I'm trying to understand scope in Javscript, and have some problem. Code:

const callback = (arg) => {
  console.log(arg);
};

function mainFunc(z) {
  const cb = z;
  insideFunc();
}

function insideFunc() {
  cb('1');
}


mainFunc(callback);

Why insideFunc cannot find cb variable?

  cb('1');
  ^

ReferenceError: cb is not defined
    at insideFunc (file:///usr/src/app/file.js:42:3)
    at mainFunc (file:///usr/src/app/file.js:38:3)
    at file:///usr/src/app/file.js:46:1