--- a/Makefile	Mon Oct 30 11:56:07 2017 +0800
+++ b/Makefile	Wed Nov 01 00:50:57 2017 +0800
@@ -1,13 +1,22 @@
+coffeefiles := $(wildcard coffee/*.coffee)
+jsfiles := $(coffeefiles:coffee/%.coffee=js/%.js)
+mapfiles := $(jsfiles:.js=.js.map)
+npm := npm update && sed -i -e 's@^\#!/usr/bin/env node$$@&js@' ./node_modules/coffee-script/bin/coffee
+
+
+
 .PHONY: all
 all: build
 
-
 .PHONY: devel
 devel:
-	npm update
-	sed -i -e 's@^#!/usr/bin/env node$$@&js@' ./node_modules/coffee-script/bin/coffee
+	$(npm)
 
+node_modules/.bin/coffee:
+	$(npm)
+
+js/%.js js/%.js.map: coffee/%.coffee node_modules/.bin/coffee
+	./node_modules/.bin/coffee -o ./js/ -cm $<
 
 .PHONY: build
-build:
-	./node_modules/.bin/coffee -o ./js/ -cm ./coffee/
+build: $(jsfiles) $(mapfiles)