Evaluate some String and Array instance methods at compile time (#505)

* Evaluate some String and Array instance methods at compile time

* Fix a few bugs and increase test coverage

* npm run format

* undefined -> void 0

* undefined -> void 0 in tests

* `isUndef(thing)` checks if `thing` is `undefined`

* I can typo!

* Rename `other` handler

* Pacify prettier

* npm run format

* To trigger CircleCI

Author: j-f1

packages/babel-plugin-minify-constant-folding/README.md

@@ -1,6 +1,6 @@
 # babel-plugin-minify-constant-folding
 
-Tries to evaluate expressions and inline the result. For now only deals with numbers and strings.
+Tries to evaluate expressions and inline the result.
 
 ## Example
 
@@ -10,7 +10,22 @@ Tries to evaluate expressions and inline the result. For now only deals with num
 "a" + "b"
 2 * 3;
 4 | 3;
-"b" + a + "c" + "d" + g + z + "f" + "h" + "z"
+"b" + a + "c" + "d" + g + z + "f" + "h" + "i"
+
+[a, b, c].concat([d, e], f, g, [h]);
+["a", "b", "c"].join();
+["a", "b", "c"].join('@');
+[1, 2, 3].length;
+[1, 2, 3][1];
+[1, 2, 3].shift();
+[1, 2, 3].slice(0, 2);
+[a, b, c].pop();
+[a, b, c].reverse();
+"a,b,c".split(",");
+"abc"[0];
+"abc".charAt();
+"abc".charAt(1);
+"abc".length;
 ```
 
 **Out**
@@ -19,7 +34,23 @@ Tries to evaluate expressions and inline the result. For now only deals with num
 "ab";
 6;
 7;
-"b" + a + "cd" + g + z + "fhz";
+"b" + a + "cd" + g + z + "fhi";
+
+[a, b, c, d, e, f, g, h];
+"a,b,c";
+"a@b@c";
+3;
+2;
+2;
+[1, 2];
+c;
+[c, b, a];
+["a", "b", "c"];
+"a";
+"a";
+"a";
+"b";
+3;
 ```
 
 ## Installation

packages/babel-plugin-minify-constant-folding/__tests__/constant-folding-test.js

@@ -103,4 +103,142 @@ describe("constant-folding-plugin", () => {
       `);
     expect(transform(source)).toBe(expected);
   });
+
+  it("should handle Array methods on array literals", () => {
+    const source = unpad(
+      `
+      [1, 2, 3].concat([4, 5, 6]);
+      [a, b, c].concat([d, e], f, g, [h]);
+      [1, 2, 3]["concat"]([4, 5, 6]);
+      [1, 2, 3].push([4, 5, 6]);
+
+      [1, 2, 3].join();
+      ["a", "b", "c"].join();
+      ["a", "b", "c"].join("@");
+
+      [1, 2, 3].length;
+      [1, 2, 3][1];
+      [1, 2, 3]["1"];
+      [1, 2, 3][4];
+
+      [].shift();
+      [1, 2, 3].shift();
+
+      [1, 2, 3].slice();
+      [1, 2, 3].slice(1);
+      [1, 2, 3].slice(0, 2);
+      [1, 2, 3].slice(0, -1);
+
+      [1, 2, 3].pop();
+      [a, b, c].pop();
+      [].pop();
+
+      [a, b, c].reverse();
+      [1, 2, 3].reverse();
+
+      [1, 2, 3].splice(1);
+      [1, 2, 3, 4].splice(1, 2);
+    `
+    );
+    const expected = unpad(
+      `
+      [1, 2, 3, 4, 5, 6];
+      [a, b, c, d, e, f, g, h];
+      [1, 2, 3, 4, 5, 6];
+      4;
+
+      "1,2,3";
+      "a,b,c";
+      "a@b@c";
+
+      3;
+      2;
+      2;
+      void 0;
+
+      void 0;
+      2;
+
+      [1, 2, 3];
+      [2, 3];
+      [1, 2];
+      [1, 2, 3].slice(0, -1);
+
+      3;
+      c;
+      void 0;
+
+      [c, b, a];
+      [3, 2, 1];
+
+      [2, 3];
+      [2, 3];
+    `
+    );
+    expect(transform(source)).toBe(expected);
+  });
+  it("should ignore bad calls to array expression methods", () => {
+    const source = unpad(
+      `
+      [1, 2, 3][concat]([4, 5, 6]);
+      [a, "b", "c"].join();
+      ["a", "b", "c"].join(a);
+      [1, 2, 3].splice("a");
+    `
+    );
+    expect(transform(source)).toBe(source);
+  });
+  it("should ignore bad calls to string expression methods", () => {
+    const source = unpad(
+      `
+      "abc".something;
+      "abc"["something"];
+    `
+    );
+    expect(transform(source)).toBe(source);
+  });
+  it("should handle String methods on string literals", () => {
+    const source = unpad(
+      `
+      "a,b,c".split(",");
+      "a,b,c".split("");
+      "a,b,c".split();
+      "abc"[0];
+      "abc"["0"];
+      "abc"[4];
+      "abc".charAt();
+      "abc".charAt(1);
+      "abc".charCodeAt();
+      "abc".charCodeAt(1);
+      "abc".length;
+
+      "\u{1f44d}".charCodeAt();
+      "\u{1f44d}".charCodeAt(1);
+      "\u{1f44d}".codePointAt();
+      "\u{1f44d}".codePointAt(1);
+    `
+    );
+
+    const expected = unpad(
+      `
+      ["a", "b", "c"];
+      ["a", ",", "b", ",", "c"];
+      ["a,b,c"];
+      "a";
+      "a";
+      void 0;
+      "a";
+      "b";
+      97;
+      98;
+      3;
+
+      ${0xd83d};
+      ${0xdc4d};
+      ${0x1f44d};
+      ${0xdc4d};
+    `
+    );
+    expect(transform(source)).toBe(expected);
+  });
 });

packages/babel-plugin-minify-constant-folding/src/index.js

@@ -2,8 +2,45 @@
 
 const evaluate = require("babel-helper-evaluate-path");
 
-module.exports = ({ types: t, traverse }) => {
+const { FALLBACK_HANDLER } = require("./replacements");
+
+function getName(member) {
+  if (member.computed) {
+    switch (member.property.type) {
+      case "StringLiteral":
+      case "NumericLiteral":
+        return member.property.value;
+      case "TemplateLiteral":
+        return;
+    }
+  } else {
+    return member.property.name;
+  }
+}
+
+function swap(path, member, handlers, ...args) {
+  const key = getName(member);
+  if (key === undefined) return;
+  let handler = handlers[key];
+  if (typeof handler !== "function") {
+    if (typeof handlers[FALLBACK_HANDLER] === "function") {
+      handler = handlers[FALLBACK_HANDLER].bind(member.object, key);
+    } else {
+      return false;
+    }
+  }
+  const replacement = handler.apply(member.object, args);
+  if (replacement) {
+    path.replaceWith(replacement);
+    return true;
+  }
+  return false;
+}
+
+module.exports = babel => {
+  const replacements = require("./replacements.js")(babel);
   const seen = Symbol("seen");
+  const { types: t, traverse } = babel;
 
   return {
     name: "minify-constant-folding",
@@ -124,6 +161,21 @@ module.exports = ({ types: t, traverse }) => {
           node[seen] = true;
           path.replaceWith(node);
         }
+      },
+      CallExpression(path) {
+        const { node } = path;
+        const { callee: member } = node;
+        if (t.isMemberExpression(member)) {
+          const helpers = replacements[member.object.type];
+          if (!helpers || !helpers.calls) return;
+          swap(path, member, helpers.calls, ...node.arguments);
+        }
+      },
+      MemberExpression(path) {
+        const { node: member } = path;
+        const helpers = replacements[member.object.type];
+        if (!helpers || !helpers.members) return;
+        swap(path, member, helpers.members);
       }
     }
   };