WIP: Transpiling

.gitignore

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+node_modules/

transpile/package.json

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+{
+  "scripts": {
+    "debug": "yarn jscodeshift test.js --dry --print"
+  },
+  "dependencies": {
+    "acorn": "^5.5.0",
+    "jscodeshift": "^0.4.1",
+    "recast": "^0.14.4"
+  }
+}

transpile/parser.js

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+/**
+ * Modified https://github.com/angelozerr/acorn-es7/blob/master/acorn-es7.js to do the same thing
+ * but for function declarations instead of class declarations
+ */
+
+const acorn = require('acorn')
+
+function getTokenType(p, loose) {
+  return loose ? p.tok.type : p.type;
+}
+
+var extendsAcorn = function (pp) {
+  var loose = pp == (acorn.LooseParser && acorn.LooseParser.prototype);
+
+  pp.parseDecorator = function() {
+    var node = this.startNode();
+    this.next();
+    node.expression = this.parseMaybeAssign();
+    return this.finishNode(node, "Decorator");
+  }
+
+  return function(instance) {
+    instance.decorators = [];
+
+    instance.extend('getTokenFromCode', function(inner) {
+      return function(code) {
+        if (code == 64) {
+          ++this.pos; return this.finishToken(tt.at); 
+        }
+        return inner.call(this, code);
+      };
+    }); 
+
+    instance.extend('parseStatement', function(inner) {
+      return function(declaration, topLevel) {
+        switch(getTokenType(this, loose)) {
+          case tt.at:
+            while (getTokenType(this, loose) === tt.at) {
+              this.decorators.push(this.parseDecorator());
+            }
+            if (!loose && getTokenType(this, loose) !== tt._function) {
+              this.raise(this.start, "Leading decorators must be attached to a function declaration");
+            }
+          case tt._function:
+            if (!loose && !declaration) this.unexpected()
+            if (this.decorators.length) {
+              var node = inner.call(this, declaration, topLevel);
+              node.decorators = this.decorators;
+              // adjust start of FunctionDeclaration with start of the first decorator (helpful for ES7 walk).
+              node.start = node.decorators[0].start; 
+              this.decorators = [];
+              return node;
+            }          
+          }       
+          return inner.call(this, declaration, topLevel);
+      };
+    });
+  }
+}
+
+var tt = acorn.tokTypes;
+tt.at = new acorn.TokenType('@');
+// acorn 
+acorn.plugins.es7 = extendsAcorn(acorn.Parser.prototype);
+// acorn loose
+if(acorn.LooseParser) acorn.pluginsLoose.es7 = extendsAcorn(acorn.LooseParser.prototype);
+
+module.exports = {
+  parse(source) {
+    return acorn.parse(source, {
+      // Specify use of the plugin
+      plugins:{es7:true},
+      // Specify the ecmaVersion
+      ecmaVersion:7
+    })
+  }
+}

transpile/test.js

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+/** Classic decorator */
+function memoize(func) {
+  const results = new Map();
+  return (arg, ...rest) => {
+    if (results.has(arg)) {
+      return results.get(arg);
+    }
+    const value = func(arg, ...rest);
+    results.set(arg, value);
+    return value;
+  }
+}
+
+/** Parametrized decorator */
+function partial(...preargs) {
+  return func => (...args) => func(...preargs, ...args);
+}
+
+const USERS = {
+  42: {
+    name: "Dan"
+  }
+}
+
+/** Wrap in block */
+function init() {
+  /** Hoisted call */
+  search(42);
+
+  /** Classic decorator appliance */
+  @memoize
+  /** Decorator factory appliance */
+  @partial(USERS)
+  function search(users, id) {
+    return users[id];
+  }
+
+  return search;
+}
+
+init();

transpile/transform.js

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+const parser = require('./parser')
+
+module.exports = function (fileInfo, api, options) {
+  const { j } = api
+  return j(fileInfo.source)
+    .find(j.FunctionDeclaration)
+    .filter(path => path.node.decorators)
+    .map(path => {
+      const { node } = path
+      const functionExpression = j.functionExpression(
+        node.id,
+        node.params,
+        node.body
+      )
+      const resolved = node.decorators.reduceRight((acc, decorator) => {
+        return j.callExpression(decorator.expression, [acc])
+      }, functionExpression)
+      path.replace(j.variableDeclaration('var', [
+        j.variableDeclarator(
+          node.id,
+          resolved,
+        )
+      ]))
+      return path
+    })
+    .toSource();
+}
+
+module.exports.parser = parser