Mutable Data

Symptom

Data structures whose fields are reassigned across the codebase, with no clear owner of the mutation.

Goal

Mutation happens in one place behind a named function (or returns a new value), so the moment of change is clear.

Smellier version

const order = { total: 100 };
applyDiscount(order); // mutates total
addTax(order);        // mutates total

Fresher version

const order = { total: 100 };
const final = addTax(applyDiscount(order));

Savings

Bugs stop being 'who set this field?' and become local to the encapsulating method.

Note

Reasoning about state at any moment requires tracing every writer; concurrent code becomes a hazard area.