introduce backtracking, and also marking join/slice calls

javascript/ql/src/Statements/IgnoreArrayResult.qhelp

@@ -4,16 +4,18 @@
 <qhelp>
 <overview>
 <p>
-The <code>concat</code> method on is pure and does not modify any of the input 
-arrays. It is therefore generally an error to ignore the return value from a 
-call to  <code>concat</code>. 
+The <code>concat</code>, <code>join</code> and <code>slice</code> methods are
+pure and does not modify any of the inputs or the array the method was called 
+on. It is therefore generally an error to ignore the return value from a call
+to one of these methods. 
 </p>
 
 </overview>
 <recommendation>
 
 <p>
-Use the returned value from the call to <code>concat</code>. 
+Use the returned value from the calls to <code>concat</code>, <code>join</code> 
+or <code>slice</code>. 
 </p>
 
 </recommendation>
@@ -26,19 +28,21 @@ function uses the <code>concat</code> method to add elements to the
 effect as the return value from <code>concat</code> is ignored.
 </p>
 
-<sample src="examples/IgnoreConcat.js" />
+<sample src="examples/IgnoreArrayResult.js" />
 
 <p>
 Assigning the returned value from the call to <code>concat</code> to the 
 <code>arr</code> variable fixes the error.
 </p>
 
-<sample src="examples/IgnoreConcatFixed.js" />
+<sample src="examples/IgnoreArrayResultFixed.js" />
 
 </example>
 <references>
 
 <li>Mozilla Developer Network: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/concat">Array concat</a>.</li>
+<li>Mozilla Developer Network: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/slice">Array slice</a>.</li>
+<li>Mozilla Developer Network: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/join">Array join</a>.</li>
 
 </references>
 </qhelp>

javascript/ql/src/Statements/IgnoreArrayResult.ql

@@ -0,0 +1,45 @@
+/**
+ * @name Ignoring result from pure array method
+ * @description The array methods do not modify the array, ignoring the result of such a call is therefore generally an error.
+ * @kind problem
+ * @problem.severity warning
+ * @id js/ignore-array-result
+ * @tags maintainability,
+ *       correctness
+ * @precision high
+ */
+
+import javascript
+import Expressions.ExprHasNoEffect
+
+DataFlow::SourceNode callsArray(DataFlow::TypeBackTracker t, DataFlow::MethodCallNode call) {
+  isIgnoredPureArrayCall(call) and
+  (
+    t.start() and
+    result = call.getReceiver()
+    or
+    exists(DataFlow::TypeBackTracker t2 | result = callsArray(t2, call).backtrack(t2, t))
+  )
+}
+
+DataFlow::SourceNode callsArray(DataFlow::MethodCallNode call) {
+  result = callsArray(DataFlow::TypeBackTracker::end(), call)
+}
+
+predicate isIgnoredPureArrayCall(DataFlow::MethodCallNode call) {
+  inVoidContext(call.asExpr()) and
+  (
+    call.getMethodName() = "concat" and
+    call.getNumArgument() = 1
+    or
+    call.getMethodName() = "join" and
+    call.getNumArgument() < 2
+    or
+    call.getMethodName() = "slice" and
+    call.getNumArgument() < 3
+  )
+}
+
+from DataFlow::MethodCallNode call
+where callsArray(call) instanceof DataFlow::ArrayCreationNode
+select call, "Result from call to " + call.getMethodName() + " ignored."

javascript/ql/src/Statements/IgnoreConcatReturn.ql

@@ -1,42 +0,0 @@
-/**
- * @name Ignoring return from concat
- * @description The concat method does not modify an array, ignoring the result of a call to concat is therefore generally an error.
- * @kind problem
- * @problem.severity warning
- * @id js/ignore-result-from-concat
- * @tags maintainability,
- *       correctness
- * @precision high
- */
-
-import javascript
-import Expressions.ExprHasNoEffect
-
-DataFlow::SourceNode array(DataFlow::TypeTracker t) {
-  t.start() and
-  result instanceof DataFlow::ArrayCreationNode
-  or
-  exists (DataFlow::TypeTracker t2 |
-    result = array(t2).track(t2, t)
-  )
-}
-
-DataFlow::SourceNode array() { result = array(DataFlow::TypeTracker::end()) }
-
-predicate isArrayMethod(DataFlow::MethodCallNode call) {
-  call.getReceiver().getALocalSource() = array()
-}
-
-predicate isIncomplete(DataFlow::Node node) {
-  any(DataFlow::Incompleteness cause | node.analyze().getAValue().isIndefinite(cause)) != "global"
-}
-
-from DataFlow::CallNode call
-where 
-  isArrayMethod(call) and
-  call.getCalleeName() = "concat" and
-  call.getNumArgument() = 1 and 
-  (call.getArgument(0).getALocalSource() = array() or isIncomplete(call.getArgument(0))) and 
-  not call.getArgument(0).asExpr().(ArrayExpr).getSize() = 0 and 
-  inVoidContext(call.asExpr())
-select call, "Return value from call to concat ignored."