Better validation and test coverage for background colours

lib/colour.js

@@ -83,18 +83,22 @@ function toColorspace (colorspace) {
  * Update a colour attribute of the this.options Object.
  * @private
  * @param {String} key
- * @param {String|Object} val
- * @throws {Error} Invalid key
+ * @param {String|Object} value
+ * @throws {Error} Invalid value
  */
-function _setColourOption (key, val) {
-  if (is.object(val) || is.string(val)) {
-    const colour = color(val);
-    this.options[key] = [
-      colour.red(),
-      colour.green(),
-      colour.blue(),
-      Math.round(colour.alpha() * 255)
-    ];
+function _setBackgroundColourOption (key, value) {
+  if (is.defined(value)) {
+    if (is.object(value) || is.string(value)) {
+      const colour = color(value);
+      this.options[key] = [
+        colour.red(),
+        colour.green(),
+        colour.blue(),
+        Math.round(colour.alpha() * 255)
+      ];
+    } else {
+      throw is.invalidParameterError('background', 'object or string', value);
+    }
   }
 }
 
@@ -111,7 +115,7 @@ module.exports = function (Sharp) {
     toColourspace,
     toColorspace,
     // Private
-    _setColourOption
+    _setBackgroundColourOption
   });
   // Class attributes
   Sharp.colourspace = colourspace;