Allow negative indexes in json operators (#7849)

* Add unit tests for negative indexes in json ops

* Allow negative indexes in JSON operators

Negative indexes will be treated as counting from the end, so -1 means
last item of the array, -2 means next-to-last item, and so on.

* Add documentation for negative indexes

core/modules/filters/json-ops.js

@@ -213,6 +213,18 @@ function getDataItemType(data,indexes) {
 	}
 }
 
+function getItemAtIndex(item,index) {
+	if($tw.utils.hop(item,index)) {
+		return item[index];
+	} else if($tw.utils.isArray(item)) {
+		index = $tw.utils.parseInt(index);
+		if(index < 0) { index = index + item.length };
+		return item[index]; // Will be undefined if index was out-of-bounds
+	} else {
+		return undefined;
+	}
+}
+
 /*
 Given a JSON data structure and an array of index strings, return the value at the end of the index chain, or "undefined" if any of the index strings are invalid
 */
@@ -225,7 +237,7 @@ function getDataItem(data,indexes) {
 	for(var i=0; i<indexes.length; i++) {
 		if(item !== undefined) {
 			if(item !== null && ["number","string","boolean"].indexOf(typeof item) === -1) {
-				item = item[indexes[i]];
+				item = getItemAtIndex(item,indexes[i]);
 			} else {
 				item = undefined;
 			}
@@ -249,16 +261,18 @@ function setDataItem(data,indexes,value) {
 	// Traverse the JSON data structure using the index chain
 	var current = data;
 	for(var i = 0; i < indexes.length - 1; i++) {
-		var index = indexes[i];
+		current = getItemAtIndex(current,indexes[i]);
-		if($tw.utils.hop(current,index)) {
+		if(current === undefined) {
-			current = current[index];
-		} else {
 			// Return the original JSON data structure if any of the index strings are invalid
 			return data;
 		}
 	}
 	// Add the value to the end of the index chain
 	var lastIndex = indexes[indexes.length - 1];
+	if($tw.utils.isArray(current)) {
+		lastIndex = $tw.utils.parseInt(lastIndex);
+		if(lastIndex < 0) { lastIndex = lastIndex + current.length };
+	}
 	current[lastIndex] = value;
 	return data;
 }

editions/test/tiddlers/tests/test-json-filters.js

@@ -53,6 +53,11 @@ describe("json filter tests", function() {
 		expect(wiki.filterTiddlers("[{First}jsonget[d],[f],[2]]")).toEqual(["true"]);
 		expect(wiki.filterTiddlers("[{First}jsonget[d],[f],[3]]")).toEqual(["false"]);
 		expect(wiki.filterTiddlers("[{First}jsonget[d],[f],[4]]")).toEqual(["null"]);
+		expect(wiki.filterTiddlers("[{First}jsonget[d],[f],[-5]]")).toEqual(["five"]);
+		expect(wiki.filterTiddlers("[{First}jsonget[d],[f],[-4]]")).toEqual(["six"]);
+		expect(wiki.filterTiddlers("[{First}jsonget[d],[f],[-3]]")).toEqual(["true"]);
+		expect(wiki.filterTiddlers("[{First}jsonget[d],[f],[-2]]")).toEqual(["false"]);
+		expect(wiki.filterTiddlers("[{First}jsonget[d],[f],[-1]]")).toEqual(["null"]);
 	});
 
 	it("should support the jsonextract operator", function() {
@@ -70,6 +75,11 @@ describe("json filter tests", function() {
 		expect(wiki.filterTiddlers("[{First}jsonextract[d],[f],[2]]")).toEqual(["true"]);
 		expect(wiki.filterTiddlers("[{First}jsonextract[d],[f],[3]]")).toEqual(["false"]);
 		expect(wiki.filterTiddlers("[{First}jsonextract[d],[f],[4]]")).toEqual(["null"]);
+		expect(wiki.filterTiddlers("[{First}jsonextract[d],[f],[-5]]")).toEqual(['"five"']);
+		expect(wiki.filterTiddlers("[{First}jsonextract[d],[f],[-4]]")).toEqual(['"six"']);
+		expect(wiki.filterTiddlers("[{First}jsonextract[d],[f],[-3]]")).toEqual(["true"]);
+		expect(wiki.filterTiddlers("[{First}jsonextract[d],[f],[-2]]")).toEqual(["false"]);
+		expect(wiki.filterTiddlers("[{First}jsonextract[d],[f],[-1]]")).toEqual(["null"]);
 	});
 
 	it("should support the jsonindexes operator", function() {
@@ -85,6 +95,11 @@ describe("json filter tests", function() {
 		expect(wiki.filterTiddlers("[{First}jsonindexes[d],[f],[2]]")).toEqual([]);
 		expect(wiki.filterTiddlers("[{First}jsonindexes[d],[f],[3]]")).toEqual([]);
 		expect(wiki.filterTiddlers("[{First}jsonindexes[d],[f],[4]]")).toEqual([]);
+		expect(wiki.filterTiddlers("[{First}jsonindexes[d],[f],[-5]]")).toEqual([]);
+		expect(wiki.filterTiddlers("[{First}jsonindexes[d],[f],[-4]]")).toEqual([]);
+		expect(wiki.filterTiddlers("[{First}jsonindexes[d],[f],[-3]]")).toEqual([]);
+		expect(wiki.filterTiddlers("[{First}jsonindexes[d],[f],[-2]]")).toEqual([]);
+		expect(wiki.filterTiddlers("[{First}jsonindexes[d],[f],[-1]]")).toEqual([]);
 	});
 
 	it("should support the jsontype operator", function() {
@@ -101,6 +116,11 @@ describe("json filter tests", function() {
 		expect(wiki.filterTiddlers("[{First}jsontype[d],[f],[2]]")).toEqual(["boolean"]);
 		expect(wiki.filterTiddlers("[{First}jsontype[d],[f],[3]]")).toEqual(["boolean"]);
 		expect(wiki.filterTiddlers("[{First}jsontype[d],[f],[4]]")).toEqual(["null"]);
+		expect(wiki.filterTiddlers("[{First}jsontype[d],[f],[-5]]")).toEqual(["string"]);
+		expect(wiki.filterTiddlers("[{First}jsontype[d],[f],[-4]]")).toEqual(["string"]);
+		expect(wiki.filterTiddlers("[{First}jsontype[d],[f],[-3]]")).toEqual(["boolean"]);
+		expect(wiki.filterTiddlers("[{First}jsontype[d],[f],[-2]]")).toEqual(["boolean"]);
+		expect(wiki.filterTiddlers("[{First}jsontype[d],[f],[-1]]")).toEqual(["null"]);
 	});
 
 	it("should support the jsonset operator", function() {
@@ -115,6 +135,7 @@ describe("json filter tests", function() {
 		expect(wiki.filterTiddlers("[{First}jsonset:null[id]]")).toEqual(['{"a":"one","b":"","c":1.618,"d":{"e":"four","f":["five","six",true,false,null]},"id":null}']);
 		expect(wiki.filterTiddlers("[{First}jsonset:array[d],[f],[5]]")).toEqual(['{"a":"one","b":"","c":1.618,"d":{"e":"four","f":["five","six",true,false,null,[]]}}']);
 		expect(wiki.filterTiddlers("[{First}jsonset:object[d],[f],[5]]")).toEqual(['{"a":"one","b":"","c":1.618,"d":{"e":"four","f":["five","six",true,false,null,{}]}}']);
+		expect(wiki.filterTiddlers("[{First}jsonset:number[d],[f],[-1],[42]]")).toEqual(['{"a":"one","b":"","c":1.618,"d":{"e":"four","f":["five","six",true,false,42]}}']);
 		expect(wiki.filterTiddlers("[{First}jsonset[missing],[id],[Antelope]]")).toEqual(['{"a":"one","b":"","c":1.618,"d":{"e":"four","f":["five","six",true,false,null]}}']);
 		expect(wiki.filterTiddlers("[{First}jsonset:json[\"Antelope\"]]")).toEqual(['"Antelope"']);
 		expect(wiki.filterTiddlers("[{First}jsonset:json[id],[{\"a\":313}]]")).toEqual(['{"a":"one","b":"","c":1.618,"d":{"e":"four","f":["five","six",true,false,null]},"id":{"a":313}}']);

editions/tw5.com/tiddlers/filters/jsonextract.tid

@@ -53,6 +53,14 @@ The <<.op jsonextract>> operator uses multiple operands to specify the indexes o
 [<jsondata>jsonextract[d],[g]] --> {"x":"max","y":"may","z":"maize"}
 ```
 
+<<.from-version "5.3.2">> Negative indexes into an array are counted from the end, so -1 means the last item, -2 the next-to-last item, and so on:
+
+```
+[<jsondata>jsonextract[d],[f],[-1]] --> null
+[<jsondata>jsonextract[d],[f],[-2]] --> false
+[<jsondata>jsonextract[d],[f],[-4]] --> "six"
+```
+
 Indexes can be dynamically composed from variables and transclusions:
 
 ```

editions/tw5.com/tiddlers/filters/jsonget.tid

@@ -51,6 +51,14 @@ The <<.op jsonget>> operator uses multiple operands to specify the indexes of th
 [<jsondata>jsonget[d],[f],[0]] --> "five"
 ```
 
+<<.from-version "5.3.2">> Negative indexes into an array are counted from the end, so -1 means the last item, -2 the next-to-last item, and so on:
+
+```
+[<jsondata>jsonget[d],[f],[-1]] --> null
+[<jsondata>jsonget[d],[f],[-2]] --> false
+[<jsondata>jsonget[d],[f],[-4]] --> "six"
+```
+
 Indexes can be dynamically composed from variables and transclusions:
 
 ```

editions/tw5.com/tiddlers/filters/jsonset.tid

@@ -51,6 +51,14 @@ The <<.op jsonset>> operator uses multiple operands to specify the indexes of th
 [<jsondata>jsonset[d],[f],[Panther]] --> {"a": "one","b": "","c": "three","d": "{"e": "four","f": "Panther","g": {"x": "max","y": "may","z": "maize"}}"}
 ```
 
+Negative indexes into an array are counted from the end, so -1 means the last item, -2 the next-to-last item, and so on:
+
+```
+[<jsondata>jsonset[d],[f],[-1],[Elephant]] --> {"a": "one","b": "","c": "three","d": "{"e": "four","f": ["five","six",true,false,"Elephant"],"g": {"x": "max","y": "may","z": "maize"}}"}
+[<jsondata>jsonset[d],[f],[-2],[Elephant]] --> {"a": "one","b": "","c": "three","d": "{"e": "four","f": ["five","six",true,"Elephant",null],"g": {"x": "max","y": "may","z": "maize"}}"}
+[<jsondata>jsonset[d],[f],[-4],[Elephant]] --> {"a": "one","b": "","c": "three","d": "{"e": "four","f": ["five","Elephant",true,false,null],"g": {"x": "max","y": "may","z": "maize"}}"}
+```
+
 Indexes can be dynamically composed from variables and transclusions:
 
 ```

editions/tw5.com/tiddlers/filters/jsontype.tid

@@ -61,6 +61,14 @@ The <<.op jsontype>> operator uses multiple operands to specify the indexes of t
 [<jsondata>jsontype[d],[f],[2]] --> "boolean"
 ```
 
+<<.from-version "5.3.2">> Negative indexes into an array are counted from the end, so -1 means the last item, -2 the next-to-last item, and so on:
+
+```
+[<jsondata>jsontype[d],[f],[-1]] --> "null"
+[<jsondata>jsontype[d],[f],[-2]] --> "boolean"
+[<jsondata>jsontype[d],[f],[-4]] --> "string"
+```
+
 Indexes can be dynamically composed from variables and transclusions:
 
 ```