Build a HTML5 game like “Knife Hit” with Phaser 3 using only tweens and trigonometry

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Did you play the latest Ketchapp Games top game, Knife Hit? « The ultimate knife challenge! Throw the knives into the logs to break them. Slash the apples and unlock new knives. Each 5th stage is defended by a boss – beat them to get exclusive knives! Be careful to not hit the knives or the spikes. Time your actions, target carefully and become the knife master! Can you beat all the bosses? » I am about to show you a Phaser 3 version made only using tweens and trigonometry. No physics engines. This is the concept: * The rotating target is a rotating sprite * The knife thrown by the player is moved by a tween to a specific y position * Once the knife this the target (that is, the tween is completed), we create a new knife in the very same place of the thrown knife * We make the newly added knife rotate around target center using good old trigonometry * The thrown knife returns to its original position And that’s it. Have a look:
Click or tap to throw a knife. If you have a mobile device, play directly on this link. And this is the line by line commented source code:

// the game itself
var game;

// global game options
var gameOptions = {

    // target rotation speed, in degrees per frame
    rotationSpeed: 3,

    // knife throwing duration, in milliseconds
    throwSpeed: 150
}

// once the window loads...
window.onload = function() {

    // game configuration object
    var gameConfig = {

        // render type
       type: Phaser.CANVAS,

       // game width, in pixels
       width: 750,

       // game height, in pixels
       height: 1334,

       // game background color
       backgroundColor: 0x444444,

       // scenes used by the game
       scene: [playGame]
    };

    // game constructor
    game = new Phaser.Game(gameConfig);

    // pure javascript to give focus to the page/frame and scale the game
    window.focus()
    resize();
    window.addEventListener("resize", resize, false);
}

// PlayGame scene
class playGame extends Phaser.Scene{

    // constructor
    constructor(){
        super("PlayGame");
    }

    // method to be executed when the scene preloads
    preload(){

        // loading assets
        this.load.image("target", "target.png");
        this.load.image("knife", "knife.png");
    }

    // method to be executed once the scene has been created
    create(){

        // can the player throw a knife? Yes, at the beginning of the game
        this.canThrow = true;

        // group to store all rotating knives
        this.knifeGroup = this.add.group();

        // adding the knife
        this.knife = this.add.sprite(game.config.width / 2, game.config.height / 5 * 4, "knife");

        // adding the target
        this.target = this.add.sprite(game.config.width / 2, 400, "target");

        // moving the target on front
        this.target.depth = 1;

        // waiting for player input to throw a knife
        this.input.on("pointerdown", this.throwKnife, this);
    }

    // method to throw a knife
    throwKnife(){

        // can the player throw?
        if(this.canThrow){

            // player can't throw anymore
            this.canThrow = false;

            // tween to throw the knife
            this.tweens.add({

                // adding the knife to tween targets
                targets: [this.knife],

                // y destination
                y: this.target.y + this.target.width / 2,

                // tween duration
                duration: gameOptions.throwSpeed,

                // callback scope
                callbackScope: this,

                // function to be executed once the tween has been completed
                onComplete: function(tween){

                    // player can now throw again
                    this.canThrow = true;

                    // adding the rotating knife in the same place of the knife just landed on target
                    var knife = this.add.sprite(this.knife.x, this.knife.y, "knife");

                    // adding the rotating knife to knifeGroup group
                    this.knifeGroup.add(knife);

                    // bringing back the knife to its starting position
                    this.knife.y = game.config.height / 5 * 4;
                }
            });
        }
    }

    // method to be executed at each frame
    update(){

        // rotating the target
        this.target.angle += gameOptions.rotationSpeed;

        // getting an array with all rotating knives
        var children = this.knifeGroup.getChildren();

        // looping through rotating knives
        for (var i = 0; i < children.length; i++){

            // rotating the knife
            children[i].angle += gameOptions.rotationSpeed;

            // turning knife angle in radians
            var radians = Phaser.Math.DegToRad(children[i].angle + 90);

            // trigonometry to make the knife rotate around target center
            children[i].x = this.target.x + (this.target.width / 2) * Math.cos(radians);
            children[i].y = this.target.y + (this.target.width / 2) * Math.sin(radians);
        }

    }
}

// pure javascript to scale the game
function resize() {
    var canvas = document.querySelector("canvas");
    var windowWidth = window.innerWidth;
    var windowHeight = window.innerHeight;
    var windowRatio = windowWidth / windowHeight;
    var gameRatio = game.config.width / game.config.height;
    if(windowRatio < gameRatio){
        canvas.style.width = windowWidth + "px";
        canvas.style.height = (windowWidth / gameRatio) + "px";
    }
    else{
        canvas.style.width = (windowHeight * gameRatio) + "px";
        canvas.style.height = windowHeight + "px";
    }
}
Making this prototype only required a few lines as you can see. Next time I’ll show you how to check if you hit a knife, obviously only using trigonometry. Meanwhile download the source code.

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