Resolving 'Type' Attribute Errors with Two-Way Binding in Svelte

The first time I created a custom <input> component for a SvelteKit application, I ran square into the 'type' attribute cannot be dynamic if input uses two-way binding error. If you're facing the same issue, this guide will help you understand and resolve it.

Understanding the Error

When creating an <input> component, you might use two-way binding to bind the value of an input element dynamically. However, if you try to bind the type attribute dynamically as well, you'll run into an error.

<!-- lib/Input.svelte -->
<script lang="ts">
  export let type: 'text' | 'number' | 'email' | 'password' = 'text';
  export let value = '';
</script>

<!-- This will throw an error -->
<input {type} bind:value />

This error occurs because Svelte requires the type attribute of an <input> to be statically defined when using two-way binding with the bind:value directive.

The Correct Approach: Using Spread Attributes

To avoid this error, you should use the spread operator to bind the type attribute dynamically:

<!-- lib/Input.svelte -->
<script lang="ts">
  export let type: 'text' | 'number' | 'email' | 'password' = 'text';
  export let value = '';
</script>

<!-- This works correctly -->
<input {...{ type }} bind:value />

Explanation

• Here, the type attribute is part of an object being spread onto the <input> element. Svelte processes this spread object and applies each key-value pair as an attribute.
• Svelte allows this because it can see the complete set of attributes and ensure that type is applied before setting up the two-way binding (bind:value).

Code Example

<!-- lib/Input.svelte -->
<script lang="ts">
  export let type: 'text' | 'number' | 'email' | 'password' = 'text';
  export let value = '';
</script>

<input {...{ type }} bind:value />

<!-- src/routes/+page.svelte -->
<script lang="ts">
  import Input from '$lib/Input.svelte';

  let inputValue = "";
</script>

<Input type="text" bind:value={inputValue} />
<p>Input Value: {inputValue}</p>

Summary

• Error Cause: The error 'type' attribute cannot be dynamic if input uses two-way binding occurs due to the dynamic binding of the type attribute.
• Correct Syntax: Use <input {...{ type }} bind:value /> to avoid the error.

That's it! Happy coding.