how 'mousemove' event work on html5 canvas?

I'm trying to reappear a 'select-function' of windows mspaint or other drawing tools on canvas, which works like:

1. when mouse is down, mark this point
2. when mouse is moving, draw a rect from mousedown point to now mouse position
3. when mouse is up, preserve the rect on canvas and end select

I'm now confused of how 'mousemove' event working when mouse moves? When mouse is moving, it draws lots of rects on the canvas.

code as:

const canvas = document.getElementById("canvas");
const ctx = canvas.getContext("2d");

function windowToCanvas(x,y) {
    let rect = canvas.getBoundingClientRect();
    x = x - rect.left * (canvas.width / rect.width);
    y = y - rect.top * (canvas.height / rect.height);
    return [x,y];
}
let mRect = {x:0,y:0,w:0,h:0};
function handleMouseMove(e){
    let temp = windowToCanvas(e.clientX,e.clientY);
    [mRect.w,mRect.h] = [temp[0]-mRect.x,temp[1]-mRect.y];
    ctx.clearRect(0,0,canvas.width,canvas.height);  // this line seems not working
    ctx.rect(mRect.x,mRect.y,mRect.w,mRect.h);
    ctx.stroke();
    
}
function handleMouseDown(e){
    [mRect.x,mRect.y]=windowToCanvas(e.clientX,e.clientY)
    canvas.addEventListener('mousemove',handleMouseMove);
    canvas.addEventListener('mouseup',()=>{
        console.log('mouseup')
        canvas.removeEventListener('mousemove',handleMouseMove)
    },{once:true});
}
canvas.addEventListener('mousedown',handleMouseDown);