: 1, x: -101, y: -50 },
            { c: 1, x: -301, y: -19 },
            { c: 2, x: 127, y: -22 },
            { c: 3, x: 120, y: -100 },
            { c: 2, x: -50, y: -200 },
            { c: 3, x: 800, y: 100 },
            { c: 3, x: 150, y: -150 },
            { c: 3, x: 70, y: -80 },
            { c: 3, x: 70, y: -500 },
            { c: 3, x: 200, y: -300 },
            { c: 2, x: -50, y: -150 },
            { c: 3, x: 111, y: -200 },
            { c: 3, x: 301, y: -310 },
            { c: 3, x: 200, y: -151 },
            { c: 1, x: -122, y: -99 },
            { c: 1, x: -301, y: -499 },
            { c: 1, x: 401, y: -1 },
        ];

        function start() {

            let classColor = "";

            setValues();

            drawCurrentPoint();

            let c = knnClassify();

            if (c == 1)
                classColor = "red";
            else if (c == 2)
                classColor = "green";
            else
                classColor = "purple";

            alert(classColor);
        }

        function knnClassify() {

            var classificationResults = [

                { c: 1, count: 0 },
                { c: 2, count: 0 },
                { c: 3, count: 0 }
            ];

            let distances = [];

            for (let i = 0; i < datas.length; i++) {

                let distance = l2Distance(datas[i], point);
                let distanceToClass = { c: datas[i].c, d: distance };

                distances.push(distanceToClass);
            }

            distances.sort(function (a, b) {

                return a.d - b.d;
            });

            for (let i = 0; i < k; i++) {

                for (let k = 0; k < classificationResults.length; k++) {

                    if (classificationResults[k].c == distances[i].c) {

                        classificationResults[k].count++;
                    }
                }
            }

            classificationResults.sort(function (a, b) {

                return b.count - a.count;
            });

            return classificationResults[0].c;
        }

        function l2Distance(point1, point2) {

            return Math.sqrt(Math.pow(point1.x - point2.x, 2) + Math.pow(point1.y - point2.y, 2));
        }

        function drawPoints() {

            var canvas = document.getElementById("g");

            for (let i = 0; i < datas.length; i++) {

                let circle = document.createElementNS('http://www.w3.org/2000/svg', 'circle');;
                circle.setAttributeNS(null, "cx", datas[i].x);
                circle.setAttributeNS(null, "cy", datas[i].y);
                circle.setAttributeNS(null, "r", 3);

                if (datas[i].c == 1)
                    circle.setAttributeNS(null, "fill", "red");
                else if (datas[i].c == 2)
                    circle.setAttributeNS(null, "fill", "green");
                else
                    circle.setAttributeNS(null, "fill", "purple");

                canvas.appendChild(circle);
            }
        }

        function drawCurrentPoint() {

            var canvas = document.getElementById("g");

            let circle = document.createElementNS('http://www.w3.org/2000/svg', 'circle');
            circle.setAttributeNS(null, "cx", point.x);
            circle.setAttributeNS(null, "cy", point.y);
            circle.setAttributeNS(null, "r", 3);

            circle.setAttributeNS(null, "fill", "orange");

            canvas.appendChild(circle);
        }
                   
        function setValues() {

            k = parseFloat(document.getElementById("pk").value);
            point.x = parseFloat(document.getElementById("pX").value);
            point.y = parseFloat(document.getElementById("pY").value);
        }

HTML Codes:

    <div>
        <input id="pk" type="text" value="3" />->k
    </div>
    <div>
        <input id="pX" type="text" value="0" />->X
    </div>
    <div>
        <input id="pY" type="text" value="0" />->Y
    </div>

    <div id="resultDiv" style="margin-top: 20px;"></div>
    <div style="margin-top: 20px;">
        <button onclick="start()" type="button">CLASSIFY</button>
    </div>

    <div style="margin-top: 50px;">
        <svg height="500" id="canvas" width="500">
            <g id="g" transform="translate(250 250) scale(1,-1)">
                <line style="stroke-width: 1; stroke: rgb(0, 0, 255);" x1="-1000" x2="1000" y1="0" y2="0"></line>
                <line style="stroke-width: 1; stroke: rgb(0, 0, 255);" x1="0" x2="0" y1="-1000" y2="1000"></line>

            </g>
        </svg>
    </div>