how do we know what was written in the function in javascript code example

Example 1: How to create a function in javascript

function addfunc(a, b) {
  return a + b;
  // standard long function
}

addfunc = (a, b) => { return a + b; }
// cleaner faster way creating functions!

Example 2: function js

function myFunc(param) {
  return param
}
console.log(myFunc("Hello World"))