Proxy objects

Sometimes you get a string value from an input field or third-party library, and want it to be automatically converted and updated with a non-string value in your schema. This is what proxies are for.

import {
  // The primitives return a Writable<string>:
  // The type of the other three depends on the field:
} from 'sveltekit-superforms';

The usage for all of them is practically the same. You can use the form store, or the whole superForm as input, which allows you to set a tainted option, in case you don’t want to taint the form when it updates.

import { superForm, intProxy } from 'sveltekit-superforms';

// Assume the following schema:
// { id: number }

const superform = superForm(data.form);
const { form, errors, enhance } = superform;

// Returns a Writable<string>
const idProxy = intProxy(form, 'id');

// Use the whole superForm object to prevent tainting
const idProxy2 = intProxy(superform, 'id', { taint: false });

Now if you bind to $idProxy instead of $, the value will be converted to and from an integer, and $ will be updated automatically.

Note that this kind of conversion will usually happen automatically with bind:value. intProxy and numberProxy are rarely needed, as Svelte handles this automatically. But proxies may still be useful if you want to set the value to undefined or null when the value is falsy, in which case you can use the empty option.

Nested proxies

You can use a proxy for nested data, like '', in which case parent objects will be created if they don’t exist.

Date input issues

The date input type is a bit special. Its underlying data is a string in yyyy-mm-dd format, but the dateProxy returns an ISO date string as default, so you need to use the format option to return the date part only:

const proxyDate = dateProxy(form, 'date', { format: 'date' });
  aria-invalid={$ ? 'true' : undefined}
  min={$, 10)}
  max={$, 10)}

We’re also taking using the min and max constraints to limit the date picker selection. The following example limits the date from today and forward, and also uses the empty option of the proxy, to set an invalid date to undefined:

  date: undefined
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