Overview
The setFromArray() decoder validates arrays using a given decoder and returns the result as an ES6 Set<T>. This automatically removes duplicates and provides Set-specific methods.
Basic Usage
import { setFromArray, string } from 'decoders';
const tagsDecoder = setFromArray(string);
const result = tagsDecoder.verify(['typescript', 'validation', 'decoders', 'typescript']);
// Type: Set<string>
// Result: Set { 'typescript', 'validation', 'decoders' }
// Note: Duplicate 'typescript' was automatically removed
result.has('typescript'); // true
result.size; // 3
Type Signature
function setFromArray<T>(decoder: Decoder<T>): Decoder<Set<T>>;
Parameters
Decoder to validate each element in the array. The validated array is then converted to a Set.
Return Value
Returns a Decoder<Set<T>> that validates arrays and converts them to ES6 Set instances.
Examples
Unique identifiers
import { setFromArray, number } from 'decoders';
const userIdsDecoder = setFromArray(number);
const userIds = userIdsDecoder.verify([101, 102, 103, 101, 102]);
// Type: Set<number>
// Result: Set { 101, 102, 103 }
console.log(userIds.size); // 3 (duplicates removed)
Complex objects
import { setFromArray, object, string } from 'decoders';
const categoriesDecoder = setFromArray(
object({
id: string,
name: string,
})
);
const categories = categoriesDecoder.verify([
{ id: 'tech', name: 'Technology' },
{ id: 'sci', name: 'Science' },
{ id: 'tech', name: 'Technology' }, // Duplicate
]);
// Type: Set<{ id: string; name: string }>
// Result: Set with 2 or 3 items (depending on object reference equality)
When using setFromArray() with objects, be aware that Set uses reference equality. Two objects with the same values are considered different if they’re different instances:const set = new Set([{ id: 1 }, { id: 1 }]);
console.log(set.size); // 2 (not 1!)
Enum values
import { setFromArray, string } from 'decoders';
const validRoles = ['admin', 'user', 'guest'] as const;
const roleDecoder = oneOf(validRoles);
const rolesSetDecoder = setFromArray(roleDecoder);
const roles = rolesSetDecoder.verify(['admin', 'user', 'admin']);
// Type: Set<'admin' | 'user' | 'guest'>
// Result: Set { 'admin', 'user' }
Converting Set back to array
import { setFromArray, number } from 'decoders';
const numbersDecoder = setFromArray(number);
const uniqueNumbers = numbersDecoder.verify([1, 2, 3, 2, 1]);
// Type: Set<number>
// Convert back to array if needed
const arrayAgain = Array.from(uniqueNumbers);
// or
const arrayAgain2 = [...uniqueNumbers];
Implementation Details
The setFromArray() decoder is built on top of array() and uses the .transform() method to convert the validated array to a Set:
// Simplified implementation
export function setFromArray<T>(decoder: Decoder<T>): Decoder<Set<T>> {
return array(decoder).transform((items) => new Set(items));
}
When to Use
Use setFromArray() instead of array() when:
- You need to ensure uniqueness (no duplicates)
- You need Set-specific methods like
.has(), .add(), .delete()
- You want to check membership efficiently (O(1) average case)
- Order doesn’t matter or you want to de-duplicate
If you need to preserve all elements including duplicates, or if order matters, use array() instead. Error Handling
setFromArray() inherits error handling from array(), providing detailed error messages for invalid elements:
import { setFromArray, number } from 'decoders';
const decoder = setFromArray(number);
try {
decoder.verify([1, 2, 'invalid', 4]);
} catch (error) {
console.error(error.message);
// Error indicates which array index failed validation
}
Set Operations
Once you have a Set, you can use all standard Set methods:
import { setFromArray, string } from 'decoders';
const tagsDecoder = setFromArray(string);
const tags = tagsDecoder.verify(['a', 'b', 'c']);
// Membership test
tags.has('a'); // true
// Add elements
tags.add('d');
// Remove elements
tags.delete('b');
// Iterate
for (const tag of tags) {
console.log(tag);
}
// Size
console.log(tags.size);
array - Validates arrays and returns them as arraysmapping - Validates objects and returns ES6 Map instancestuple - For fixed-length arrays with specific types
See Also
