Fix style updates to avoid unnecessary allocation (#2312)

* Fix style updates to avoid unnecessary allocation

- Drive-by: properly censor `cssFloat` -> css `float:` to match DOM
  behavior

* Document `style` syntax + custom property support. [skip ci]

* Add a missing test

* Add a few more descriptive comments.

* Update changelog [skip ci]

docs/change-log.md

@@ -72,7 +72,7 @@
 - docs: tweaks: ([#2104](https://github.com/MithrilJS/mithril.js/pull/2104) [@mikeyb](https://github.com/mikeyb), [#2205](https://github.com/MithrilJS/mithril.js/pull/2205), [@cavemansspa](https://github.com/cavemansspa), [#2250](https://github.com/MithrilJS/mithril.js/pull/2250) [@isiahmeadows](https://github.com/isiahmeadows), [#2265](https://github.com/MithrilJS/mithril.js/pull/2265), [@isiahmeadows](https://github.com/isiahmeadows))
 - render/core: avoid touching `Object.prototype.__proto__` setter with `key: "__proto__"` in certain situations ([#2251](https://github.com/MithrilJS/mithril.js/pull/2251))
 - render/core: Vnodes stored in the dom node supplied to `m.render()` are now normalized [#2266](https://github.com/MithrilJS/mithril.js/pull/2266)
-- render/core: CSS vars can now be specified in `{style}` attributes [#2192](https://github.com/MithrilJS/mithril.js/pull/2192)
+- render/core: CSS vars can now be specified in `{style}` attributes ([#2192](https://github.com/MithrilJS/mithril.js/pull/2192) [@barneycarroll](https://github.com/barneycarroll)), ([#2311](https://github.com/MithrilJS/mithril.js/pull/2311) [@porsager](https://github.com/porsager)), ([#2312](https://github.com/MithrilJS/mithril.js/pull/2312) [@isiahmeadows](https://github.com/isiahmeadows))
 - request: don't modify params, call `extract`/`serialize`/`deserialize` with correct `this` value ([#2288](https://github.com/MithrilJS/mithril.js/pull/2288))
 
 ---

docs/hyperscript.md

@@ -262,6 +262,8 @@ m("div[style=background:red]")
 
 Using a string as a `style` would overwrite all inline styles in the element if it is redrawn, and not only CSS rules whose values have changed.
 
+You can use both hyphenated CSS property names (like `background-color`) and camel cased DOM `style` property names (like `backgroundColor`). You can also define [CSS custom properties](https://developer.mozilla.org/en-US/docs/Web/CSS/Using_CSS_variables), if your browser supports them.
+
 Mithril does not attempt to add units to number values. It simply stringifies them.
 
 ---

render/render.js

@@ -775,38 +775,45 @@ module.exports = function($window) {
 		) && key in vnode.dom
 	}
 
-	var matchUpperCase = /[A-Z]/g
-	function prependDashAndLowerCase(string){
-		return "-" + string.toLowerCase()
-	}
-
-	function normalizeProp(prop) {
-		return "-" && prop[1] === "-"
-			? prop
-			: prop.replace(matchUpperCase, prependDashAndLowerCase)
-	}
-
 	//style
+	var uppercaseRegex = /[A-Z]/g
+	function toLowerCase(capital) { return "-" + capital.toLowerCase() }
+	function normalizeKey(key) {
+		return key[0] === "-" && key[1] === "-" ? key :
+			key === "cssFloat" ? "float" :
+				key.replace(uppercaseRegex, toLowerCase)
+	}
 	function updateStyle(element, old, style) {
-		if (old != null && style != null && typeof old === "object" && typeof style === "object" && style !== old) {
+		if (old === style) {
+			// Styles are equivalent, do nothing.
+		} else if (style == null) {
+			// New style is missing, just clear it.
+			element.style.cssText = ""
+		} else if (typeof style !== "object") {
+			// New style is a string, let engine deal with patching.
+			element.style.cssText = style
+		} else if (old == null || typeof old !== "object") {
+			// `old` is missing or a string, `style` is an object.
+			element.style.cssText = ""
+			// Add new style properties
+			for (var key in style) {
+				var value = style[key]
+				if (value != null) element.style.setProperty(normalizeKey(key), String(value))
+			}
+		} else {
 			// Both old & new are (different) objects.
 			// Update style properties that have changed
 			for (var key in style) {
-				if (style[key] !== old[key]) element.style.setProperty(normalizeProp(key), style[key])
+				var value = style[key]
+				if (value != null && (value = String(value)) !== String(old[key])) {
+					element.style.setProperty(normalizeKey(key), value)
+				}
 			}
 			// Remove style properties that no longer exist
 			for (var key in old) {
-				if (!(key in style)) element.style.removeProperty(normalizeProp(key))
-			}
-			return
-		}
-		if (old === style) element.style.cssText = "", old = null
-		if (style == null) element.style.cssText = ""
-		else if (typeof style === "string") element.style.cssText = style
-		else {
-			if (typeof old === "string") element.style.cssText = ""
-			for (var key in style) {
-				element.style.setProperty(normalizeProp(key), style[key])
+				if (old[key] != null && style[key] == null) {
+					element.style.removeProperty(normalizeKey(key))
+				}
 			}
 		}
 	}

render/tests/test-createElement.js

@@ -45,6 +45,13 @@ o.spec("createElement", function() {
 
 		o(vnode.dom.style["--cssVar"]).equals("red")
 	})
+	o("censors cssFloat to float", function() {
+		var vnode = {tag: "a", attrs: {style: {cssFloat: "left"}}}
+
+		render(root, [vnode])
+
+		o(vnode.dom.style.float).equals("left")
+	})
 	o("creates children", function() {
 		var vnode = {tag: "div", children: [{tag: "a"}, {tag: "b"}]}
 		render(root, [vnode])