Spread & immutable updates
Make a copy with a purpose
Lesson 14 of 20 · 11 min · Module 5: Reliable data pipelines
Two names can refer to the same object. An update through either name then changes the same data. A deliberate copy lets you create a new version instead.
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By the end, you’ll be able to…
- Recognize shared object references
- Copy each level being updated
The idea
Use object spread to make a shallow copy. Later properties override earlier ones: { ...settings, theme: "dark" }. Nested objects still need their own copies when you change them.
const before = { theme: "light", reminders: true };
const after = { ...before, theme: "dark" };01Assignment does not clone
const copy = original gives the same object another name. Both variables can observe later mutations.
02Spread is shallow
A new outer object does not automatically copy its nested preferences. Copy the nested level before assigning its new dailyGoal value.
03Test the promise to callers
Keep a reference to the original preferences, call the function, and inspect both versions. A useful test checks identity as well as values.
Your turn
withDailyGoal(profile, goal). Return a new profile and a new nested preferences object with dailyGoal set to goal. Preserve other preferences and every other profile field. preferences may be missing. Do not mutate the input.What the checks look for
- Preserves other profile fields and preferences
- Creates separate objects without changing the input
- Handles missing preferences
Take this with you
Copy exactly the levels you change; keep the update explicit.
Go a little further
Add a second nested preference update and test that the previous version remains unchanged.