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The Node.js `util` module

Siddharth
13. Coding for a hobby
・3 min read

There are a lot of builtin modules in Node.js. Most of us only use a few of them (I'm pretty sure the path, fs, and the server related modules are the most used ones).

But one of the more overlooked modules is the util module. Here, I'm going to show you some of the helpful functions from this module.

util.callbackify(original)

I don't think I will use this a lot because I love promises, but I think some people might love this.

Just like it's name, it basically "Takes an async function (or a function that returns a Promise) and returns a function following the error-first callback style, i.e. taking an (err, value) => ... callback as the last argument"

const util = require('util');

async function fn() {
  return 'hello world';
}
const callbackFunction = util.callbackify(fn);

callbackFunction((err, ret) => {
  if (err) throw err;
  console.log(ret);
});
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util.promisify(original)

I love this method. No more callback functions and more promises!

Just like it's name, it basically "Takes a function following the common error-first callback style, i.e. taking an (err, value) => ... callback as the last argument, and returns a version that returns promises."

const util = require('util');
const fs = require('fs');

const stat = util.promisify(fs.stat);
stat('.').then((stats) => {
  // Do something with `stats`
}).catch((error) => {
  // Houston, we have a problem
});
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util.deprecate(fn, msg[, code])

Basically, this function takes in a deprecated function and a deprecation message (and an optional deprecation code) and it returns a new function (which is basically the same as the original function). However, whenever the function returned is called, it will emit a DeprecationWarning (which will basically be logged to the console). You may have seen this before.

const util = require('util');

exports.obsoleteFunction = util.deprecate(() => {
  // Do something here.
}, 'obsoleteFunction() is deprecated. Use newShinyFunction() instead.');
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util.inspect(object[, options])

This function logs an object to the console, but instead of directly logging it, it applies some transformations to the object.

const util = require('util')

const obj = {
  foo: '{}',
  bar: {
    baz: undefined
  }
}

util.inspect(obj)
// => { foo: '{}', bar: { baz: undefined } }
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Now, you may think it is just an equivalent of console.log(JSON.stringify(obj)), but there is a lot more functionality available using the options object. Some of the more useful options are:

  • depth (number): useful when you have a deep nested structure and want to limit the number of recursions
  • colors (boolean): whether to add syntax highlighting to the output
  • compact (boolean): whether to show the output in a compact manner (defaults to true). Setting to false will break each property to a newline.

util.isDeepStrictEqual(val1, val2)

Think of it like the toBe Jest matcher — it uses Object.is() on primitives and compares objects checking their keys recursively.

const util = require('util')

util.isDeepStrictEqual(
  { someKey: 'someValue', somThingElse: { nested: true } },
  { somThingElse: { nested: true }, someKey: 'someValue' }
)
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util.types

It's basically a type checker — you have functions like util.types.isAsyncFunction(value), util.types.isPromise(value), util.types.isGeneratorFunction(value), util.types.isDate(value), and even some crazy ones like util.types.isArgumentsObject(value), util.types.isCryptoKey(value), util.types.isBooleanObject(value), and more.

A lot more!

I only chose the ones which seemed like they could be used by almost anyone, but there is a lot more. Debug logs, printf like string formatting, Text Decoding and Encoding, and more. You should totally check it out here

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