TwoStep

Is a simple control-flow library for node.js that makes serial execution, parallel execution and error handling painless. Inspired by step twostep was based on Tim Caswell (step author) gist, but later it was refactored and almost totally rewritten. Twostep follows and extends the ideas behind the step.

Features

• serial and parallel (with results grouping support) execution
• simple error handling
• simplified error handling for common use case (when error handled at the last step)
• ability to pass the arbitrary values between steps (using this.pass())
• pure js code (no dependencies, < 200 lines of code)
• full test coverage (100%)
• super battle-tested (we ubiquitously use twostep in a big project (50 000+ lines of code))

Installation

npm install twostep

Usage

All steps (functions) passed to the Step will be executed in series. Inside each step this.slot() for wating of async call, this.pass() for passing value to the next step or this.makeGroup() for creating group (for having results grouped to array) can be called.

var Step = require('twostep').Step,
	fs = require('fs');

Step(
	function() {
		// pass filename to the next step as second argument (error always first)
		this.pass(__filename + '.bak');
		// read the file content, it will be passed as third argument (because
		// this.pass was called before)
		fs.readFile(__filename, 'utf8', this.slot());
	},
	function(err, bakFile, content) {
		// if we have an error, throw it to skip this step
		if (err) throw err;
		this.pass(bakFile);
		fs.readdir(__dirname, this.slot());
		fs.writeFile(bakFile, content, this.slot())
		// `readdir` and `writeFile` will be exeuted in parallel when both of
		// them done next step will be called
	},
	function(err, bakFile, dirContent) {
		if (err) throw err;
		console.log('%s successfully written', bakFile);
		this.pass(dirContent);
		// make the group
		var group = this.makeGroup();
		// wait for ordered results using group.slot() - all results
		// will be passed as an array with same order (as at calls)
		dirContent.forEach(function(name) {
			fs.stat(name, group.slot());
		});
	},
	function(err, dirContent, stats) {
		if (err) throw err;
		var fileNames = dirContent.filter(function(name, i) {
			return stats[i].isFile();
		});
		console.log('files in dir: %s', fileNames);
		// don't call next step manually, it's the last step if no error
	},
	function(err) {
		// this step will be called only if an error was occured at any step
		// above, so here we can handle the error.
		console.log('Error occured: ', err.stack || err);
		process.exit(1);
	}
);

In the example above we did if (err) throw err; at the start of every step to pass the error (if it exists) to the next step for handle the error at the last step. We can avoid writing this annoying line using Steppy. With Steppy example described above transforms to

var Steppy = require('twostep').Steppy,
	fs = require('fs');

Steppy(
	function() {
		this.pass(__filename + '.bak');
		fs.readFile(__filename, 'utf8', this.slot());
	},
	function(err, bakFile, content) {
		this.pass(bakFile);
		fs.readdir(__dirname, this.slot());
		fs.writeFile(bakFile, content, this.slot())
	},
	function(err, bakFile, dirContent) {
		console.log('%s successfully written', bakFile);
		this.pass(dirContent);
		var group = this.makeGroup();
		dirContent.forEach(function(name) {
			fs.stat(name, group.slot());
		});
	},
	function(err, dirContent, stats) {
		var fileNames = dirContent.filter(function(name, i) {
			return stats[i].isFile();
		});
		console.log('files in dir: %s', fileNames);
	},
	function(err) {
		console.log('Error occured: ', err.stack || err);
		process.exit(1);
	}
);

API

Step(step1, step2, stepN...)

Steps container accepts functions and executes them in series. If error is occured inside step it will be passed to the next step as first argument. First argument of the step is always an error (falsy if no error), subsequent arguments - values passed to the reserved slots (created via this.pass(), this.slot() or this.makeGroup()) of previous step in the order the slots were reserved.

Steppy(step1, step2, stepN...)

Same steps container as Step but it also automatically wraps every single step with error check and calls the last step if error occurs.

Methods which can be called inside each step

this.slot()

Reserves one slot at the current step. Next step will be called when all reserved slots of current step will be filled with data or the error occurs. Returns callback function(err, data) to fill the slot with data.

this.pass(value1, value2, valueN...)

Passes one or several synchronous values to the next step.

this.makeGroup()

Reserves slot, creates and returns a group, all results of which will be passed into the reserved slot as a single array. pass, slot methods can be called for created group. If group methods were not called empty array will be passed into reserved slot.

Tests

into cloned repository run

npm install

after installtion run

npm test

for run tests and generate coverage report run

npm run testAndCover

detailed coverage report will be saved at ./coverage/index.html

License

MIT