Fixes peggyjs#493. Allows null, undefined, or empty array as allowedS…

…tartRules, all of which mean the default (first) rule

CHANGELOG.md

@@ -12,13 +12,15 @@ Unreleased
 - [#509](https://github.com/peggyjs/peggy/pull/509) Add and implement ES6 export button
 
 ### Bug fixes
+- [#493](https://github.com/peggyjs/peggy/issues/493) Allow use of an empty
+  array, null, or undefined as allowedStartRules option
 - [#507](https://github.com/peggyjs/peggy/pull/507) Remove stray semicolon in CSS
 - [#508](https://github.com/peggyjs/peggy/pull/508) Fix broken text input
-- [#512](https://github.com/peggyjs/peggy/pull/512) Add "StartRules" to peg.d.ts.
-- [#513](https://github.com/peggyjs/peggy/pull/513) Allow whitespace between
+- [#512](https://github.com/peggyjs/peggy/issues/512) Add "StartRules" to peg.d.ts.
+- [#513](https://github.com/peggyjs/peggy/issues/513) Allow whitespace between
   plucked word and its pattern.
-- [#520](https://github.com/peggyjs/peggy/pull/520) Grammar with token "constructor" fails to generate
-- [#522](https://github.com/peggyjs/peggy/pull/522) Switched from puppeteer
+- [#520](https://github.com/peggyjs/peggy/issues/520) Grammar with token "constructor" fails to generate
+- [#522](https://github.com/peggyjs/peggy/issues/522) Switched from puppeteer
   to playwright for web tests, and added them to CI.
 
 ### Documentation

lib/compiler/index.js

@@ -83,8 +83,9 @@ const compiler = {
   compile(ast, passes, options) {
     options = options !== undefined ? options : {};
 
+    const defaultStartRules = [ast.rules[0].name];
     options = processOptions(options, {
-      allowedStartRules: [ast.rules[0].name],
+      allowedStartRules: defaultStartRules,
       cache: false,
       dependencies: {},
       exportVar: null,
@@ -93,11 +94,16 @@ const compiler = {
       trace: false,
     });
 
+    if (options.allowedStartRules === null
+        || options.allowedStartRules === undefined) {
+      options.allowedStartRules = defaultStartRules;
+    }
+
     if (!Array.isArray(options.allowedStartRules)) {
       throw new Error("allowedStartRules must be an array");
     }
     if (options.allowedStartRules.length === 0) {
-      throw new Error("Must have at least one start rule");
+      options.allowedStartRules = defaultStartRules;
     }
     const allRules = ast.rules.map(r => r.name);
     // "*" means all rules are start rules.  "*" is not a valid rule name.

test/unit/compiler.spec.js

@@ -10,12 +10,18 @@ describe("Peggy compiler", () => {
   it("checks start rules", () => {
     const ast = parser.parse("foo='1'");
     expect(compiler.compile(ast, compiler.passes)).to.be.an("object");
-    expect(() => compiler.compile(ast, compiler.passes, {
+    expect(compiler.compile(ast, compiler.passes, {
       allowedStartRules: null,
-    })).to.throw("allowedStartRules must be an array");
-    expect(() => compiler.compile(ast, compiler.passes, {
+    })).to.be.an("object");
+    expect(compiler.compile(ast, compiler.passes, {
+      allowedStartRules: undefined,
+    })).to.be.an("object");
+    expect(compiler.compile(ast, compiler.passes, {
       allowedStartRules: [],
-    })).to.throw("Must have at least one start rule");
+    })).to.be.an("object");
+    expect(() => compiler.compile(ast, compiler.passes, {
+      allowedStartRules: {},
+    })).to.throw("allowedStartRules must be an array");
     expect(() => compiler.compile(ast, compiler.passes, {
       allowedStartRules: ["bar"],
     })).to.throw('Unknown start rule "bar"');