children

function children(fn: () => JSX.Element): () => ResolvedChildren

The children's helper is used for more complex interactions with props. When you're not just passing children to another component using props.children once in JSX, you should use children. Props are normally passed in via a getter for props.children in this manner:

const resolved = children(() => props.children)

The return value is a memo evaluating to the resolved children, which updates whenever the children change. Using this memo instead of accessing props.children directly has some important advantages in some scenarios. The underlying issue is that, when you specify component children via JSX, Solid automatically defines props.children as a property getter, so that the children are created (in particular, DOM is created) whenever props.children gets accessed.

Two particular consequences:

1. If you access props.children multiple times, the children (and associated DOM) get created multiple times. This is useful if you want the DOM to be duplicated (as DOM nodes can appear in only one parent element), but in many cases it creates redundant DOM nodes. If you instead call resolved() multiple times, you re-use the same children.

2. If you access props.children outside of a tracking scope (e.g., in an event handler), then you create children that will never be cleaned up. If you instead call resolved(), you re-use the already resolved children. You also guarantee that the children are tracked in the current component, as opposed to another tracking scope such as another component.

In addition, the children helper "resolves" children by calling argumentless functions and flattening arrays of arrays into an array. For example, a child specified with JSX like {signal() * 2} gets wrapped into a getter function () => count() * 2 in props.children, but gets evaluated to an actual number in resolved, properly depending on a count signal.

If the given props.children is not an array (which occurs when the JSX tag has a single child), then the children helper will not normalize it into an array. This is useful behavior e.g. when the intention is to pass a single function as a child, which can be detected via typeof resolved() === 'function'. If you want to normalize to an array, the returned memo has a toArray method (new in 1.5).

In most cases, you don't need (and in some cases, don't want) to use the children helper if you're just passing props.children on to another component or element via JSX:

const Wrapper = (props) => {
  return <div>{props.children}</div>
}

An important aspect of the children helper is that it forces the children to be created and resolved, as it accesses props.children immediately. This can be undesirable for conditional rendering, e.g., when using the children within a <Show> component. For example, the following code always evaluates the children:

const resolved = children(() => props.children)

return <Show when={visible()}>{resolved()}</Show>

To evaluate the children only when <Show> would render them, you can push the call to children inside a component or a function within <Show>, which only evaluates its children when the when condition is true. Another nice workaround is to pass props.children to the children helper only when you actually want to evaluate the children:

const resolved = children(() => visible() && props.children)