SwiftUI search field from a local JSON file

To start, I recommend making a struct to represent your JSON structure. Try the following:

struct Person: Codable, Hashable {
    let name: String
    let lastname: String
    let age: String
}

typealias People = [Person]

I usually make a typealias to handle the array version of my data. Once you have defined your data structure to match your JSON, I usually extend my data to make loading from JSON easy.

extension Person {
    static func load(fromJson json: URL) -> People {
        guard let data = try? Data(contentsOf: json) else {
            preconditionFailure("Unable to read data from URL")
        }
        let jsonDecoder = JSONDecoder()
        var people = People()
        do {
            people = try jsonDecoder.decode(People.self, from: data)
        } catch {
            print(error)
        }
        return people
    }
}

There are more generic ways to do this to support a wider swath of models, but for your example, this is quick and easy.

Now that you have an easy way to work with your model, an easy solution to what you have in mind could be done by extending the Array type like so:

extension Array where Element == Person {
    func retrievePeople(byName name: String) -> People {
        return self.filter { $0.name == name }
    }

    func retrievePeople(byLastName lastname: String) -> People {
        return self.filter { $0.lastname == lastname }
    }

    func retrievePeople(byAge age: String) -> People {
        return self.filter { $0.age == age }
    }
}

This will allow you to query the entire range of objects by any of the elements and in turn, return the array of matches. If you're certain that there's only one return, you could use the following code to get the first element:

// Let's assume your array of people is stored in this variable
var myPeople: People
if let person = myPeople.retrievePeople(byName: "John").first {
   // Do the things you want with this person object here
}

The nice thing about this style of loading/working with data is that it's easy to generate quickly and it will support returning 0 objects, 1 object, and multiple objects. This will also allow you to move the model over to use the features of SwiftUI/Combine (which is what it looks like you're hoping to do above).