How to de-normalize numbers (reverse normalize) in javascript?

I have a function thats giving me a number between 0 and 1, for the x
and i also have another function thats giving me a number between 0 and 1, for the y coordinate

then i want to visualize it using canvas, so i would want to turn the numbers into pixel values

for example:

i have a a canvas thats 300x400px,
if the normalized number is 0.5, for the x and the y

what math would i need to do to make it 150x, 200y?

my code to visualize it (this part works, its just for refrence)

ctx.beginPath()
ctx.arc(xval, yval, 5, 0, 2 * Math.PI)
ctx.fillStyle = "#FF0000";
ctx.fill()

ctx.beginPath();
ctx.moveTo(0, xval);
ctx.lineTo(300, xval);
ctx.stroke();

ctx.beginPath();
ctx.moveTo(yval, 0);
ctx.lineTo(yval, 400);
ctx.stroke();

thanks!