Setting a checkbox "check" property in React

Question

I am having a very annoying issue with React and checkboxes. The application I am working with requires a list of checkboxes that represent settings that are persisted in the back-end. There is an option to restore the settings to their original state.

At first, I created a component that has an object like a map of settings. Each setting has a key and a boolean value. Hence:

{
    bubbles: true,
    gregory: false
}

Is to be represented as:

<input type="checkbox" value="bubbles" checked="checked" />
<input type="checkbox" value="gregory" />

Now, it seems React is ignorant about how a checkbox is supposed to work. I don't want to set the checkboxes' values, I want the "checked" property.

Yet, if I try something like this:

<input
    type="checkbox"
    value={setting}
    checked={this.settings[setting]}
    onChange={this.onChangeAction.bind(this)}
/>

I get this warning:

Warning: AwesomeComponent is changing an uncontrolled input of type checkbox to be controlled. Input elements should not switch from uncontrolled to controlled (or vice versa). Decide between using a controlled or uncontrolled input element for the lifetime of the component. More info: [some useless docs page I read several times to no avail]

So I decided to create another component to wrap each individual checkbox and I got this:

<input
    type="checkbox"
    checked={this.state.checked}
    onChange={this.onChangeAction.bind(this)}
/>

Now the checked is a property present directly in my state.

This yields the same warning, so I tried using defaultChecked:

<input
    type="checkbox"
    defaultChecked={this.state.checked}
    onChange={this.onChangeAction.bind(this)}
/>

Which makes the warning disappear, but now it is unable to reset the checked value to the default one. So I tried playing with the method componentWillReceiveProps, this way I am quite sure my state is correct, this.state.checked is correct and the component renders again.

And it does. But the checkbox remains as it was originally. For now I left that ugly warning and I am using checked. How do I fix this thing so the warning goes away?

I was thinking that perhaps there is a way to force-re-render the component, so it captures the new defaultChecked value and uses it. But I don't know how to do that. Perhaps suppress the warning only for this component? Is that possible? Perhaps there is something else that can be done?

Answer 1

The problem arises if you set the checked property of your checkbox to null or undefined.

Those are a "falsely" values in JS, however React treats a value of null as if the property was not set at all. Since the default state of a checkbox is unchecked, everything will work fine though. If you then set checked to true React thinks the property suddenly comes into existence! This is when React figures you switched from uncontrolled to controlled, since now the prop checked exists.

In your example you can get rid of this warning by changing checked={this.settings[setting]} to checked={!!this.settings[setting]}. Notice the double bang (!!). They will convert null or undefined to false (and leave true alone), so React will register your checked property with a value of false and start off with a controlled form component.

I had this problem too and I, too, read the docs about controlled-components serveral times to no avail, but I finally figured it out, so I thought I'd share. Also, since version 15.2.0 normal inputs are triggered to be controlled by setting value, while checkboxes are initialized as controlled by setting checked, regardless of their value property, which added a bit to the confusion.