Implement nullish coalescing operator in parser (#761)

* Implement nullish coalescing operator in parser

* Add ?? to AST

README.md

@@ -150,6 +150,7 @@ require("babylon").parse("code", {
 | `optionalCatchBinding` ([proposal](https://github.com/babel/proposals/issues/7)) | `try {throw 0;} catch{do();}` |
 | `throwExpressions` ([proposal](https://github.com/babel/proposals/issues/23)) | `() => throw new Error("")` |
 | `pipelineOperator` ([proposal](https://github.com/babel/proposals/issues/29)) | `a \|> b` |
+| `nullishCoalescingOperator` ([proposal](https://github.com/babel/proposals/issues/14)) | `a ?? b` |
 
 ### FAQ
 

ast/spec.md

@@ -791,6 +791,7 @@ enum BinaryOperator {
      | "|" | "^" | "&" | "in"
      | "instanceof"
      | "|>"
+     | "??"
 }
 ```
 

src/parser/expression.js

@@ -297,6 +297,10 @@ export default class ExpressionParser extends LValParser {
           this.state.potentialArrowAt = startPos;
         }
 
+        if (node.operator === "??") {
+          this.expectPlugin("nullishCoalescingOperator");
+        }
+
         node.right = this.parseExprOp(
           this.parseMaybeUnary(),
           startPos,

src/tokenizer/index.js

@@ -581,7 +581,10 @@ export default class Tokenizer extends LocationParser {
     // '?'
     const next = this.input.charCodeAt(this.state.pos + 1);
     const next2 = this.input.charCodeAt(this.state.pos + 2);
-    if (next === 46 && !(next2 >= 48 && next2 <= 57)) {
+    if (next === 63) {
+      // '??'
+      this.finishOp(tt.nullishCoalescing, 2);
+    } else if (next === 46 && !(next2 >= 48 && next2 <= 57)) {
       // '.' not followed by a number
       this.state.pos += 2;
       this.finishToken(tt.questionDot);

src/tokenizer/types.js

@@ -132,6 +132,7 @@ export const types: { [name: string]: TokenType } = {
   bang: new TokenType("!", { beforeExpr, prefix, startsExpr }),
   tilde: new TokenType("~", { beforeExpr, prefix, startsExpr }),
   pipeline: new BinopTokenType("|>", 0),
+  nullishCoalescing: new BinopTokenType("??", 1),
   logicalOR: new BinopTokenType("||", 1),
   logicalAND: new BinopTokenType("&&", 2),
   bitwiseOR: new BinopTokenType("|", 3),

test/fixtures/experimental/nullish-coalescing-operator/expression/actual.js

@@ -0,0 +1 @@
+foo ?? 1;

test/fixtures/experimental/nullish-coalescing-operator/expression/expected.json

@@ -0,0 +1,102 @@
+{
+  "type": "File",
+  "start": 0,
+  "end": 9,
+  "loc": {
+    "start": {
+      "line": 1,
+      "column": 0
+    },
+    "end": {
+      "line": 1,
+      "column": 9
+    }
+  },
+  "program": {
+    "type": "Program",
+    "start": 0,
+    "end": 9,
+    "loc": {
+      "start": {
+        "line": 1,
+        "column": 0
+      },
+      "end": {
+        "line": 1,
+        "column": 9
+      }
+    },
+    "sourceType": "script",
+    "body": [
+      {
+        "type": "ExpressionStatement",
+        "start": 0,
+        "end": 9,
+        "loc": {
+          "start": {
+            "line": 1,
+            "column": 0
+          },
+          "end": {
+            "line": 1,
+            "column": 9
+          }
+        },
+        "expression": {
+          "type": "BinaryExpression",
+          "start": 0,
+          "end": 8,
+          "loc": {
+            "start": {
+              "line": 1,
+              "column": 0
+            },
+            "end": {
+              "line": 1,
+              "column": 8
+            }
+          },
+          "left": {
+            "type": "Identifier",
+            "start": 0,
+            "end": 3,
+            "loc": {
+              "start": {
+                "line": 1,
+                "column": 0
+              },
+              "end": {
+                "line": 1,
+                "column": 3
+              },
+              "identifierName": "foo"
+            },
+            "name": "foo"
+          },
+          "operator": "??",
+          "right": {
+            "type": "NumericLiteral",
+            "start": 7,
+            "end": 8,
+            "loc": {
+              "start": {
+                "line": 1,
+                "column": 7
+              },
+              "end": {
+                "line": 1,
+                "column": 8
+              }
+            },
+            "extra": {
+              "rawValue": 1,
+              "raw": "1"
+            },
+            "value": 1
+          }
+        }
+      }
+    ],
+    "directives": []
+  }
+}

test/fixtures/experimental/nullish-coalescing-operator/expression/options.json

@@ -0,0 +1,3 @@
+{
+  "plugins": ["nullishCoalescingOperator"]
+}

test/fixtures/experimental/nullish-coalescing-operator/no-plugin-error/actual.js

@@ -0,0 +1 @@
+foo ?? 1;

test/fixtures/experimental/nullish-coalescing-operator/no-plugin-error/options.json

@@ -0,0 +1,3 @@
+{
+  "throws": "This experimental syntax requires enabling the parser plugin: 'nullishCoalescingOperator' (1:7)"
+}