Traditional JavaScript and Node.js API will utilize the concept of asynchronous callbacks for virtually every input/output operation. Whether it's reading a disk file, accessing a database table, or calling a web service, the results are not directly returned to the caller; instead they are passed into a separate function referred to as a callback. Because of JavaScript design, the only way to "freeze" a computation and have the "rest of it" execute latter (asynchronously) is to put "the rest of it" inside a callback.
For certain types of applications, this can be quite useful and performant; however, it also creates the problem more commonly known as Callback Hell. A lot of code ends up looking like this:
fs.readdir(source, function (err, files) {
if (err) {
console.log('Error finding files: ' + err)
} else {
files.forEach(function (filename, fileIndex) {
console.log(filename)
gm(source + filename).size(function (err, values) {
if (err) {
console.log('Error identifying file size: ' + err)
} else {
console.log(filename + ' : ' + values)
aspect = (values.width / values.height)
widths.forEach(function (width, widthIndex) {
height = Math.round(width / aspect)
console.log('resizing ' + filename + 'to ' + height + 'x' + height)
this.resize(width, height).write(dest + 'w' + width + '_' + filename, function(err) {
if (err) console.log('Error writing file: ' + err)
})
}.bind(this))
}
})
})
}
}
The pyramid shape and all the }) at the end are needed because each API call returns its data into an anonymous callback function.