JS(extractor): support nullish coalescing operators

Author: Esben Sparre Andreasen

change-notes/1.20/extractor-javascript.md

@@ -0,0 +1,21 @@
+[[ condition: enterprise-only ]]
+
+# Improvements to JavaScript analysis
+
+> NOTES
+>
+> Please describe your changes in terms that are suitable for
+> customers to read. These notes will have only minor tidying up
+> before they are published as part of the release notes.
+>
+> This file is written for lgtm users and should contain *only*
+> notes about changes that affect lgtm enterprise users. Add
+> any other customer-facing changes to the `studio-java.md`
+> file.
+>
+
+## General improvements
+
+## Changes to code extraction
+
+* The extractor now supports [Nullish Coalescing](https://github.com/tc39/proposal-nullish-coalescing) expressions.

javascript/extractor/src/com/semmle/jcorn/Parser.java

@@ -508,8 +508,12 @@ private Token readToken_dot() {
 	private Token readToken_question() { // '?'
 		int next = charAt(this.pos + 1);
 		int next2 = charAt(this.pos + 2);
-		if (this.options.esnext() && next == '.' && !('0' <= next2 && next2 <= '9')) // '?.', but not '?.X' where X is a digit
-			return this.finishOp(TokenType.questiondot, 2);
+		if (this.options.esnext()) {
+			if (next == '.' && !('0' <= next2 && next2 <= '9')) // '?.', but not '?.X' where X is a digit
+				return this.finishOp(TokenType.questiondot, 2);
+			if (next == '?') // '??'
+				return this.finishOp(TokenType.questionquestion, 2);
+		}
 		return this.finishOp(TokenType.question, 1);
 	}
 

javascript/extractor/src/com/semmle/jcorn/TokenType.java

@@ -123,6 +123,7 @@ public void updateContext(Parser parser, TokenType prevType) {
 		}
 	},
 	prefix = new TokenType(new Properties("prefix").beforeExpr().prefix().startsExpr()),
+	questionquestion = new TokenType(binop("??", 1)),
 	logicalOR = new TokenType(binop("||", 1)),
 	logicalAND = new TokenType(binop("&&", 2)),
 	bitwiseOR = new TokenType(binop("|", 3)),

javascript/extractor/src/com/semmle/js/extractor/CFGExtractor.java

@@ -331,9 +331,11 @@ public Node visit(MemberDefinition<?> nd, Void v) {
 			return nd.getKey().accept(this, v);
 		}
 
-		// for binary operators, the operands come first (but not for LogicalExpression, see above)
+		// for binary operators, the operands come first (but not for short-circuiting expressions), see above)
 		@Override
 		public Node visit(BinaryExpression nd, Void v) {
+			if ("??".equals(nd.getOperator()))
+				return nd;
 			return nd.getLeft().accept(this, v);
 		}
 
@@ -1583,8 +1585,16 @@ protected void visitAssign(INode assgn, INode lhs, Expression rhs) {
 
 		@Override
 		public Void visit(BinaryExpression nd, SuccessorInfo i) {
-			this.seq(nd.getLeft(), nd.getRight(), nd);
-			succ(nd, i.getGuardedSuccessors(nd));
+			if ("??".equals(nd.getOperator())) {
+				// the nullish coalescing operator is short-circuiting, but we do not add guards for it
+				succ(nd, First.of(nd.getLeft()));
+				Object leftSucc = union(First.of(nd.getRight()), i.getAllSuccessors());  // short-circuiting happens with both truthy and falsy values
+				visit(nd.getLeft(), leftSucc, null);
+				nd.getRight().accept(this, i);
+			} else {
+				this.seq(nd.getLeft(), nd.getRight(), nd);
+				succ(nd, i.getGuardedSuccessors(nd));
+			}
 			return null;
 		}
 

javascript/extractor/src/com/semmle/js/extractor/ExprKinds.java

@@ -72,6 +72,7 @@ public class ExprKinds {
 		binOpKinds.put("&=", 58);
 		binOpKinds.put("**", 87);
 		binOpKinds.put("**=", 88);
+		binOpKinds.put("??", 107);
 	}
 
 	private static final Map<String, Integer> unOpKinds = new LinkedHashMap<String, Integer>();

javascript/extractor/src/com/semmle/js/extractor/Main.java

@@ -41,7 +41,7 @@ public class Main {
 	 * such a way that it may produce different tuples for the same file under the same
 	 * {@link ExtractorConfig}.
 	 */
-	public static final String EXTRACTOR_VERSION = "2018-11-22_a";
+	public static final String EXTRACTOR_VERSION = "2018-11-23";
 
 	public static final Pattern NEWLINE = Pattern.compile("\n");
 

javascript/extractor/tests/esnext/input/nullish-coalescing.js

@@ -0,0 +1,7 @@
+x1 ?? y1;
+
+x2 || y2 ?? z2;
+x3 ?? y3 || z3;
+
+x4 && y4 ?? z4;
+x5 ?? y5 && z5;