Referentially Opaque Expressions
An expression is referentially transparent when it cannot be replaced with its return value.
At the time of writing the execution of
'2021-09-22T12:45:25.657Z'. This result will change over time, so we cannot replace
const isoDate = today() with
const isoDate = '2021-09-22T12:45:25.657Z' which makes this expression referentially opaque.