如果矩形中没有写入任何内容，如何让 if 类什么都不做？

Question

So I'm making Tic Tac Toe and I made it so that when I click on the empty rectangles (tiles) it puts the label X or O on the tile, but if the tile is labeled (not empty) I want the code to not write anything.

That's how the piece of code looks currently:

 if (!this.empty ()) {
        exit ();
    }

The issue is that when I press on a tile that is labeled the program stops entirely and I can no longer see a X or an O when clicking on an empty tile. Is there an alternative for exit(); that doesn't do that?

Here's the complete code (this code is actually not by me it's an practice from KhanAcademy): https://www.khanacademy.org/computing/computer-programming/programming-games-visualizations/memory-game/pc/challenge-tic-tac-toe

var playerTurn = 0;
var NUM_COLS = 3;
var NUM_ROWS = 3;
var SYMBOLS =["X","O"];
var tiles = [];

var checkWin = function() {

};

var Tile = function(x, y) {
    this.x = x;
    this.y = y;
    this.size = width/NUM_COLS;
    this.label = "";
};

Tile.prototype.draw = function() {
    fill(214, 247, 202);
    strokeWeight(2);
    rect(this.x, this.y, this.size, this.size, 10);
    textSize(100);
    textAlign(CENTER, CENTER);
    fill(0, 0, 0);
    text(this.label, this.x+this.size/2, this.y+this.size/2);
};

Tile.prototype.empty = function() {
    return this.label === "";
};

Tile.prototype.onClick = function() {
    // If the tile is not empty, exit the function
    if (!this.empty ()) {
        exit();
    }
    // Put the player's symbol on the tile

        this.label = SYMBOLS[playerTurn];

    // Change the turn
    playerTurn ++;
    if (playerTurn >= 1) {
        playerTurn = 0;
    }
};

Tile.prototype.handleMouseClick = function(x, y) {
    // Check for mouse clicks inside the tile
    if (x >= this.x && x <= this.x + this.size &&
    y >= this.y && y <= this.y + this.size)
    {
        this.onClick();
    }
};

for (var i = 0; i < NUM_COLS; i++) {
    for (var j = 0; j < NUM_ROWS; j++) {
        tiles.push(new Tile(i * (width/NUM_COLS-1), j * (height/NUM_ROWS-1)));
    }
}

var drawTiles = function() {
    for (var i in tiles) {
        tiles[i].draw();
    }
};

mouseReleased = function() {
    for (var i in tiles) {
        tiles[i].handleMouseClick(mouseX, mouseY);
    }
};

draw = function() {
    background(143, 143, 143);
    drawTiles();

};

Answer 1

The function exit is not a native function to javascript. It means nothing in this context as it has not been defined anywhere. You can use the word return to end execution of a function - it does what I think you're trying to accomplish with exit()

Tile.prototype.onClick = function() {
    // If the tile is not empty, exit the function
    if (!this.empty ()) {
        return
    } else {
    // Put the player's symbol on the tile

        this.label = SYMBOLS[playerTurn];

    // Change the turn

    playerTurn ++;
    if (playerTurn >= 1) {
        playerTurn = 0;
    }

  }

};

Answer 2

Okay I solved it indirectly xD

What I did was change

Tile.prototype.onClick = function() {
    // If the tile is not empty, exit the function
    if (!this.empty ()) {
        exit();
    }
    // Put the player's symbol on the tile

        this.label = SYMBOLS[playerTurn];

    // Change the turn
    playerTurn ++;
    if (playerTurn >= 1) {
        playerTurn = 0;
    }
};

to

Tile.prototype.onClick = function() {
    // If the tile is not empty, exit the function
    if (this.empty ()) {
        // Put the player's symbol on the tile
        this.label = SYMBOLS[0];
        SYMBOLS.reverse();
    }
};

So instead of making an if-class for all the not-empty tiles that should do nothing and the rest is done ouside of the if-class, I made an if class for the empty tiles.

Yeah I know it was stupid but heyyy at least it's solved now.