add js/hexColor.js file

js/hexColor.js

@@ -0,0 +1,32 @@
+/* 用于填充字符串 `str` 到指定长度 `len`，默认为 2
+例如：padZero('F', 2) => '0F' */
+function padZero(str, len) {
+	len = len || 2;
+	const zeros = new Array(len).join("0");
+	return (zeros + str).slice(-len);
+}
+function invertColor(hex, bw) {
+	if (hex.indexOf("#") === 0) {
+		hex = hex.slice(1);
+	}
+	// convert 3-digit hex to 6-digits.
+	if (hex.length === 3) {
+		hex = hex[0] + hex[0] + hex[1] + hex[1] + hex[2] + hex[2];
+	}
+	if (hex.length !== 6) {
+		throw new Error("Invalid HEX color.");
+	}
+	let r = parseInt(hex.slice(0, 2), 16),
+		g = parseInt(hex.slice(2, 4), 16),
+		b = parseInt(hex.slice(4, 6), 16);
+	if (bw) {
+		// https://stackoverflow.com/a/3943023/112731
+		return r * 0.299 + g * 0.587 + b * 0.114 > 186 ? "#000000" : "#FFFFFF";
+	}
+	// invert color components
+	r = (255 - r).toString(16);
+	g = (255 - g).toString(16);
+	b = (255 - b).toString(16);
+	// pad each with zeros and return
+	return "#" + padZero(r) + padZero(g) + padZero(b);
+}