Referentially Opaque Expressions

An expression is referentially transparent when it cannot be replaced with its return value.

Example:

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function today(): string {
  return new Date().toISOString();
}

const isoDate = today();

At the time of writing the execution of today() returned '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.