我不知道为什么我得到一个"未定义不是函数"错误

Question

所以我不知道为什么,在第35行,我得到这个错误,有人可以解释为什么这不起作用,因为我似乎无法弄清楚为什么？

这是其上传的网页的链接,您可以在控制台中看到错误消息,以及所有代码.http://matthew-hoyle.co.uk/files/beta/projects/platformer.php

window.onload = function() {
var canvas = document.getElementById("canvas");
var c = canvas.getContext("2d");

canvas.width  = 800;
canvas.height = 600;

function Player() {
    this.width = 20;
    this.height = 20;
    this.color = "red";
    this.posY = (canvas.height / 2) - (this.height / 2);
    this.posX = (canvas.width / 2) - (this.width / 2);
    this.volY = 0;
    this.volX = 0;
    this.gravity = 0.5;
    this.onGround = true;
    this.draw = function() {this.posY += this.volY;
        this.posX += this.volX;
        this.volY += this.gravity;

        c.fillStyle = this.color;
        c.fillRect(this.posX, this.posY, this.width, this.height);
    };
}

function Obstacle(x,y,w,h,c) {
    this.posY = y;
    this.posX = x;
    this.width = w;
    this.height = h;
    this.color = c;
    this.draw = function() {
        c.fillStyle = this.color;
        c.fillRect(this.posX, this.posY, this.width, this.height);
    };
}

//objects
var player = new Player();
var ground = new Obstacle(0, canvas.height-20,canvas.width,20,"red");

//game update loop
window.setInterval(function() {
    //clears screen
    c.fillStyle = "lightblue";
    c.fillRect(0,0, canvas.width, canvas.height);

    //drawing objects
    player.draw();
    ground.draw();
}, 30);

};

Answer 1

var ground = new Obstacle(0, canvas.height-20,canvas.width,20,"red");

在这里,您初始化ground为Obstacle5个参数.第五个c是字符串"red".所以,当你以后调用ground.draw()你试图调用时"red".fillRect,那不是字符串的有效成员函数.

根本的问题是,该参数c的的Obstacle构造函数隐藏全局c变量,因为它是一个更近的范围,因此躲在你打算使用画布上下文的引用.