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react-state-basics.md

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React State

"state" without React?

CodeSandBox: Counter in variables

Comments:

  • Keep state in local variable (inside of component) or external variable (outside of component).
  • Result: Component won't re-render.

Using useState: Counter re-renders

CodeSandBox: useState hook

Question: Why does the countOutside value also change (with the state value) but inner countInside does not?

Show Answer

Answer: The inner count gets initialized (and set to 0) on every render which is triggered by the state value.

useState hook uses strict equality (===) comparison to decide whether to return a new value

CodeSandBox: useState with mutable vs. immutable state features broken and correct ways to update state.

  • A new state value is created if the value passed to the setter function is not strictly equal the current value (referential equality in case of complex data types)
  • Every successful state upate causes:
    1. The component function to be re-run
    2. A new state value to be returned by the useState hook

Note:

  • states are const (not let)! CodeSandBox: Also objects are entirely new objects, not just copied references (JS))

  • Questions: Why do the broken increment functions increase the value without updating the UI?

  • Answer: In case of the broken click handlers the useState hook's strict equality check returns true the count value in the existing state variable objectCount does indeed change its value, but since it won't cause the component to re-render, both useEffect hooks won't run and the function won't return a new UI to be rendered. See the appendix at the bottom for a code example concerning equality checks in JS.

  • Note: Since the state value and the setter function are const values they can't be re-assigned.

TODO: Review this one: CodeSandBox: One state change causes all states to be new! (JS)

Appendix: Shallow copy vs. deep copy:

// current state
const obj1 = { val: 1 };

// Mutation: Change val: 1 to val: 2
obj1.val = 2
const obj2 = obj1
console.log(obj1 === obj2) // true

// New Object: Change val: 1 to val: 2
const obj3 = { val: 2 }
console.log(obj2 === obj3) // false