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Svelteframework~10 mins

Readonly props in Svelte - Step-by-Step Execution

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Concept Flow - Readonly props
Parent Component
Pass prop to Child
Child Component receives prop
Child tries to modify prop?
YesError or Warning
No
Child uses prop as readonly value
Parent sends data to child as props. Child can read but cannot change props. Trying to change props causes error.
Execution Sample
Svelte
<script>
  export let count;
</script>

<p>{count}</p>
Child component receives a prop 'count' and displays it without changing it.
Execution Table
StepActionProp 'count' ValueModification AttemptResult
1Parent sets count=55NoProp passed to child as 5
2Child renders with count=55NoDisplays 5
3Child tries count = 105YesError: Cannot assign to 'count' - readonly prop
4Child uses count without change5NoDisplays 5
5Parent updates count=77NoChild re-renders with 7
💡 Child cannot modify 'count' prop; it remains readonly.
Variable Tracker
VariableStartAfter Step 1After Step 3After Step 5
countundefined55 (unchanged, error on modify)7
Key Moments - 2 Insights
Why can't the child component change the value of a prop?
Props are readonly in Svelte. The execution_table row 3 shows an error when the child tries to assign a new value to 'count'. Props are meant only for passing data down.
What happens if the parent changes the prop value?
As shown in execution_table row 5, when the parent updates 'count' to 7, the child automatically re-renders with the new value.
Visual Quiz - 3 Questions
Test your understanding
Look at the execution_table, what is the value of 'count' after step 3 when the child tries to modify it?
A5
B10
Cundefined
D7
💡 Hint
Check the 'Prop 'count' Value' column at step 3 in execution_table.
At which step does the parent update the prop value to 7?
AStep 2
BStep 3
CStep 5
DStep 4
💡 Hint
Look at the 'Action' column in execution_table for parent updates.
If the child tries to modify the prop, what will happen?
AThe prop value changes successfully
BAn error occurs preventing modification
CThe parent prop also changes
DNothing happens, silently ignored
💡 Hint
Refer to the 'Modification Attempt' and 'Result' columns in execution_table step 3.
Concept Snapshot
Readonly props in Svelte:
- Parent passes data as props to child
- Child can read but cannot modify props
- Attempting to assign to a prop causes an error
- Parent updates cause child to re-render
- Props are one-way data flow from parent to child
Full Transcript
In Svelte, props are values passed from a parent component to a child component. The child can use these props to display or compute things but cannot change them. If the child tries to assign a new value to a prop, Svelte will throw an error because props are readonly. When the parent changes the prop value, the child automatically updates to reflect the new value. This ensures a clear one-way data flow and avoids confusion about where data changes happen.