Re-engineered FieldContainer to extend successfully array with its method;

Overridden splice, push and pop methods and length prop for FieldsContainer;
Renamed FieldsContainer to FieldsArray (also file);

src/fieldsArray.js

@@ -0,0 +1,72 @@
+const schema = require("./schema");
+const debug = require("debug")("passkit:fields");
+
+/**
+ * Class to represent lower-level keys pass fields
+ * @see https://apple.co/2wkUBdh
+ */
+
+const uniqueKeys = new Set();
+
+class FieldsArray extends Array {
+	constructor(...items) {
+		super(...items);
+	}
+
+	/**
+	 * Like `Array.prototype.push` but will alter
+	 * also uniqueKeys set.
+	 */
+
+	push(...fieldsData) {
+		let validFields = fieldsData.reduce((acc, current) => {
+			if (!(typeof current === "object") || !schema.isValid(current, "field")) {
+				return acc;
+			}
+
+			if (acc.some(e => e.key === current.key) || uniqueKeys.has(current.key)) {
+				debug(`UNIQUE field key CONSTRAINT VIOLATED. Fields keys must be unique in pass scope. Field key: "${current.key}"`);
+			} else {
+				uniqueKeys.add(current.key);
+				acc.push(current);
+			}
+
+			return acc;
+		}, []);
+
+		return Array.prototype.push.call(this, ...validFields);
+	}
+
+	/**
+	 * Like `Array.prototype.pop`, but will alter
+	 * also uniqueKeys set
+	 */
+
+	pop() {
+		const element = Array.prototype.pop.call(this);
+		uniqueKeys.delete(element.key);
+		return element;
+	}
+
+	/**
+	 * Like `Array.prototype.splice` but will alter
+	 * also uniqueKeys set
+	 */
+
+	splice(start, deleteCount, ...items) {
+		let removeList = this.slice(start, deleteCount+start);
+		removeList.forEach(item => uniqueKeys.delete(item.key));
+
+		return Array.prototype.splice.call(this, start, deleteCount, items);
+	}
+
+	get length() {
+		return this.length;
+	}
+
+	static emptyUnique() {
+		uniqueKeys.clear();
+	}
+}
+
+module.exports = FieldsArray;