JSFiddle - React, Tailwind, and code Playground

HTML

<p>Open the console to see the outputs</p>

JavaScript

var myMatrix = [
    ['-','-','-','-','-','-','-','-','-','-'],
    ['-','-','-','-','-','-','-','-','-','-'],
    ['-','-','-','-','-','-','-','-','-','-'],
    ['-','-','-','-','-','-','-','-','-','-'],
    ['-','-','-','-','-','-','-','-','-','-'],
    ['-','-','-','-','-','-','-','-','-','-'],
    ['-','-','-','-','-','-','-','-','-','-'],
    ['-','-','-','-','-','-','-','-','-','-'],
    ['-','-','-','-','-','-','-','-','-','-'],
    ['-','-','-','-','-','-','-','-','-','-']
];

function stepOne(){
	console.log('Step One');
    
    for( var i = 0; i < myMatrix.length; i++ ){
        console.log(myMatrix[i].join(' '));
    }
}

//stepOne();

function stepTwo(coordinates) {
    console.log('Step Two');
    for(var j =0; j < coordinates.length; j ++) {
        myMatrix[coordinates[j].y][coordinates[j].x] = 'x';
    }
    for(var i = 0; i < myMatrix.length; i++) {
        console.log(myMatrix[i].join(' '));
    }
    
     
}

var coordinatesArray = [
    {x: 4, y: 0},
    {x: 5, y: 1},
    {x: 8, y: 2},
    {x: 8, y: 3},
    {x: 3, y: 4},
    {x: 7, y: 6},
    {x: 4, y: 8},
    {x: 7, y: 9},
];

//stepTwo(coordinatesArray);

function stepThree(points) {
    points.forEach(function(entry) {
        var row = myMatrix[entry.y];
        
        if(!row) { // if no row continue to next iteration
            return;
        }

        var fillNum = entry.x > row.length ? row.length : entry.x; // if the number to fill is higher than length use length
        
        myMatrix[entry.y] = new Array(fillNum + 1) // create an array which is bigger by 1 from places to fill
            .join('x') // join it using x's getting the right number
            .split('') // split it to create new array
            .concat(row.slice(fillNum)); // add the leftovers
    });
}

var pointsArray = [
    {x: 5, y: 0},
    {x: 6, y: 1},
    {x: 8, y: 2},
    {x: 8, y: 3},
    {x: 3, y: 4},
    {x: 6, y: 6},
    {x: 4, y: 8},
    {x: 7, y: 9},
];

stepThree(pointsArray);

stepOne();