JS: add utility SyntacticConstants::isNullOrUndefined

javascript/ql/src/Declarations/DeadStoreOfLocal.ql

@@ -25,25 +25,6 @@ predicate deadStoreOfLocal(VarDef vd, PurelyLocalVariable v) {
   not exists (SsaExplicitDefinition ssa | ssa.defines(vd, v))
 }
 
-/**
- * Holds if `e` is an expression evaluating to `null` or `undefined`.
- *
- * This includes not only direct references to `null` and `undefined`, but
- * also `void` expressions and assignments of the form `x = rhs`, where `rhs`
- * is itself an expression evaluating to `null` or `undefined`.
- */
-predicate isNullOrUndef(Expr e) {
-  exists (Expr inner |
-    inner = e.stripParens() |
-    // `null` or `undefined`
-    inner instanceof NullLiteral or
-    inner.(VarAccess).getName() = "undefined" or
-    inner instanceof VoidExpr or
-    // recursive case to catch multi-assignments of the form `x = y = null`
-    isNullOrUndef(inner.(AssignExpr).getRhs())
-  )
-}
-
 /**
  * Holds if `e` is an expression that may be used as a default initial value,
  * such as `0` or `-1`, or an empty object or array literal.
@@ -77,7 +58,7 @@ where deadStoreOfLocal(dead, v) and
         not fd = outer.getBody().(BlockStmt).getAStmt()
       ) and
       // don't flag overwrites with `null` or `undefined`
-      not isNullOrUndef(dead.getSource()) and
+      not SyntacticConstants::isNullOrUndefined(dead.getSource()) and
       // don't flag default inits that are later overwritten
       not (isDefaultInit(dead.getSource()) and dead.isOverwritten(v)) and
       // don't flag assignments in externs

javascript/ql/src/semmle/javascript/Constants.qll

@@ -132,6 +132,27 @@ module SyntacticConstants {
 
   }
 
+  /**
+   * Holds if `c` evaluates to `undefined`.
+   */
+  predicate isUndefined(SyntacticConstant c) {
+    c.getUnderlyingValue() instanceof UndefinedConstant
+  }
+
+  /**
+   * Holds if `c` evaluates to `null`.
+   */
+  predicate isNull(SyntacticConstant c) {
+    c.getUnderlyingValue() instanceof NullConstant
+  }
+
+  /**
+   * Holds if `c` evaluates to `null` or `undefined`.
+   */
+  predicate isNullOrUndefined(SyntacticConstant c) {
+    isUndefined(c) or isNull(c)
+  }
+
 }
 
 /**

javascript/ql/test/library-tests/Constants/Constants.expected

@@ -58,7 +58,6 @@
 | tst.js:47:5:47:5 | 1 |
 | tst.js:48:1:48:7 | x.p = 1 |
 | tst.js:48:7:48:7 | 1 |
-| tst.js:49:1:49:6 | x += 1 |
 | tst.js:49:6:49:6 | 1 |
 | tst.ts:1:13:1:21 | <number>1 |
 | tst.ts:1:21:1:21 | 1 |