Add option to merge into an array using an index

README.md

@@ -178,7 +178,7 @@ Merge a value by a dot path.
 > The target value must be an object, array, null, or undefined.
 
  * If target is an object, Object.assign({}, target, param) is used.
- * If target an array, target.concat(param) is used.
+ * If target an array, target.concat(param) is used, unless an index is given in the opts param.
  * If target is null or undefined, the value is simply set.
 
 ```javascript
@@ -193,6 +193,10 @@ var arr = {foo: { bar: [1, 2] } };
 // merge array
 dotProp.merge(arr, 'foo.bar', [3, 4] );
 //=> {foo: { bar:[1, 2, 3, 4 ] }
+
+// merge array at index
+dotProp.merge(arr, 'foo.bar', [1.5, 1.6], {index: 1} );
+//=> {foo: { bar:[1, 1.5, 1.6, 2, 3, 4 ] }
 ```
 ## License
 

index.js

@@ -113,17 +113,29 @@ module.exports.toggle = function(obj, prop) {
 /**
  * Merges a value.  The target value must be an object, array, null, or undefined.
  * If target is an object, Object.assign({}, target, param) is used.
- * If target an array, target.concat(param) is used.
+ * If target an array, target.concat(param) is used, unless an index is given in the opts param.
  * If target is null or undefined, the value is simply set.
  * @param obj The object to evaluate.
  * @param prop The path to the value.
  * @param val The value to merge into the target value.
+ * @param opts Options for the merge:  
+ * <li> index: if merging into an array,
+ * insert at the specified index (if it is a valid index) instead of appending.
  */
-module.exports.merge = function(obj, prop, val) {
+module.exports.merge = function(obj, prop, val, opts={}) {
 	var curVal = this.get(obj, prop);
 	if (typeof curVal === 'object') {
 		if (Array.isArray(curVal)){
-			return this.set(obj, prop, curVal.concat(val));
+			if (typeof opts.index === 'number') {
+				if (opts.index >= 0 && opts.index <= curVal.length) {
+					const newList = curVal.slice(0, opts.index).concat(val).concat(curVal.slice(opts.index));
+					return this.set(obj, prop, newList);
+				} else{
+					return obj;
+				}
+			} else {
+				return this.set(obj, prop, curVal.concat(val));
+			}
 		} else if (curVal === null){
 			return this.set(obj, prop, val);
 		}

test/dot-prop-immutable-merge.spec.js

@@ -65,6 +65,76 @@ describe('dot-prop-immutable.merge.spec.js', function () {
 
 		});
 
+
+		describe('merge an array value into array with index', () => {
+
+			before(function () {
+				result = dotProp.merge(obj, 'c', [3, 4], {index: 1});
+			});
+
+			it('should merge prop', () => {
+				expect(result).to.eql({
+					a: 1,
+					b: {
+						x: 1,
+						y: 2
+					},
+					c: [1, 3, 4, 2],
+					d: null,
+					'b.x': 10
+				});
+			});
+
+			it('invariant', objInvariant);
+
+		});
+
+		describe('merge an array value into array with index at end', () => {
+
+			before(function () {
+				result = dotProp.merge(obj, 'c', [3, 4], {index: 2});
+			});
+
+			it('should merge prop', () => {
+				expect(result).to.eql({
+					a: 1,
+					b: {
+						x: 1,
+						y: 2
+					},
+					c: [1, 2, 3, 4],
+					d: null,
+					'b.x': 10
+				});
+			});
+
+			it('invariant', objInvariant);
+
+		});
+
+		describe('merge an array value into array with index out of bounds', () => {
+
+			before(function () {
+				result = dotProp.merge(obj, 'c', [3, 4], {index: 3});
+			});
+
+			it('should merge prop', () => {
+				expect(result).to.eql({
+					a: 1,
+					b: {
+						x: 1,
+						y: 2
+					},
+					c: [1, 2],
+					d: null,
+					'b.x': 10
+				});
+			});
+
+			it('invariant', objInvariant);
+
+		});
+
 		describe('merge an object value into null', () => {
 
 			before(function () {