Merge pull request #2203 from erik-krogh/ignorePureFunction

Approved by max-schaefer, mchammer01

javascript/ql/src/Statements/IgnoreArrayResult.qhelp

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+<!DOCTYPE qhelp PUBLIC
+  "-//Semmle//qhelp//EN"
+  "qhelp.dtd">
+<qhelp>
+<overview>
+<p>
+The <code>concat</code>, <code>join</code> and <code>slice</code> methods are
+pure and do not modify any of the inputs or the array the method is 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 calls to <code>concat</code>, <code>join</code> 
+or <code>slice</code>. 
+</p>
+
+</recommendation>
+<example>
+
+<p>
+A function <code>extend</code> is defined in the following example. The 
+function uses the <code>concat</code> method to add elements to the 
+<code>arr</code> array. However, the <code>extend</code> function has no 
+effect as the return value from <code>concat</code> is ignored:
+</p>
+
+<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/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

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+/**
+ * @name Ignoring result from pure array method
+ * @description Ignoring the result of an array method that does not modify its receiver is 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().getALocalSource()
+  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 and
+  not call.getReceiver().asExpr().(ArrayExpr).getSize() = 0
+select call, "Result from call to " + call.getMethodName() + " ignored."

javascript/ql/src/Statements/examples/IgnoreArrayResult.js

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+var arr = [1,2,3];
+
+function extend(others) {
+	arr.concat(others);
+}

javascript/ql/src/Statements/examples/IgnoreArrayResultFixed.js

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+var arr = [1,2,3];
+
+function extend(others) {
+	arr = arr.concat(others);
+}