Why is fillRect not a function in the loop?

Question

Canvas

Good afternoon, tell me why when using cycles it says that fillRect is not a function?
https://jsfiddle.net/f4ocguv4/20/

Answer 1

Because you overwrote it (wtf?):

if(arr[i][j] == "w") {
  ctx.fillRect = "white"; // чооооо?
} else {ctx.fillRect = "black" // чооооо?}

First write the color to fillStyle and then call fillRect()

ctx.fillStyle = "#FF0000";
ctx.fillRect(20,20,150,100);

Result
Bonus