JavaScript 'is not a function' - object instance

Question

I get the error:

Uncaught TypeError: rect.drawIt is not a function

I don't understand why this error happens? I mean i am doing the same as here: http://www.w3schools.com/js/js_object_prototypes.asp

  var context = document.getElementById("canv").getContext("2d");
    
    for (var k = 0; k < 4; k++) {
        var rect = new Recta();
        rect.drawIt();
    }
    
    function Recta() {
        this.x1 = Math.floor(Math.random() * context.canvas.width);
        this.y1 = Math.floor(Math.random() * context.canvas.height);
        this.x2 = Math.floor(Math.random() * context.canvas.width);
        this.y2 = Math.floor(Math.random() * context.canvas.height);
        this.x3 = Math.floor(Math.random() * context.canvas.width);
        this.y3 = Math.floor(Math.random() * context.canvas.height);
        this.x4 = Math.floor(Math.random() * context.canvas.width);
        this.y4 = Math.floor(Math.random() * context.canvas.height);
    };
    
    Recta.drawIt = function () {
        context.moveTo(this.x1, this.y1);
        context.lineTo(this.x2, this.y2);
        context.stroke();
    
        context.moveTo(this.x2, this.y2);
        context.lineTo(this.x3, this.y3);
        context.stroke();
    
        context.moveTo(this.x3, this.y3);
        context.lineTo(this.x4, this.y4);
        context.stroke();
    
        context.moveTo(this.x4, this.y4);
        context.lineTo(this.x1, this.y1);
        context.stroke();
    };

<canvas id="canv"></canvas>

you try calling drawIt before adding it to Recta.prototype — Grundy
– Grundy, Commented May 8, 2015 at 6:34

Zee · Accepted Answer · 2015-05-08 06:42:05Z

4

First define the Recta.prototype.drawIt then use it.

 function Recta() {
    this.x1 = Math.floor(Math.random() * context.canvas.width);
    this.y1 = Math.floor(Math.random() * context.canvas.height);
    this.x2 = Math.floor(Math.random() * context.canvas.width);
    this.y2 = Math.floor(Math.random() * context.canvas.height);
    this.x3 = Math.floor(Math.random() * context.canvas.width);
    this.y3 = Math.floor(Math.random() * context.canvas.height);
    this.x4 = Math.floor(Math.random() * context.canvas.width);
    this.y4 = Math.floor(Math.random() * context.canvas.height);
};

Recta.prototype.drawIt = function () {
    context.moveTo(this.x1, this.y1);
    context.lineTo(this.x2, this.y2);
    context.stroke();

    context.moveTo(this.x2, this.y2);
    context.lineTo(this.x3, this.y3);
    context.stroke();

    context.moveTo(this.x3, this.y3);
    context.lineTo(this.x4, this.y4);
    context.stroke();

    context.moveTo(this.x4, this.y4);
    context.lineTo(this.x1, this.y1);
    context.stroke();
};

    var context = document.getElementById("canv").getContext("2d");

for (var k = 0; k < 4; k++) {
    var rect = new Recta();
    rect.drawIt();
}

edited May 8, 2015 at 6:42

answered May 8, 2015 at 6:33

Zee

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2 Comments

Grundy Over a year ago

you can define function in any place in file and it would be available, but adding drawIt it more setup property in prototype than defining function

Zee Over a year ago

@Grundy You are right :) However just to give a clear picture I did that. Edited my answer :)

sanjeev · Accepted Answer · 2015-05-08 06:32:41Z

1

Write the below code at the last (after definition of the object)

    var context = document.getElementById("canv").getContext("2d");

    for (var k = 0; k < 4; k++) {
        var rect = new Recta();
        rect.drawIt();
    }

answered May 8, 2015 at 6:32

sanjeev

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