document.addEventListener("keydown",keyDown);

gameObjects = [];

const boxStyles = getComputedStyle(document.getElementById("box"))

// This accomplishes what your code was doing, but in two steps
// Placing a + before a string converts it to a positive Number value
const SCREEN_WIDTH = +boxStyles.getPropertyValue("width").replace('px',"");
const SCREEN_HEIGHT = +boxStyles.getPropertyValue("height").replace('px',"");

function createGameObject(id, x, y) {
    const retVal = {
        "id": id,
        "element": document.getElementById(id),
        "x": x,
        "y": y,
        "width": document.getElementById(id).offsetWidth,
        "height": document.getElementById(id).offsetHeight,
        "setScreenPosition": function(){this.element.style.left = this.x + "px"; this.element.style.top = this.y + "px";},
        "moveLeft": function() {this.x -= this.width; this.setScreenPosition();},
        "moveRight": function() {this.x += this.width; this.setScreenPosition();},
        "moveUp": function() {this.y -= this.height; this.setScreenPosition();},
        "moveDown": function() {this.y += this.height; this.setScreenPosition();},
        "setXY": function(x, y) {this.x = x; this.y = y; this.setScreenPosition();}
    }
    retVal.setScreenPosition();
    gameObjects.push(retVal);
    return retVal;
}

const myPlayer = createGameObject("player", 0, 0);
const myBrick = createGameObject("brick", 128, 32);
const myWall = createGameObject("wall", 256, 128);

function keyDown(e) {

    switch(e.keyCode) {
        case 37:
            moveLeft(myPlayer);
            break;
        case 38:
            moveUp(myPlayer);
            break;
        case 39:
            moveRight(myPlayer);
            break;
        case 40:
            moveDown(myPlayer);
            break;
    }
}

function moveLeft(gameObject) {
    let newX = gameObject.x - gameObject.width;

    if (newX < 0){
        console.log("Off the screen to the left.");
        return
    }
    // The following three lines perform these actions:
    // Check if the gameObj[brick] will touch gameObject[Player] (x)
    // Check if the brick is in the way in the other dimension (y)
    // Check that the object we're comparing against isn't itself
    // I'm using filter to create an array of all objects in the path and then checking the length of that array. An empty array means nothing in the way
    if (gameObjects.filter(gameObj=>gameObj.x <= newX && gameObj.x + gameObj.width >= gameObject.x && 
                                    gameObject.y >= gameObj.y && gameObject.y + gameObject.height <= gameObj.y + gameObj.height && 
                                    gameObj.id !== gameObject.id).length) {
        console.log("Hitting something on the left.");
        return;
    }
    gameObject.moveLeft();
}
function moveRight(gameObject) {
    let newX = gameObject.x + gameObject.width;

    if (newX >= SCREEN_WIDTH) {
        console.log("Off the screen to the right.");
        return;
    }
    if (gameObjects.filter(gameObj=>gameObj.x === newX && gameObject.y >= gameObj.y && 
                                    gameObject.y + gameObject.height <= gameObj.y + gameObj.height && 
                                    gameObj.id !== gameObject.id).length) {
        console.log("Hitting something on the right.")
        return;
    }
    gameObject.moveRight();
}
function moveUp(gameObject) {
    let newY = gameObject.y - gameObject.height;

    if (newY < 0) {
        console.log("Off the screen to the top.");
        return;
    }
    if (gameObjects.filter(gameObj=>gameObj.y <= newY && gameObj.y + gameObj.height >= gameObject.y &&
                                    gameObject.x >= gameObj.x && gameObject.x + gameObject.width <= gameObj.x + gameObj.width && 
                                    gameObj.id !== gameObject.id).length) {
        console.log("Hitting something above.");
        return;
    }
    gameObject.moveUp();
}
function moveDown(gameObject) {
    let newY = gameObject.y + gameObject.height;

    if (newY >= SCREEN_HEIGHT) {
        console.log("Off the screen to the bottom.");
        return;
    }
    if (gameObjects.filter(gameObj=>gameObj.y === newY && gameObject.x >= gameObj.x && 
                                    gameObject.x + gameObject.width <= gameObj.x + gameObj.width && 
                                    gameObj.id !== gameObject.id).length) {
        console.log("Hitting something below.");
        return;
    }
    gameObject.moveDown();
}

#box {
    position:absolute
    top:0px;
    left:0px;
    background-color:blue;
    width:512px;
    height:256px;
}
#player {
    position:absolute;
    background-color:red;
    width:8px;
    height:8px;
    z-index:2;
}
#world {
    position:relative;
    bottom:0px;
    left:0px;
    width:100%;
    height:100%;
    z-index:1;
}
#brick {
    position:absolute;
    background-color:black;
    width:16px;
    height:16px;
    z-index:1;
}

#wall {
    position:absolute;
    background-color:yellow;
    width:16px;
    height:48px;
    z-index:1;
}

<div id="box">
    <div id="world">
        <div id="brick"></div>
        <div id="wall"/></div>
        <div id="player"/></div>
    </div>
</div>