diff --git a/src/fields.js b/src/fields.js
deleted file mode 100644
index 3639c6a..0000000
--- a/src/fields.js
+++ /dev/null
@@ -1,82 +0,0 @@
-const schema = require("./schema");
-const debug = require("debug")("passkit:fields");
-
-/**
- * Class to represent lower-level keys pass fields
- * @see https://apple.co/2wkUBdh
- */
-
-let uniqueKeys = [];
-
-class FieldsContainer {
-	constructor() {
-		this.fields = [];
-	}
-
-	/**
-	 * A wrapper of Array.prototype.push to validate the pushed content with the schema.
-	 * Accepts also one array of objects.
-	 *
-	 * @method push
-	 * @params {Object[]} fields - the fields to be checked and pushed
-	 * @params {schema.field} fields[].* - each key must be compliant with schema.field structure
-	 * @returns {Number} - the amount of pushed elements (for checks)
-	 */
-
-	push(...fieldsData) {
-		if (fieldsData[0] instanceof Array && fieldsData[0].length) {
-			fieldsData = fieldsData[0];
-		}
-
-		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.includes(current.key)) {
-				debug(`UNIQUE field key CONSTRAINT VIOLATED. Fields keys must be unique in pass scope. Field key: "${current.key}"`);
-				return acc;
-			}
-
-			acc.push(current);
-			return acc;
-		}, []);
-
-		uniqueKeys.push(...validFields.map(v => v.key));
-		this.fields.push(...validFields);
-
-		return validFields.length;
-	}
-
-	/**
-	 * A wrapper of Array.prototype.pop and Array.prototype.slice to pop
-	 * last element or n elements starting from the end.
-	 *
-	 * @method pop
-	 * @params {Number} [quantity=-1] - the amount of elements to be removed
-	 * @returns {Number} - the amount of removed elements
-	 */
-
-	pop(amount = -1) {
-		if (!this.fields.length) {
-			return undefined;
-		}
-
-		if (amount > -1) {
-			let removedElements = this.fields.slice(amount);
-			this.fields = this.fields.slice(0, this.fields.length - amount);
-			this._uniqueKeys = this._uniqueKeys.slice(0, this._uniqueKeys - amount);
-
-			return removedElements;
-		}
-
-		this._uniqueKeys.pop();
-		return this.fields.pop();
-	}
-
-	static emptyUnique() {
-		uniqueKeys = [];
-	}
-}
-
-module.exports = FieldsContainer;
diff --git a/src/fieldsArray.js b/src/fieldsArray.js
new file mode 100644
index 0000000..dfa2031
--- /dev/null
+++ b/src/fieldsArray.js
@@ -0,0 +1,72 @@
+const schema = require("./schema");
+const debug = require("debug")("passkit:fields");
+
+/**
+ * Class to represent lower-level keys pass fields
+ * @see https://apple.co/2wkUBdh
+ */
+
+const uniqueKeys = new Set();
+
+class FieldsArray extends Array {
+	constructor(...items) {
+		super(...items);
+	}
+
+	/**
+	 * Like `Array.prototype.push` but will alter
+	 * also uniqueKeys set.
+	 */
+
+	push(...fieldsData) {
+		let validFields = fieldsData.reduce((acc, current) => {
+			if (!(typeof current === "object") || !schema.isValid(current, "field")) {
+				return acc;
+			}
+
+			if (acc.some(e => e.key === current.key) || uniqueKeys.has(current.key)) {
+				debug(`UNIQUE field key CONSTRAINT VIOLATED. Fields keys must be unique in pass scope. Field key: "${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;
diff --git a/src/pass.js b/src/pass.js
index 7de2422..df6d684 100644
--- a/src/pass.js
+++ b/src/pass.js
@@ -14,7 +14,7 @@ const loadDebug = debug("passkit:load");
 
 const schema = require("./schema");
 const formatMessage = require("./messages");
-const FieldsContainer = require("./fields");
+const FieldsArray = require("./fieldsArray");
 
 const readdir = promisify(fs.readdir);
 const readFile = promisify(fs.readFile);
@@ -37,7 +37,7 @@ class Pass {
 
 		this._fields = ["primaryFields", "secondaryFields", "auxiliaryFields", "backFields", "headerFields"];
 
-		this._fields.forEach(a => this[a] = new FieldsContainer());
+		this._fields.forEach(a => this[a] = new FieldsArray());
 		this._transitType = "";
 
 		// Assigning model and _props to this
@@ -212,7 +212,7 @@ class Pass {
 
 				archive.pipe(passStream);
 
-				FieldsContainer.emptyUnique();
+				FieldsArray.emptyUnique();
 
 				return archive.finalize().then(() => passStream);
 			});
@@ -633,11 +633,11 @@ class Pass {
 		}
 
 		this._fields.forEach(area => {
-			if (this[area].fields.length) {
+			if (this[area].length) {
 				if (this.shouldOverwrite) {
-					passFile[this.type][area] = this[area].fields;
+					passFile[this.type][area] = [...this[area]];
 				} else {
-					passFile[this.type][area].push(...this[area].fields);
+					passFile[this.type][area].push(...this[area]);
 				}
 			}
 		});