Zustand State and set Explained

1. Ways to Update State Using set

Zustand provides two ways to update state inside the store:

a) Functional Update (set((state) => {...}))

• Use this when your update depends on the previous state.
• It ensures that you always modify the latest state.

set((state) => ({ count: state.count + 1 }));

Why? The function receives state, so we can safely modify count based on the latest state.

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b) Direct Object Update (set({...}))

• Use this when updating state without depending on the previous state.

set({ user: newUser });

Why? We don’t need the previous state here, just replacing user.

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2. Parentheses in set((state) => {...})

• When returning an object in an arrow function, wrap it in () to avoid JavaScript confusion.

✅ Correct:

set((state) => ({ count: state.count + 1 })); // ✅ Parentheses required

❌ Incorrect (No return, JavaScript thinks {} is a function block)

set((state) => { count: state.count + 1 }); // ❌ Syntax error!

If using a function block {}, explicitly use return:

set((state) => {
  return { count: state.count + 1 };
});

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3. When to Use Which?

Approach	When to Use?	Example
set((state) => ({ ... }))	If updating based on previous state	set((state) => ({ count: state.count + 1 }))
set({ ... })	If replacing state without needing the old state	set({ user: newUser })

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4. Why No Parentheses in Some Cases?

• If an arrow function uses {}, it’s treated as a function block and requires an explicit return.
• If it’s one line returning an object, wrap it in ().

✅ With {} (Needs return)

set((theme) => {
  const toggle = theme.isDarkmode === "dark" ? "light" : "dark";
  localStorage.setItem("theme", toggle);
  return { isDarkmode: toggle }; // ✅ Explicit return needed
});

✅ With () (Implicit Return)

set((state) => ({ count: state.count + 1 })); // ✅ No `return` needed

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Summary

• Use set((state) => ({ ... })) when updating based on previous state, and set({ ... }) when setting a new value directly.
• Use () around objects when using an implicit return in arrow functions.
• If using {}, you must use return.

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🚀 Now you have a complete reference for Zustand state updates!