Implement support for declare on class fields with Flow

packages/babel-generator/test/fixtures/flow/declare-field/input.js

@@ -0,0 +1,4 @@
+class A {
+  declare x
+  declare static y
+}

packages/babel-generator/test/fixtures/flow/declare-field/options.json

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

packages/babel-generator/test/fixtures/flow/declare-field/output.js

@@ -0,0 +1,4 @@
+class A {
+  declare x;
+  declare static y;
+}

packages/babel-parser/src/parser/statement.js

@@ -1217,50 +1217,59 @@ export default class StatementParser extends ExpressionParser {
     return this.finishNode(classBody, "ClassBody");
   }
 
-  parseClassMember(
+  // returns true if the current identifier is a method/field name,
+  // false if it is a modifier
+  parseClassMemberFromModifier(
     classBody: N.ClassBody,
     member: N.ClassMember,
-    state: { hadConstructor: boolean },
-    constructorAllowsSuper: boolean,
-  ): void {
-    let isStatic = false;
+  ): boolean {
     const containsEsc = this.state.containsEsc;
+    const key = this.parseIdentifier(true); // eats the modifier
 
-    if (this.match(tt.name) && this.state.value === "static") {
-      const key = this.parseIdentifier(true); // eats 'static'
+    if (this.isClassMethod()) {
+      const method: N.ClassMethod = (member: any);
 
-      if (this.isClassMethod()) {
-        const method: N.ClassMethod = (member: any);
+      // a method named like the modifier
+      method.kind = "method";
+      method.computed = false;
+      method.key = key;
+      method.static = false;
+      this.pushClassMethod(
+        classBody,
+        method,
+        false,
+        false,
+        /* isConstructor */ false,
+        false,
+      );
+      return true;
+    } else if (this.isClassProperty()) {
+      const prop: N.ClassProperty = (member: any);
 
-        // a method named 'static'
-        method.kind = "method";
-        method.computed = false;
-        method.key = key;
-        method.static = false;
-        this.pushClassMethod(
-          classBody,
-          method,
-          false,
-          false,
-          /* isConstructor */ false,
-          false,
-        );
-        return;
-      } else if (this.isClassProperty()) {
-        const prop: N.ClassProperty = (member: any);
-
-        // a property named 'static'
-        prop.computed = false;
-        prop.key = key;
-        prop.static = false;
-        classBody.body.push(this.parseClassProperty(prop));
-        return;
-      } else if (containsEsc) {
-        throw this.unexpected();
-      }
+      // a property named like the modifier
+      prop.computed = false;
+      prop.key = key;
+      prop.static = false;
+      classBody.body.push(this.parseClassProperty(prop));
+      return true;
+    } else if (containsEsc) {
+      throw this.unexpected();
+    }
 
-      // otherwise something static
-      isStatic = true;
+    return false;
+  }
+
+  parseClassMember(
+    classBody: N.ClassBody,
+    member: N.ClassMember,
+    state: { hadConstructor: boolean },
+    constructorAllowsSuper: boolean,
+  ): void {
+    const isStatic = this.isContextual("static");
+
+    if (isStatic && this.parseClassMemberFromModifier(classBody, member)) {
+      // a class element named 'static'
+      return;
     }
 
     this.parseClassMemberWithIsStatic(

packages/babel-parser/src/plugins/flow.js

@@ -51,6 +51,10 @@ const FlowErrors = Object.freeze({
   AmbiguousDeclareModuleKind:
     "Found both `declare module.exports` and `declare export` in the same module. Modules can only have 1 since they are either an ES module or they are a CommonJS module",
   AssignReservedType: "Cannot overwrite reserved type %0",
+  DeclareClassElement:
+    "The `declare` modifier can only appear on class fields.",
+  DeclareClassFieldInitializer:
+    "Initializers are not allowed in fields with the `declare` modifier.",
   DuplicateDeclareModuleExports: "Duplicate `declare module.exports` statement",
   EnumBooleanMemberNotInitialized:
     "Boolean enum members need to be initialized. Use either `%0 = true,` or `%0 = false,` in enum `%1`.",
@@ -2082,6 +2086,39 @@ export default (superClass: Class<Parser>): Class<Parser> =>
       }
     }
 
+    parseClassMember(
+      classBody: N.ClassBody,
+      member: any,
+      state: { hadConstructor: boolean },
+      constructorAllowsSuper: boolean,
+    ): void {
+      const pos = this.state.start;
+      if (this.isContextual("declare")) {
+        if (this.parseClassMemberFromModifier(classBody, member)) {
+          // 'declare' is a class element name
+          return;
+        }
+
+        member.declare = true;
+      }
+
+      super.parseClassMember(classBody, member, state, constructorAllowsSuper);
+
+      if (member.declare) {
+        if (
+          member.type !== "ClassProperty" &&
+          member.type !== "ClassPrivateProperty"
+        ) {
+          this.raise(pos, FlowErrors.DeclareClassElement);
+        } else if (member.value) {
+          this.raise(
+            member.value.start,
+            FlowErrors.DeclareClassFieldInitializer,
+          );
+        }
+      }
+    }
+
     // ensure that inside flow types, we bypass the jsx parser plugin
     getTokenFromCode(code: number): void {
       const next = this.input.charCodeAt(this.state.pos + 1);

packages/babel-parser/test/fixtures/flow/class-private-property/declare-field-initializer/input.js

@@ -0,0 +1,3 @@
+class A {
+  declare #foo = 2
+}

packages/babel-parser/test/fixtures/flow/class-private-property/declare-field-initializer/output.json

@@ -0,0 +1,158 @@
+{
+  "type": "File",
+  "start": 0,
+  "end": 30,
+  "loc": {
+    "start": {
+      "line": 1,
+      "column": 0
+    },
+    "end": {
+      "line": 3,
+      "column": 1
+    }
+  },
+  "errors": [
+    "SyntaxError: Initializers are not allowed in fields with the `declare` modifier. (2:17)"
+  ],
+  "program": {
+    "type": "Program",
+    "start": 0,
+    "end": 30,
+    "loc": {
+      "start": {
+        "line": 1,
+        "column": 0
+      },
+      "end": {
+        "line": 3,
+        "column": 1
+      }
+    },
+    "sourceType": "module",
+    "interpreter": null,
+    "body": [
+      {
+        "type": "ClassDeclaration",
+        "start": 0,
+        "end": 30,
+        "loc": {
+          "start": {
+            "line": 1,
+            "column": 0
+          },
+          "end": {
+            "line": 3,
+            "column": 1
+          }
+        },
+        "id": {
+          "type": "Identifier",
+          "start": 6,
+          "end": 7,
+          "loc": {
+            "start": {
+              "line": 1,
+              "column": 6
+            },
+            "end": {
+              "line": 1,
+              "column": 7
+            },
+            "identifierName": "A"
+          },
+          "name": "A"
+        },
+        "superClass": null,
+        "body": {
+          "type": "ClassBody",
+          "start": 8,
+          "end": 30,
+          "loc": {
+            "start": {
+              "line": 1,
+              "column": 8
+            },
+            "end": {
+              "line": 3,
+              "column": 1
+            }
+          },
+          "body": [
+            {
+              "type": "ClassPrivateProperty",
+              "start": 12,
+              "end": 28,
+              "loc": {
+                "start": {
+                  "line": 2,
+                  "column": 2
+                },
+                "end": {
+                  "line": 2,
+                  "column": 18
+                }
+              },
+              "declare": true,
+              "static": false,
+              "key": {
+                "type": "PrivateName",
+                "start": 20,
+                "end": 24,
+                "loc": {
+                  "start": {
+                    "line": 2,
+                    "column": 10
+                  },
+                  "end": {
+                    "line": 2,
+                    "column": 14
+                  }
+                },
+                "id": {
+                  "type": "Identifier",
+                  "start": 21,
+                  "end": 24,
+                  "loc": {
+                    "start": {
+                      "line": 2,
+                      "column": 11
+                    },
+                    "end": {
+                      "line": 2,
+                      "column": 14
+                    },
+                    "identifierName": "foo"
+                  },
+                  "name": "foo"
+                }
+              },
+              "variance": null,
+              "value": {
+                "type": "NumericLiteral",
+                "start": 27,
+                "end": 28,
+                "loc": {
+                  "start": {
+                    "line": 2,
+                    "column": 17
+                  },
+                  "end": {
+                    "line": 2,
+                    "column": 18
+                  }
+                },
+                "extra": {
+                  "rawValue": 2,
+                  "raw": "2"
+                },
+                "value": 2
+              }
+            }
+          ]
+        }
+      }
+    ],
+    "directives": []
+  }
+}

packages/babel-parser/test/fixtures/flow/class-private-property/declare-field/input.js

@@ -0,0 +1,3 @@
+class A {
+  declare #foo
+}