nel

Node.js Evaluation Loop (NEL)

NEL is an npm module for running Node.js REPL sessions.

NEL is a spin-off library from IJavascript. This fact explains some of the design decisions in NEL such as returning results in MIME format, and the functionality provided for completion and inspection of Javascript expressions. See the section on usage for more details.

Please, consider this repository as an alpha release. The API is likely to change.

Main Features

• Run Javascript code within a Node.js session. The result can be formatted as:
  • plain text
  • other MIME formats (HTML, SVG, PNG...)
• Run Javascript code asynchronously.
• Generate a list of completion options for an incomplete piece of Javascript code.
• Inspect a Javascript expression and return information such as type or even documentation (currently, only for Javascript builtins).

Announcements

• NEL is in the process of being refactored to maximise the amount shared amongst ijavascript, jp-babel and jp-coffeescript.
• NEL v0.3: New API (simplify API by hiding type module:nel~Task)
• NEL v0.2: API change (removed Session#executionCount)
• NEL v0.1.1: New experimental $$mimer$$ and $$defaultMimer$$
• NEL v0.1: Output change (changed function output)
• NEL v0.0: Initial release

Install

npm install nel

Usage

Hello, World!

// Load `nel` module
var nel = require("nel");

// Setup a new Javascript session
var session = new nel.Session();

// Define callbacks to handle results and errors
var onSuccess = function printResult(result) { console.log(result); }
var onError = function printError(error) { console.log(error); }

// Example of an execution request
// Output:
// { mime: { 'text/plain': '\'Hello, World!\'' } }
var code = "['Hello', 'World!'].join(', ');";
session.execute(code, onSuccess, onError);

Exceptions

// Example of throwing an exception
// Output:
// { error:
//  { ename: 'Error',
//    evalue: 'Hello, World!',
//    traceback:
//     [ 'Error: Hello, World!',
//       '    at evalmachine.<anonymous>:1:7',
//       '    at run ([eval]:182:19)',
//       '    at onMessage ([eval]:63:41)',
//       '    at process.EventEmitter.emit (events.js:98:17)',
//       '    at handleMessage (child_process.js:318:10)',
//       '    at Pipe.channel.onread (child_process.js:345:11)' ] } }
code = "throw new Error('Hello, World!');";
session.execute(code, onSuccess, onError);

stdout and stderr

// Example of use of console.log()
// Output:
// { mime: { 'text/plain': 'undefined' } }
code = "console.log('Hello, World!');";
session.execute(code, onSuccess, onError);

// Example of reading the session stdout
// Output:
// Hello, World!
process.stdout.write(session.stdout.read());

MIME output

A session may return results in MIME formats other than 'text/plain'.

// HTML example
// Output:
// { mime: { 'text/html': '<div style=\'background-color:olive;width:50px;height:50px\'></div>' } }
code = "$$html$$ = \"<div style='background-color:olive;width:50px;height:50px'></div>\";";
session.execute(code, onSuccess, onError);

// SVG example
// Output:
// { mime: { 'image/svg+xml': '<svg><rect width=80 height=80 style=\'fill: orange;\'/></svg>' } }
code = "$$svg$$ = \"<svg><rect width=80 height=80 style='fill: orange;'/></svg>\";";
session.execute(code, onSuccess, onError);

// PNG example
code = "$$png$$ = require('fs').readFileSync('image.png').toString('base64');";
session.execute(code, onSuccess, onError);

// JPEG example
code = "$$jpeg$$ = require('fs').readFileSync('image.jpg').toString('base64');";
session.execute(code, onSuccess, onError);

// MIME example
code = "$$mime$$ = {\"text/html\": \"<div style='background-color:olive;width:50px;height:50px'></div>\"};";
session.execute(code, onSuccess, onError);

Generate a completion list

NEL can parse simple Javascript variable expressions and generate a list of completion options:

session.complete(
    "set",     // code
    3,         // cursorPos
    onSuccess, // onSucess
    onError    // onError
);

// Output:
// { completion:
//    { list: [ 'setImmediate', 'setInterval', 'setTimeout' ],
//      code: 'set',
//      cursorPos: 3,
//      matchedText: 'set',
//      cursorStart: 0,
//      cursorEnd: 3 } }

Note that the cursor position can be located anywhere within the Javascript code:

session.complete(
    "set",     // code
    2,         // cursorPos
    onSuccess, // onSucess
    onError    // onError
);

// Output:
// { completion:
//    { list: [ 'setImmediate', 'setInterval', 'setTimeout' ],
//      code: 'set',
//      cursorPos: 2,
//      matchedText: 'se',
//      cursorStart: 0,
//      cursorEnd: 3 } }

Inspect an expression

NEL can parse simple Javascript variable expressions and inspect their value:

code = "var a = [1, 2, 3];";
session.execute(code, null, onError);
session.inspect(
    code,      // code
    5,         // cursorPos
    onSuccess, // onSucess
    onError    // onError
);

// Output:
// { inspection:
//    { string: '[ 1, 2, 3 ]',
//      type: 'Array',
//      constructorList: [ 'Array', 'Object' ],
//      length: 3,
//      code: 'var a = [1, 2, 3];',
//      cursorPos: 5,
//      matchedText: 'a' } }

NEL can also provide relevant documentation (currently only available for Javascript builtins):

session.inspect(
    "parseInt", // code
    8,          // cursorPos
    onSuccess,  // onSucess
    onError     // onError
);

// Output:
// { inspection:
//    { string: '[Function: parseInt]',
//      type: 'Object',
//      constructorList: [ 'Function', 'Object' ],
//      length: 2,
//      code: 'parseInt',
//      cursorPos: 8,
//      matchedText: 'parseInt' },
//   doc:
//    { description: 'The parseInt() function parses a string argument and returns an integer of the specified radix (the base in mathematical numeral systems).',
//      url: 'https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/parseInt',
//      usage: 'parseInt(string, radix);' } }

Callbacks beforeRequest and afterRequest

var beforeRequest = function() { console.log("This callback runs first"); }
code = "'I run next'";
var afterRequest = function() { console.log("This callback runs last"); }
session.execute(code, onSuccess, onError, beforeRequest, afterRequest);

// Output:
// This callback runs first
// { mime: { 'text/plain': '\'I run next\'' } }
// This callback runs last

Contributions

First of all, thank you for taking the time to contribute. Please, read CONTRIBUTING.md and use the issue tracker for any contributions: support requests, bug reports, enhancement requests, pull requests, ...

TODO

• Implement $$text$$, $$update$$()
• Add tests for $$async$$ and $$done()$$
• Add tests for $$html$$, $$png$$, $$jpeg$$, $$mime$$, $$mimer$$, ...
• Session#complete and Session#inspect: make cursorPos argument default to code.length
• Add Node.js documentation