Let’s say you have a function that takes two or more arrays or strings, merge them and return a new array:
//using spread syntax
function arrayMerge(...arrays){
let a = [];
arrays.forEach(e=>{
Array.isArray(e)
? Array.prototype.push.apply(a,e)
: a.push(e);
});
return a;
}
To convert above function to module, we need to save it in a separate file and then export it using the exports statement, as shown in the following code:
//arrays.js
module.exports.arrayMerge = arrayMerge;
function arrayMerge(...arrays){
let a = [];
arrays.forEach(e=>{
Array.isArray(e)
? Array.prototype.push.apply(a,e)
: a.push(e);
});
return a;
}
To use arrayMerge in a Node app, we need to import the module as shown in following code:
//app.js
let arrays = require('./arrays');
let arrayMerge = arrays.arrayMerge;
/* or
let {arrayMerge} = require('./arrays');
*/
let str = 'hello';
let arr = ['a','b','c'];
let newArr = arrayMerge(str,arr);
console.log(newArr);
//[ 'hello', 'a', 'b', 'c' ]
Let’s make our arrays.js module a little bigger, growing it to three functions:
//arrays.js
module.exports.arrayMerge = arrayMerge;
module.exports.hasValue = hasValue;
module.exports.delValue = delValue;
function hasValue(val,arr){
return 'hasValue function';
}
function delValue(val,arr){
return 'delValue function';
}
function arrayMerge(...arrays){
...
}
//minimum code
Next, we’ll import the module. Destructuring assignment (curly braces around the variables) is a great way to assign many functions exported by a required module to their local variables in a single line.
//app.js
Destructuring assignment
let {arrayMerge,hasValue,delValue} = require('./arrays');
let str = 'hello';
let arr = ['a','b','c'];
let newArr = arrayMerge(str,arr);
console.log(hasValue(str,arr));
//hasValue function
console.log(delValue(str,arr));
//delValue function
exports vs module.exports
As you inspect the code of Node modules you might see that some modules export their functionality using module.exports, while others simply use exports:
//arrays.js exports.arrayMerge = arrayMerge; exports.hasValue = hasValue; exports.delValue = delValue; /* vs. */ module.exports.arrayMerge = arrayMerge; module.exports.hasValue = hasValue; module.exports.delValue = delValue;
Both methods given above will do the same thing. The exports is just a reference to module.exports. Node’s core modules are defined using the module.exports statement. So, to be safe, just use the module.exports.
Exporting our FileWalker class as a module
We’ll just add the module.exports = before the class keyword, as shown:
//walker.js
const fs = require('fs'),
path = require('path'),
{EventEmitter} = require('events');
//class FileWalker extends EventEmitter {
module.exports = class FileWalker extends EventEmitter {
constructor (entry){
super();
this.readTree(entry);
}
readTree (entry) {
entry = entry || this._entry;
fs.lstat(entry, (err,stat) => {
if (err){
this.emit('error',err,entry,stat);
return;
}
if (stat.isFile()){
this.emit('file',entry,stat);
}
else if (stat.isDirectory()){
this.emit('dir',entry,stat);
fs.readdir(entry, (err,files) => {
if (err){
this.emit('error',err,entry,stat);
return;
}
files.forEach( file => {
this.readTree(path.join(entry,file));
});
});
}
});
}
}
In next tutorial we’ll expand our FileWalker module by adding some methods and events for filtering dirs and files. For example, if (dir == 'some/path') //skip or if (file.extension == 'ext') //skip.