how do i place a circle on specific places in grid (canvas)

so I am trying to make thisgame called GO and I need to place circles in the edges like this. I've made the grid and now need to place the circles in there. I 'v tried different things like using the arc in canvas with mouse position and placing it but it isn't working something else I tried is to make an array that checks where the lines cross but it still didn't do anything. but i may have done it wrong. I'm not sure what's wrong so hope you guys could help me find a way to place the circles on the board every time I click the mouse. I've deleted the things that didn't work and this is my code now:

const canvas = document.getElementById("myCanvas")
const ctx = canvas.getContext("2d")
//canvas.style.border = "1px solid black"
let w = ctx.canvas.width
let h = ctx.canvas.height


goBoard = []
goCheckBoard = []

function drawGrid(w, h) {

    for (x = 0; x <= w; x += 40) {
        for (y = 0; y <= h; y += 40) {
            ctx.moveTo(x, 0);
            ctx.lineTo(x, h);
            ctx.stroke();
            ctx.moveTo(0, y);
            ctx.lineTo(w, y);
            ctx.stroke();
        }
    }
}

drawGrid(400, 400)

this just makes the grid

one example i've tried is this:

mouseClicked = function () {
    ctx.beginPath()
    ctx.strokeStyle = "black"
    ctx.ellipse(mouseX, mouseY, 20, 20);
    ctx.stroke()
};

another:

function rtn(n, r){
    return Math.round(n/r)*r;
}
canvas.onclick = function(event){
    ellipse(rtn(event.clientX, 40), rtn(event.clientY, 40), 180, 180);
};

im not sure if that is the correct way to do it

maybe make an array of all the possible places to place the circle but im not sure how

here is a fiddle if you want to test it out https://jsfiddle.net/pwe0vx7o/

Solution

you first need to change the ellipse(...) to ctx.ellipse(...), because ellipse isn't a global function, but a metod of the CanvasRenderingContext2D.

Through the docs I also found that this function takes 7 arguments so added ones for radiusX, radiusY, rotation, startAngle, endAngle. Here is also a fiddle for the finished result: https://jsfiddle.net/c6udn9gf/8/