acorn-es7-plugin

acorn-es7-plugin

acorn-es7-plugin is a plugin for the Acorn parser that generates ESTrees following the 'experimental' specification for asynchronous functions.

npm install --save acorn-es7-plugin

Usage

Adding the plugin

// Require acorn as usual
var acorn = require("acorn");
// Add the es7-plugin
require('./acorn-es7-plugin')(acorn) ;

Using the plugin

var code = "async function x(){ if (x) return await(x-1) ; return 0 ; }\n";	
var ast = acorn.parse(code,{
	// Specify use of the plugin
	plugins:{asyncawait:true},
	// Specify the ecmaVersion
	ecmaVersion:7
}) ;
// Show the AST
console.log(JSON.stringify(ast,null,2)) ;

Output:

{
	"type": "Program",
	"body": [
	{
	  "type": "FunctionDeclaration",
	  "id": {
	    "type": "Identifier",
	    "name": "x"
	  },
	  "generator": false,
	  "expression": false,
	  "params": [],
	  "body": {
	    "type": "BlockStatement",
	    "body": [
	      {
	        "type": "IfStatement",
	        "test": {
	          "type": "Identifier",
	          "name": "x"
	        },
	        "consequent": {
	          "type": "ReturnStatement",
	          "argument": {
	            "type": "AwaitExpression",
	            "operator": "await",
	            "argument": {
	              "type": "BinaryExpression",
	              "left": {
	                "type": "Identifier",
	                "name": "x"
	              },
	              "operator": "-",
	              "right": {
	                "type": "Literal",
	                "value": 1,
	                "raw": "1"
	              }
	            }
	          }
	        },
	        "alternate": null
	      },
	      {
	        "type": "ReturnStatement",
	        "argument": {
	          "type": "Literal",
	          "value": 0,
	          "raw": "0"
	        }
	      }
	    ]
	  },
	  "async": true
	}
	],
	"sourceType": "script"
}

Options & Compliance

The parser attempts to enforce strict contextualisation of async and await. Specifically, async is only a keyword if it precedes a function declaration, function expression or arrow function. await is only a keyword inside an async function. Outside of these contexts, both tokens are treated as identifiers (as they were in ES6 and earlier).

When using the plugin, you can supply an object in place of the 'true' flag with the following options.

flag	meaning
awaitAnywhere	If await is used outside of an async function and could not be an identifier, generate an AwaitExpression node. This typically means you can use await anywhere except when its argument would require parentheses, as this parses to a call to 'await(....)'.
asyncExits	Allow the additional sequences async return <optional-expression> and async throw <expression>. These sequences are used with nodent. In each case, as with async functions, a standard ReturnStatement (or ThrowStatement) node is generated, with an additional member 'async' set to true.