Properly transform enum values, rewrite enum implementation

Fixes #620

The previous enum implementation was a bit tangled, handling several different
cases in one big `while` loop. This PR refactors the code into three distinct
cases: string enum members, calculated/constant enum members, and
autoincrementing enum members, and the transform details are handled separately
for each. Each case still needs to think about the case where we save a variable
and the case where we don't, but hopefully the whole thing is more
understandable now, and certainly better-documented.

This refactor makes it much easier to fix #620, where we weren't actually
passing the RHS expressions to the root transformer for their own transforms. In
particular, that meant that imported values weren't transformed correctly. To
fix, we now apply the enum transform in a purely left-to-right way rather than
needing to save and re-emit a region of code.

As part of the refactor, I also changed the emitted code so that when
autoincrementing a previous value, we always reference it rather than copying
the entire expression. This should reduce cases where we need to copy a large
range of code, which is generally questionable in Sucrase.

src/transformers/TypeScriptTransformer.ts

@@ -1,3 +1,4 @@
+import type {Token} from "../parser/tokenizer";
 import {TokenType as tt} from "../parser/tokenizer/types";
 import type TokenProcessor from "../TokenProcessor";
 import isIdentifier from "../util/isIdentifier";
@@ -69,88 +70,209 @@ export default class TypeScriptTransformer extends Transformer {
   }
 
   /**
-   * Rather than try to compute the actual enum values at compile time, we just create variables for
-   * each one and let everything evaluate at runtime. There's some additional complexity due to
-   * handling string literal names, including ones that happen to be valid identifiers.
+   * Transform an enum into equivalent JS. This has complexity in a few places:
+   * - TS allows string enums, numeric enums, and a mix of the two styles within an enum.
+   * - Enum keys are allowed to be referenced in later enum values.
+   * - Enum keys are allowed to be strings.
+   * - When enum values are omitted, they should follow an auto-increment behavior.
    */
   processEnumBody(enumName: string): void {
-    let isPreviousValidIdentifier = false;
-    let lastValueReference = null;
+    // Code that can be used to reference the previous enum member, or null if this is the first
+    // enum member.
+    let previousValueCode = null;
     while (true) {
       if (this.tokens.matches1(tt.braceR)) {
         break;
       }
-      const nameToken = this.tokens.currentToken();
-      let name;
-      let nameStringCode;
-      if (nameToken.type === tt.name) {
-        name = this.tokens.identifierNameForToken(nameToken);
-        nameStringCode = `"${name}"`;
-      } else if (nameToken.type === tt.string) {
-        name = this.tokens.stringValueForToken(nameToken);
-        nameStringCode = this.tokens.code.slice(nameToken.start, nameToken.end);
-      } else {
-        throw new Error("Expected name or string at beginning of enum element.");
-      }
-      const isValidIdentifier = isIdentifier(name);
+      const {nameStringCode, variableName} = this.extractEnumKeyInfo(this.tokens.currentToken());
       this.tokens.removeInitialToken();
 
-      let valueIsString;
-      let valueCode;
-
-      if (this.tokens.matches1(tt.eq)) {
-        const rhsEndIndex = this.tokens.currentToken().rhsEndIndex!;
-        if (rhsEndIndex == null) {
-          throw new Error("Expected rhsEndIndex on enum assign.");
-        }
-        this.tokens.removeToken();
-        if (
-          this.tokens.matches2(tt.string, tt.comma) ||
-          this.tokens.matches2(tt.string, tt.braceR)
-        ) {
-          valueIsString = true;
-        }
-        const startToken = this.tokens.currentToken();
-        while (this.tokens.currentIndex() < rhsEndIndex) {
-          this.tokens.removeToken();
-        }
-        valueCode = this.tokens.code.slice(
-          startToken.start,
-          this.tokens.tokenAtRelativeIndex(-1).end,
-        );
+      if (
+        this.tokens.matches3(tt.eq, tt.string, tt.comma) ||
+        this.tokens.matches3(tt.eq, tt.string, tt.braceR)
+      ) {
+        this.processStringLiteralEnumMember(enumName, nameStringCode, variableName);
+      } else if (this.tokens.matches1(tt.eq)) {
+        this.processExplicitValueEnumMember(enumName, nameStringCode, variableName);
       } else {
-        valueIsString = false;
-        if (lastValueReference != null) {
-          if (isPreviousValidIdentifier) {
-            valueCode = `${lastValueReference} + 1`;
-          } else {
-            valueCode = `(${lastValueReference}) + 1`;
-          }
-        } else {
-          valueCode = "0";
-        }
+        this.processImplicitValueEnumMember(
+          enumName,
+          nameStringCode,
+          variableName,
+          previousValueCode,
+        );
       }
       if (this.tokens.matches1(tt.comma)) {
         this.tokens.removeToken();
       }
 
-      let valueReference;
-      if (isValidIdentifier) {
-        this.tokens.appendCode(`const ${name} = ${valueCode}; `);
-        valueReference = name;
+      if (variableName != null) {
+        previousValueCode = variableName;
       } else {
-        valueReference = valueCode;
+        previousValueCode = `${enumName}[${nameStringCode}]`;
       }
+    }
+  }
 
-      if (valueIsString) {
-        this.tokens.appendCode(`${enumName}[${nameStringCode}] = ${valueReference};`);
-      } else {
-        this.tokens.appendCode(
-          `${enumName}[${enumName}[${nameStringCode}] = ${valueReference}] = ${nameStringCode};`,
-        );
+  /**
+   * Detect name information about this enum key, which will be used to determine which code to emit
+   * and whether we should declare a variable as part of this declaration.
+   *
+   * Some cases to keep in mind:
+   * - Enum keys can be implicitly referenced later, e.g. `X = 1, Y = X`. In Sucrase, we implement
+   *   this by declaring a variable `X` so that later expressions can use it.
+   * - In addition to the usual identifier key syntax, enum keys are allowed to be string literals,
+   *   e.g. `"hello world" = 3,`. Template literal syntax is NOT allowed.
+   * - Even if the enum key is defined as a string literal, it may still be referenced by identifier
+   *   later, e.g. `"X" = 1, Y = X`. That means that we need to detect whether or not a string
+   *   literal is identifier-like and emit a variable if so, even if the declaration did not use an
+   *   identifier.
+   * - Reserved keywords like `break` are valid enum keys, but are not valid to be referenced later
+   *   and would be a syntax error if we emitted a variable, so we need to skip the variable
+   *   declaration in those cases.
+   *
+   * The variableName return value captures these nuances: if non-null, we can and must emit a
+   * variable declaration, and if null, we can't and shouldn't.
+   */
+  extractEnumKeyInfo(nameToken: Token): {nameStringCode: string; variableName: string | null} {
+    if (nameToken.type === tt.name) {
+      const name = this.tokens.identifierNameForToken(nameToken);
+      return {
+        nameStringCode: `"${name}"`,
+        variableName: isIdentifier(name) ? name : null,
+      };
+    } else if (nameToken.type === tt.string) {
+      const name = this.tokens.stringValueForToken(nameToken);
+      return {
+        nameStringCode: this.tokens.code.slice(nameToken.start, nameToken.end),
+        variableName: isIdentifier(name) ? name : null,
+      };
+    } else {
+      throw new Error("Expected name or string at beginning of enum element.");
+    }
+  }
+
+  /**
+   * Handle an enum member where the RHS is just a string literal (not omitted, not a number, and
+   * not a complex expression). This is the typical form for TS string enums, and in this case, we
+   * do *not* create a reverse mapping.
+   *
+   * This is called after deleting the key token, when the token processor is at the equals sign.
+   *
+   * Example 1:
+   * someKey = "some value"
+   * ->
+   * const someKey = "some value"; MyEnum["someKey"] = someKey;
+   *
+   * Example 2:
+   * "some key" = "some value"
+   * ->
+   * MyEnum["some key"] = "some value";
+   */
+  processStringLiteralEnumMember(
+    enumName: string,
+    nameStringCode: string,
+    variableName: string | null,
+  ): void {
+    if (variableName != null) {
+      this.tokens.appendCode(`const ${variableName}`);
+      // =
+      this.tokens.copyToken();
+      // value string
+      this.tokens.copyToken();
+      this.tokens.appendCode(`; ${enumName}[${nameStringCode}] = ${variableName};`);
+    } else {
+      this.tokens.appendCode(`${enumName}[${nameStringCode}]`);
+      // =
+      this.tokens.copyToken();
+      // value string
+      this.tokens.copyToken();
+      this.tokens.appendCode(";");
+    }
+  }
+
+  /**
+   * Handle an enum member initialized with an expression on the right-hand side (other than a
+   * string literal). In these cases, we should transform the expression and emit code that sets up
+   * a reverse mapping.
+   *
+   * The TypeScript implementation of this operation distinguishes between expressions that can be
+   * "constant folded" at compile time (i.e. consist of number literals and simple math operations
+   * on those numbers) and ones that are dynamic. For constant expressions, it emits the resolved
+   * numeric value, and auto-incrementing is only allowed in that case. Evaluating expressions at
+   * compile time would add significant complexity to Sucrase, so Sucrase instead leaves the
+   * expression as-is, and will later emit something like `MyEnum["previousKey"] + 1` to implement
+   * auto-incrementing.
+   *
+   * This is called after deleting the key token, when the token processor is at the equals sign.
+   *
+   * Example 1:
+   * someKey = 1 + 1
+   * ->
+   * const someKey = 1 + 1; MyEnum[MyEnum["someKey"] = someKey] = "someKey";
+   *
+   * Example 2:
+   * "some key" = 1 + 1
+   * ->
+   * MyEnum[MyEnum["some key"] = 1 + 1] = "some key";
+   */
+  processExplicitValueEnumMember(
+    enumName: string,
+    nameStringCode: string,
+    variableName: string | null,
+  ): void {
+    const rhsEndIndex = this.tokens.currentToken().rhsEndIndex!;
+    if (rhsEndIndex == null) {
+      throw new Error("Expected rhsEndIndex on enum assign.");
+    }
+
+    if (variableName != null) {
+      this.tokens.appendCode(`const ${variableName}`);
+      this.tokens.copyToken();
+      while (this.tokens.currentIndex() < rhsEndIndex) {
+        this.rootTransformer.processToken();
+      }
+      this.tokens.appendCode(
+        `; ${enumName}[${enumName}[${nameStringCode}] = ${variableName}] = ${nameStringCode};`,
+      );
+    } else {
+      this.tokens.appendCode(`${enumName}[${enumName}[${nameStringCode}]`);
+      this.tokens.copyToken();
+      while (this.tokens.currentIndex() < rhsEndIndex) {
+        this.rootTransformer.processToken();
       }
-      lastValueReference = valueReference;
-      isPreviousValidIdentifier = isValidIdentifier;
+      this.tokens.appendCode(`] = ${nameStringCode};`);
+    }
+  }
+
+  /**
+   * Handle an enum member with no right-hand side expression. In this case, the value is the
+   * previous value plus 1, or 0 if there was no previous value. We should also always emit a
+   * reverse mapping.
+   *
+   * Example 1:
+   * someKey2
+   * ->
+   * const someKey2 = someKey1 + 1; MyEnum[MyEnum["someKey2"] = someKey2] = "someKey2";
+   *
+   * Example 2:
+   * "some key 2"
+   * ->
+   * MyEnum[MyEnum["some key 2"] = someKey1 + 1] = "some key 2";
+   */
+  processImplicitValueEnumMember(
+    enumName: string,
+    nameStringCode: string,
+    variableName: string | null,
+    previousValueCode: string | null,
+  ): void {
+    let valueCode = previousValueCode != null ? `${previousValueCode} + 1` : "0";
+    if (variableName != null) {
+      this.tokens.appendCode(`const ${variableName} = ${valueCode}; `);
+      valueCode = variableName;
     }
+    this.tokens.appendCode(
+      `${enumName}[${enumName}[${nameStringCode}] = ${valueCode}] = ${nameStringCode};`,
+    );
   }
 }

test/typescript-test.ts

@@ -571,21 +571,23 @@ describe("typescript transform", () => {
         "\\n",
         ",",
         "'",
+        "f f" = "g g"
       }
     `,
       `"use strict";
       var Foo; (function (Foo) {
         const A = 15.5; Foo[Foo["A"] = A] = "A";
         Foo[Foo["Hello world"] = A / 2] = "Hello world";
-        Foo[Foo[""] = (A / 2) + 1] = "";
+        Foo[Foo[""] = Foo["Hello world"] + 1] = "";
         const D = "foo".length; Foo[Foo["D"] = D] = "D";
         const E = D / D; Foo[Foo["E"] = E] = "E";
         Foo[Foo["debugger"] = 4] = "debugger";
         Foo[Foo["default"] = 7] = "default";
         Foo[Foo["!"] = E << E] = "!";
-        Foo[Foo["\\n"] = (E << E) + 1] = "\\n";
-        Foo[Foo[","] = ((E << E) + 1) + 1] = ",";
-        Foo[Foo["'"] = (((E << E) + 1) + 1) + 1] = "'";
+        Foo[Foo["\\n"] = Foo["!"] + 1] = "\\n";
+        Foo[Foo[","] = Foo["\\n"] + 1] = ",";
+        Foo[Foo["'"] = Foo[","] + 1] = "'";
+        Foo["f f"] = "g g";
       })(Foo || (Foo = {}));
     `,
     );
@@ -2425,4 +2427,25 @@ describe("typescript transform", () => {
     `,
     );
   });
+
+  it("correctly transforms enum expressions", () => {
+    assertTypeScriptResult(
+      `
+      import A from './A';
+      
+      enum E {
+        Foo = A.Foo,
+        Bar = A.Bar,
+      }
+    `,
+      `"use strict";${IMPORT_DEFAULT_PREFIX}
+      var _A = require('./A'); var _A2 = _interopRequireDefault(_A);
+      
+      var E; (function (E) {
+        const Foo = _A2.default.Foo; E[E["Foo"] = Foo] = "Foo";
+        const Bar = _A2.default.Bar; E[E["Bar"] = Bar] = "Bar";
+      })(E || (E = {}));
+    `,
+    );
+  });
 });