normalizeChildren: preallocate array length and perform key-consistency checks after normalization

This change preallocates the array to the input length and collapses multiple loops into a single pass. Assigning immediately after preallocation improves performance on V8 (generally neutral elsewhere).

Key checks are now performed on normalized vnodes, making the consistency validation more accurate and clarifying the correspondence between error messages and code.
Perf-sensitive comments have been clarified to reflect the original intent of commit 6c562d2.

Behavior is unchanged, except that the timing/order of related errors may differ slightly. All existing tests pass.
Additionally, bundle size is slightly reduced.

Author: kfule

render/vnode.js

@@ -10,24 +10,23 @@ Vnode.normalize = function(node) {
 	return Vnode("#", undefined, undefined, String(node), undefined, undefined)
 }
 Vnode.normalizeChildren = function(input) {
-	var children = []
-	if (input.length) {
-		var isKeyed = input[0] != null && input[0].key != null
-		// Note: this is a *very* perf-sensitive check.
-		// Fun fact: merging the loop like this is somehow faster than splitting
-		// it, noticeably so.
-		for (var i = 1; i < input.length; i++) {
-			if ((input[i] != null && input[i].key != null) !== isKeyed) {
-				throw new TypeError(
-					isKeyed && (input[i] == null || typeof input[i] === "boolean")
-						? "In fragments, vnodes must either all have keys or none have keys. You may wish to consider using an explicit keyed empty fragment, m.fragment({key: ...}), instead of a hole."
-						: "In fragments, vnodes must either all have keys or none have keys."
-				)
-			}
-		}
-		for (var i = 0; i < input.length; i++) {
-			children[i] = Vnode.normalize(input[i])
-		}
+	// Preallocate the array length (initially holey) and fill every index immediately in order.
+	// Benchmarking shows better performance on V8.
+	var children = new Array(input.length)
+	// Count the number of keyed normalized vnodes for consistency check.
+	// Note: this is a perf-sensitive check.
+	// Fun fact: merging the loop like this is somehow faster than splitting
+	// the check within updateNodes(), noticeably so.
+	var numKeyed = 0
+	for (var i = 0; i < input.length; i++) {
+		children[i] = Vnode.normalize(input[i])
+		if (children[i] != null && children[i].key != null) numKeyed++
+	}
+	if (numKeyed !== 0 && numKeyed !== input.length) {
+		throw new TypeError(children.includes(null)
+			? "In fragments, vnodes must either all have keys or none have keys. You may wish to consider using an explicit keyed empty fragment, m.fragment({key: ...}), instead of a hole."
+			: "In fragments, vnodes must either all have keys or none have keys."
+		)
 	}
 	return children
 }