Shortly about immutability vs mutability

mutants.md

Consolidated this info from various Stack Overflow questions:

Mutable data

• you need to know:
  • what data means
  • where data was first assigned
  • where data can be changed
  • when data can be changed
  • if those changes can happen concurrently and require locks, what sequence locks need to be acquired in
  • if changing the data needs coordination with others
  • if data is actually changed here
• complexity: mutable data is inherently complex, because it can change

Immutable data

• you need to know:
  • what data means
  • where it was first assigned
  • (perhaps) how it is used here
• garbage collection efficiency
• lock-free operation
• efficient substructure sharing
• safely access data from 2 threads without worrying about locks or other concurrency issues
• produces code that's easier to reason about, easier to test, and easier to be confident about its correctness

could do

//splice   MUTABLE
const array = [1,2,3,4,5];
array.splice(2);   // -> [3,4,5]

//slice   IMMUTABLE
const array2 = [1,2,3,4,5];
array2.slice(2);   // ->   [3,4,5]

console.log(array);   //  -> [1,2]    Splice mutates original array
console.log(array2);  //  -> [1,2,3,4,5]  Slice doesn't mutate anything