Moved source to src folder

src/fields.js

@@ -0,0 +1,71 @@
+const schema = require("./schema");
+
+/**
+ * Class to represent lower-level keys pass fields
+ * @see https://apple.co/2wkUBdh
+ */
+
+class FieldsContainer {
+	constructor() {
+		this._uniqueKeys = [];
+		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.filter(f => {
+			if (this._uniqueKeys.includes(f.key)) {
+				return false;
+			}
+
+			this._uniqueKeys.push(f.key);
+
+			return typeof f === "object" && schema.isValid(f, "field");
+		});
+
+		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();
+	}
+}
+
+module.exports = FieldsContainer;