Use strict TypeScript, enable TSLint, update all types to work in strict mode.

src/lib/util.js

@@ -1,19 +0,0 @@
-import { Component } from 'preact';
-
-export function updater(obj, property, value) {
-	return e => {
-		let update = {};
-		update[property] = typeof value === 'function' ? value(obj.state[property], e) : value;
-		obj.setState(update);
-	};
-}
-
-export const toggle = value => !value;
-
-export class When extends Component {
-	state = { ready: !!this.props.value };
-	render({ value, children: [child] }, { ready }) {
-		if (value && !ready) this.setState({ ready: true });
-		return ready ? (typeof child === 'function' ? child() : child) : null;
-	}
-}

src/lib/util.ts

@@ -0,0 +1,39 @@
+import { Component, ComponentProps } from 'preact';
+
+type WhenProps = ComponentProps<When> & {
+  value: boolean,
+  children?: (JSX.Element | (() => JSX.Element))[]
+};
+
+type WhenState = {
+  ready: boolean
+};
+
+export class When extends Component<WhenProps, WhenState> {
+  state: WhenState = {
+    ready: !!this.props.value
+  };
+
+  render({ value, children = [] }: WhenProps, { ready }: WhenState) {
+    let child = children[0];
+    if (value && !ready) this.setState({ ready: true });
+    return ready ? (typeof child === 'function' ? child() : child) : null;
+  }
+}
+
+/**
+ * A decorator that binds values to their class instance.
+ * @example
+ * class C {
+ *   @bind
+ *   foo () {
+ *     return this;
+ *   }
+ * }
+ * let f = new C().foo;
+ * f() instanceof C;    // true
+ */
+export function bind(target: any, propertyKey: string, descriptor: PropertyDescriptor) {
+  descriptor.value = descriptor.value.bind(target);
+  return descriptor;
+}