JS: Add UselessRangeCheck.ql

javascript/ql/src/Statements/UselessRangeCheck.qhelp

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+<!DOCTYPE qhelp PUBLIC
+  "-//Semmle//qhelp//EN"
+  "qhelp.dtd">
+<qhelp>
+<overview>
+<p>
+If a condition always evaluates to true or always evaluates to false, this often indicates
+incomplete code or a latent bug and should be examined carefully.
+</p>
+
+</overview>
+<recommendation>
+
+<p>
+Examine the surrounding code to determine why the condition is useless. If it is no
+longer needed, remove it.
+</p>
+
+<p>
+If the check is needed to guard against NaN values, insert a comment explaning the possibility of NaN.
+</p>
+
+</recommendation>
+<example>
+
+<p>
+The following example finds the index of an element in a given slice of the array:
+</p>
+
+<sample src="examples/UselessConditional.js" />
+
+<p>
+The condition <tt>i &lt; end</tt> at the end is always false, however. The code can be clarified if the
+redundant condition is removed:
+</p>
+
+<sample src="examples/UselessConditionalGood.js" />
+
+</example>
+<references>
+
+
+<li>Mozilla Developer Network: <a href="https://developer.mozilla.org/en-US/docs/Glossary/Truthy">Truthy</a>,
+<a href="https://developer.mozilla.org/en-US/docs/Glossary/Falsy">Falsy</a>.</li>
+
+</references>
+</qhelp>

javascript/ql/src/Statements/UselessRangeCheck.ql

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+/**
+ * @name Useless range check
+ * @description If a range check always fails or always succeeds it is indicative of a bug.
+ * @kind problem
+ * @problem.severity warning
+ * @id js/useless-range-check
+ * @tags correctness
+ * @precision high
+ */
+
+import javascript
+
+/**
+ * Gets the guard node with the opposite outcome of `guard`.
+ */
+ConditionGuardNode getOppositeGuard(ConditionGuardNode guard) {
+  result.getTest() = guard.getTest() and
+  result.getOutcome() = guard.getOutcome().booleanNot()
+}
+
+from ConditionGuardNode guard
+where RangeAnalysis::isContradictoryGuardNode(guard)
+
+  // Do not report conditions that themselves are unreachable because of
+  // a prior contradiction.
+  and not RangeAnalysis::isContradictoryGuardNode(getOppositeGuard(guard))
+
+select guard.getTest(), "The condition '" + guard.getTest() + "' is always " + guard.getOutcome().booleanNot()

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

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+function findValue(values, x, start, end) {
+  let i;
+  for (i = start; i < end; ++i) {
+    if (values[i] === x) {
+        return i;
+    }
+  }
+  if (i < end) {
+    return i;
+  }
+  return -1;
+}

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

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+function findValue(values, x, start, end) {
+  for (let i = start; i < end; ++i) {
+    if (values[i] === x) {
+        return i;
+    }
+  }
+  return -1;
+}