Const KeyWord Confusion in Javascript

function fib(n){
  let result =[0,1];
  for(let i=2;i<=n-1;i++){
const a =result[i-1];
const b = result[i-2];
result.push(a+b);
  }

return result;
}
console.log(fib(4));

// In the above code snippets everything is working fine only confusion is regarding the thesis behind the const keyword ,as according to ES6 we cannot change the value of variable with const keyword ,but in the above code snippets there is a continuous changing of the value for the variable a and b