Beware the return statement in Javascript

The return statement in Javascript looks innocent and intuitive, but it hides a caveat that can break the logic of your code and lead to undefined behavior. The best way to illustrate this is to use a simple example, so let’s consider this function:

function greeter(name) {
    return 'Hello ' + name;
greeter('Hugo');  // works as expected

The function above works as expected and you don’t have to worry much about it. Now let’s imagine that you have to concatenate more strings inside the greeter function and you end up with a much longer line of code like this:

function greeter(name) {
   return 'Hello, ' + name + ',\nThis is a long long long sentence.\nThis is another long sentence.';

The code above also works fine if you keep it exactly as it is. You can notice that the string is too long to fit the width of the screen, so any decent developer would care about that and try to break that long line into smaller readable chunks. One possibility is:

function greeter(name) {
       'Hello, ' + name + ',\n' +
       'This is a long long long sentence.\n' + 
       'This is another long sentence.';

B-I-N-G-O! The code above does NOT work as expected. This function returns undefined because the return statement is actually empty and Javascript doesn’t consider the next lines to be part of the return statement. There are some ways to fix this issue. One of them is to remove the line break after the return keyword, like this:

function greeter(name) {
   return 'Hello, ' + name + ',\n' +
       'This is a long long long sentence.\n' + 
       'This is another long sentence.';

You can notice that the code above is a bit unaligned. A perfectionist (like me) would add the line break after the return in order to fix the alignment problem, but this actually creates the other (much bigger) problem. Sigh.

Another way of fixing the code is to use parenthesis around the return value. In this case the alignment isn’t a problem:

function greeter(name) {
   return (
       'Hello, ' + name + ',\n' +
       'This is a long long long sentence.\n' + 
       'This is another long sentence.'

Parentheses are valid in this case because the return statement can be followed by an expression. It is quite uncommon to see such syntax, but in this case it plays an important role. This is one of the many details about the language that you have to be aware of in order to play it safe. If you develop code in other languages — like Java or C++ — you know that such code without the parentheses would work perfectly fine in those languages, so it would be fairly easy to forget to use them when you switch back to Javascript.

Typescript is better

In Typescript we could have some similar code like this:

function greeter(name: string): string {
       'Hello, ' + name + ',\n'
       'This is a long long long sentence.\n' + 
       'This is another long sentence.';

If you try to compile this code with Typescript you get the following error message:

code.ts(3,8): error TS7027: Unreachable code detected.

So it basically says that line 3 has unreachable code and now you have to fix it. This is a much better world compared to pure javascript development since you know that this issue will not waste your time later.

If you use Typescript, make sure you have the compiler option “allowUnreachableCode” enabled (otherwise you will not see the error above). Read more about this flag on the compiler options page.

One question may come to your mind at this point: should Typescript fix the code automatically by generating JS code that doesn’t contain the line break after the return statement? Nope. The number of scenarios that this issue could happen is huge, so it is much safer to just warn the developer instead of trying to fix the code silently. I think Typescript is doing a good job here (kudos to Microsoft).


The return statement issue described in this post is just one of many caveats of the Javascript language. It should remind you that Javascript alone can be a treacherous path and you should definitely consider better alternatives like Typescript in your development. Javascript alone can make the software maintenance a nightmare in the long run, so serious developers must look for better alternatives in order to detect and fix mistakes as early as possible. Be productive and stay safe.

Having fun with Typescript, ThreeJS and Ammo.js

This week I implemented a simple 3D experiment with Typescript, ThreeJS and Ammo.js. It works like an FPS game where you can navigate with the mouse pointer and W-A-S-D keys, and you can also shoot cannon balls with the mouse button.


Using Typescript

Typescript is a great language that compiles to Javascript code. If you are a web developer and have never used this language before, you should check it out immediately because it has a lot of interesting things that can make your life (as a web programmer) much easier. You know that Javascript is a powerful language, but alone it can make the maintenance of big projects a nightmare. Typescript is a great step towards high quality maintainable code because it is a typed superset of Javascript.

The challenge of using Typescript with some external libraries is that you need the type definition files of those libraries to compile your code. A type definition file is a piece of typescript code that contains the declaration of types, classes, interfaces and functions of a library. It doesn’t provide implementation details, only types and signatures. This allows Typescript to analyze your code and detect programming errors. For example, check the ThreeJS definition file here:

DefinitelyTyped is a website that aggregates lots of definition files and it is a great asset for Typescript developers. Sometimes it doesn’t contain definition files that you need and this was the case of the Ammo.js library I used in my experiment.

No definition files – What should I do?

When there is no definition file available for the library you want to use, you can build one your own. Typescript can certainly guide you through this process. The compiler errors reported by Typescript will show you exactly what is missing in the definition file. For example, let’s try to declare a variable like this:

let physicsWorld: Ammo.btDiscreteDynamicsWorld;

You can start with an empty definition file (ammo.d.ts). Compiling the code above will show this message:

Cannot find namespace 'Ammo'.

So you can use this information to fix your custom definition file for that library. In this case, you can just add an empty namespace like this:

declare namespace Ammo {

After that, if you try to compile again the error message will change to:

Module 'Ammo' has no exported member 'btDiscreteDynamicsWorld'.

So you can change your definition file to:

declare namespace Ammo {
    export class btDiscreteDynamicsWorld {

Following this pattern, you can basically add the missing information piece by piece to the definition file until your project is fully compilable.  Of course you have to keep an eye on the real library code and make sure your declarations are consistent with the real implementation. For Ammo.js, you can see below the final definition file I created. Note that there is no implementation code, only types and signatures.

declare namespace Ammo {

   export class btDefaultCollisionConfiguration {}

   export class btCollisionDispatcher {
      constructor(c: btDefaultCollisionConfiguration);

   export class btVector3 {
      x(): number;
      y(): number;
      z(): number;
      constructor(x: number, y: number, z: number);

   export class btAxisSweep3 {
      constructor(min: btVector3, max: btVector3);

   export class btSequentialImpulseConstraintSolver {}

   export class btDiscreteDynamicsWorld {
      constructor(a: btCollisionDispatcher, b: btAxisSweep3, c: btSequentialImpulseConstraintSolver, d: btDefaultCollisionConfiguration);
      setGravity(v: btVector3);
      addRigidBody(b: btRigidBody);
      stepSimulation(n1: number, n2: number);

   export class btConvexShape {
      calculateLocalInertia(n: number, v: btVector3);
      setMargin(n: number);

   export class btBoxShape extends btConvexShape {
      constructor(v: btVector3);

   export class btSphereShape extends btConvexShape {
      constructor(radius: number);

   export class btRigidBody {
      constructor(info: btRigidBodyConstructionInfo);
      setActivationState(s: number);

   export class btQuaternion {
      x(): number;
      y(): number;
      z(): number;
      w(): number;
      constructor(x: number, y: number, z: number, w: number);

   export class btTransform {
      setOrigin(v: btVector3);
      getOrigin(): btVector3;
      setRotation(q: btQuaternion);
      getRotation(): btQuaternion;

   export class btRigidBodyConstructionInfo {
      constructor(mass: number, motionState: btDefaultMotionState, shape: btConvexShape, inertia: btVector3);

   export class btDefaultMotionState {
      constructor(t: btTransform);

You should realize that my type definitions for Ammo.js are far from complete, considering what the library does. Ammo is a huge library and it contains a lot more things that my code will not use at this point. This is okay and the main point here is that the file I created is enough for me to compile my experiment and keep moving. If I need more methods from Ammo.js, I can adjust that file as needed and move on.

Typescript and browser differences

Sometimes Typescript will NOT understand your Javascript code and you will have to deal with it. Let me give you a clear example:

let m = document.body.requestPointerLock || 
        document.body.mozRequestPointerLock || 

The code above is syntactically correct and it looks for the Pointer Lock API in the current browser. Typescript complains about this code with the following error message:

Property 'mozRequestPointerLock' does not exist on type 'HTMLElement'.
Property 'webkitRequestPointerLock' does not exist on type 'HTMLElement'.

Those methods are browser-specific and typescript doesn’t recognize them. We have to fix this error message and one possible solution is to assign the document.body value to a variable of type “any”. This will prevent typescript from checking if those properties belong to the HTMLElement type:

let _body: any = document.body;
let m = _body.requestPointerLock || 
        _body.mozRequestPointerLock || 

There are other ways of handling such cases, but I won’t explain them in this post. One great thing about Typescript is that it doesn’t stop the compilation when it finds errors like the one above. It simply generates the javascript output as if the code were correct and you can clean up that error message later.


Working with Typescript is an amazing experience. I believe that Microsoft is doing a great job with this language and contributing to the future of web development with great ideas. My experiment with ThreeJS and Ammo.js is just in the beginning. I have a good experience with the BulletPhysics library (see my personal projects) and this should allow me to expand the code and build more cool demos, examples and games.


A note about the volatile keyword in C++

The volatile keyword in C++ is poorly misunderstood. Lots of developers believe that it makes the declared variable an atomic variable, but that’s wrong. The truth is that you should use std::atomic or related code (e.g., mutex) on a variable if you want to guarantee atomic operations in a multithreaded context.

What I want to discuss in this article is something else. Before I start you should remember that the volatile keyword should be used when you want to tell the compiler to not optimize a given variable. It basically says “don’t perform any optimizations on operations on this memory because something else outside the program (which the compiler is not aware of) may change it”. The most common use of volatile is in memory-mapped I/O. Gadgets and electronic peripherals (e.g., sensors, displays, etc.) may communicate with the software and optimizations could break the code.

The use of the volatile keyword has caught my attention recently and I want to show you a curious thing about it. I will go through some code examples and I will use an amazing online tool called to look at the assembly code generated by the compiler.

With the GCC compiler

Let’s start with a very simple example without the volatile keyword. As you can see in the code below, we have an int variable and a while loop. The GCC compiler realizes that the variable will never change its value and the loop will never finish. So it optimizes the assembly code by ignoring the variable check and keeps jumping forever back to the L2 label.


Now consider the same code, but let’s add volatile to the declaration of the int variable on line 4. As we can see, the GCC compiler now generates an assembly code that is totally different than the previous example and it respects the value == 5 check inside the loop.


Now let’s change this example a little bit. What would happen if we move the int variable into a struct? Consider the code below without the volatile keyword. The assembly code is the same as in the first example. The GCC compiler ignores the loop check and keeps jumping forever.


If we want to tell the compiler to not optimize the int variable, should we add the volatile keyword to the int variable declaration inside the struct? Let’s see how this would work:


As we can see, the GCC compiler simply ignored the volatile definition inside the struct and optimized away the loop again. A careless developer could easily assume that code was correct and move on to other tasks. Bugs later would require hours of debugging in order to find the source of the problem. The real solution here is to add the volatile keyword to the struct object on line 6:


I’ve tried different versions of GCC they all have the same result regarding this issue. Now let’s try CLANG and see what happens.

With the CLANG compiler

Using clang 3.9 we can start without the volatile keyword. As you can see below, the loop is optimized just like it happened with the GCC compiler.


Now let’s try the volatile keyword in the struct member variable. We have a surprise here: CLANG doesn’t optimize the loop like GCC did. I tried older versions of CLANG and the output is the same.


We get the same result when we move the volatile to the object on line 6.


So which compiler is the correct one?

It seems to me that CLANG is doing a better job than GCC because the loop deals with the struct object and its member variable. So if the volatile modifier is present either in the object or in its member, then the compiler has to respect that in the loop.

If you have more thoughts about this, please leave comments below.

EDIT 1: I posted this discussion on reddit and we have some great comments there.

EDIT 2: This issue was also discussed on the CppCast Episode 76 with Dan Saks, check it out:

Reading text files line by line: Java, C++ and C benchmark

I wanted to compare how Java, C++ and C perform when reading a text file line by line and printing the output. I’ve implemented some possibilities and, at the end, we can compare the speed of each execution.

Java 7 version (BufferedReader)


public class Main7 {

   private static final String FILENAME = "/path/to/file.txt";

   public static void main(String[] args) throws IOException {
      File file = new File(FILENAME);
      BufferedReader br = new BufferedReader(new FileReader(file));
      for (String line; (line = br.readLine()) != null; ) {

Java 8 version (Stream)

import java.nio.file.Files;
import java.nio.file.Paths;

public class Main8 {

   private static final String FILENAME = "/path/to/file.txt";

   public static void main(String[] args) throws IOException {
       try (Stream stream = Files.lines(Paths.get(FILENAME))) {

C++ (ifstream)

#include < iostream>
#include < fstream>
#include < string>

using namespace std;

int main() {
   const constexpr char* FILENAME = "/path/to/file.txt";
   ifstream file(FILENAME);

   string line;
   while (file.peek() != EOF) {
      getline(file, line);
      cout << line << '\n';
   return 0;


#include < stdio.h>
#include < stdlib.h>

int main(void)
   FILE* fp = fopen("/path/to/file.txt", "r");
   if (fp == NULL)

   char* line = NULL;
   size_t len = 0;
   while ((getline(&line, &len, fp)) != -1) {
      printf("%s", line);

   if (line)


Below you can see how each program above performs reading a TXT file with 10,440 lines (file size is 3.5Mb).

 Version  Time
 Java 7 (BufferedReader)  0.254 seconds
 Java 8 (Stream)  0.324 seconds
 C++ (ifstream)  0.429 seconds
 C (File)  0.023 seconds

As we can see, C is by far the fastest option. I am surprised that C++ isn’t faster than Java, but this is probably because of the ifstream and std::getline() implementation. This is not the first time I see the Standard Library with performance issues compared to other languages (the regular expression implementation was the first time).



Advantages of Mercurial over Subversion

I have been wondering about version control systems (VCS) recently and I wanted to build a list with clear advantages of Mercurial over Subversion. First of all, Mercurial is a distributed VCS, Subversion is not. This brings advantages like:

1) You have a full copy of the repository on your computer, so you don’t have to rely on server backups in case a server goes down.

2) If you don’t have internet connection, you can still work and commit the changes to your local repository. When the connection is back online, you can push your changes to the server.

3) Mercurial organizes revisions as changesets which allow you to very easily branch/merge the code base. Merging a branch in SVN is harder.

More UI Lessons from the Real World (Second Part)

This is the second part of my UI lessons from the real world article. It has more funny examples of what (not) to do with your UIs.

You guessed it, folks. After the huge success of my first article Ten UI Lessons from the Real World, I decided to write the second part with more examples. Again, the idea is to teach UI lessons based on real world images that are funny and interesting for this type of discussion.

Lesson 11 – Don’t redefine concepts

curvedPlease look at the image on the left. Why on earth would someone call a banana a “curved yellow fruit”?! OK, it is curved, it is yellow and it is a fruit, but WTF?

If you don’t know how to call a certain thing, please ask the business analyst. Don’t try to create a name that is meaningful to you but meaningless to the user. Applications should have standardized concepts and the whole team should understand them. FYI: this field of research is called ontology and it is a great domain analysis tool.

Lesson 12 – UIs should never surprise the user

bibleThe UI should never surprise the user. If it does, then the designer has failed to do a good job. One example of this problem is a link that doesn’t look clickable. This is actually conflicting with the idea that UIs should be clear and intuitive. How can something be surprising and intuitive at the same time?

Joel Spolsky has explained this lesson on his bookUser Interface Design for Programmers. Refer to the book if you want to see more examples on this problem.

Lesson 13 – UIs should never scare the user

Users shouldn’t be scared by the UI. If they are, they won’t use the system correctly and you will fail as a software engineer. If something can go wrong, it is your responsibility to create restrictions that will prevent wrong actions (unless the business itself is risky – e.g., trade market).


Lesson 14 – Don’t ask users to do stupid things

The UI should never indirectly imply that something stupid can be done. You somehow tease the imagination of those people that have low IQ scores and have nothing else to do in life:


Lesson 15 – UIs should be easy to read

clockOne of the ideas behind simplicity is that UIs should be understandable and easy to read. The clock on the left is an example of how things can get critical if you must read it immediately. Of course you know the position of the numbers, but imagine different situations where you must read them (e.g., the clock is rotated 90 degrees). Since you need some seconds to calculate the numbers, this will distract (and even disturb) you more than needed. By the way, what time does the clock show right now?

Lesson 16 – Users should never ask “What is this?”

whatWhich animal is this? Is this a cat? A bunny? Some people say it is a small camel! Use your imagination😛

Users should never ask questions like “What is this?” when it comes to your UI. If they do, the designer has failed to create a good intuitive interface. UAT (User Acceptance Tests) is a great tool to find problems in this sense and you should talk to your team about this. Watch how your friends use the software application and ask them to do certain tasks (e.g., open a new document; create a new invoice, etc). You should also talk to the support team and see what people have been complaining about. You will learn a lot from that, trust me.

Lesson 17 – Don’t distract users with obvious messages

Please look at the next two signs and think. Are they useful in your opinion? What is the chance that you won’t see the stairs, but will see the sign? Or what is the real value of the second sign and its warning?


Please don’t add obvious messages to your UIs because they will distract (and probably disturb) your users. UIs should guide the user, not force them to look at every detail on the screen. Their eyes should do this job seamlessly.

Lesson 18 – Accessibility should work in practice

disabledWe all know that accessibility features (in general) are used by just a small group of users. But if you decide to implement them, please make sure they are really useful in practice. If you don’t know what I mean, please look at the image on the left.

Look, there are nice toilets on the second floor! Yes, but you need a crane if you are on a wheelchair.


Lesson 19 – Know your users

aliensYou can’t create good interfaces if you don’t know your users. Things can get more difficult if they are aliens, but it is always possible to figure out what they really want. Talk to your users (maybe using a translator helmet) and try to understand how the see the world (not only our world, but also their world).


Lesson 20 – Creativity Again

The last lesson of my previous article was about creativity and its powerful effects on our brains. I will keep creativity again as the last lesson of this second part because it is really worth doing so. Please look at the AXE marketing signs displayed at shopping centers and malls… they are simple, easy to understand and – most important – have a real marketing message.



This was the second part of my article about user interfaces and their lessons from the real world. What is most fascinating is the fact that we can take lessons from almost anything visual. All we need is to think about our problems and create a bridge to real world signs and objects. Stay tuned for the next articles.

Ten UI Lessons from the Real World

Learn UI concepts from real world pictures. Signs can teach valuable lessons in this regard and you should pay attention to them.

User interface isn’t a subject specific to computers. Every object in the real world has an interface and understanding it is crucial for a correct usage. Some objects are well designed and can offer different types of good reactions when we interact with them. Others aren’t that good and the designer might have missed the point.

Today I am going to teach some UI lessons based on signs and pictures of the real world. If you pay careful attention, you can realize that they do have lessons to be learned and you can use that knowledge to improve your UIs. This is a valid approach because we – as humans – have a common way of looking at things, be it a system UI, a coffee machine, a vacuum cleaner or signs on the roads. Signs on a road are my favorites because they guide the driver pretty much like icons guide users on a system interface. Let’s start our lessons.

Lesson 1 – Review your Icons

Please look carefully at the sign on the left. What is the meaning of that sign? Are there cows that bite and eat cars?!?

Actually, if you add an icon to your application, make sure it is understandable. Don’t try to create your own if you are not prepared for this task. UI designers should handle that. The biggest issue behind this problem is the fact that users have fear of clicking on things they don’t understand. This can get even worse if their money is involved in the action.

dentistaAnother problem related to icons is best explained when we look at the image on the right. At first glance, you might see a dentist with a patient. But if you look closer, you might note a dirty second meaning. Would you let your wife go to that dentist? I don’t think so…


Lesson 2 – Unrelated information shouldn’t be displayed together

nohuntingLook at this second example. By placing both signs together, it is inevitable to imagine people hunting children. When the worker placed the second sign, he probably saw the problem, but he didn’t care. He might have thought that the problem was with him, not with other people.

Trust me, this also happens on your UI. Please keep unrelated information clearly separated from other types of information.


Lesson 3 – Use the right punctuation or appropriate separators

dieselIsn’t that yummy? This might be good for your cholesterol levels.

Note that a missing separator clearly creates a misleading scenario that can ruin your business. Your interface can also suffer from that if you don’t pay careful attention. If you are not fully convinced, remember that when you read the same sentence several times, your brain starts to ignore or misinterpret it.

Lesson 4 – Redundancy increases complexity

scheduleThis sign is in Portuguese, but I don’t think we need a translation here. The problem is right ahead and it is difficult to understand why people keep repeating the same information again and again. What worries me is the fact that they do that too often.

Redundancy might be important in some cases (especially when it comes to security), but it can be avoided in most situations. Note that when we remove redundancy we improve simplicity.


Another type of redundancy is in the meaning of words. The next sign provides a good example of how bad this can be. Maybe they are really telling the whole truth…



Lesson 5 – Identify and remove conflicting ideas

Conflicting ideas can be deadly critical. Look at the three pictures below and think about their explanations: an ant that goes to the opposite direction of the sign, police that can’t turn right while normal people can do so, or a land full of poison just to protect living animals. What could be stranger than that?

The same concepts apply to user interfaces. It is very easy to think that you wouldn’t do such mistakes, but people do that and we have to admit. So look carefully at your UIs and search for conflicting ideas.


Lesson 6 – Hide unnecessary precision

precisionSometimes too much precision can be useless. Take a closer look at the next sign and let me know if that extra half mile is important to you. Information like that is inappropriate for the situation and only adds noise and clutter. Drivers usually have just a few seconds to read a sign and the more information you add, the worse it will be. The same applies to software UI.

Lesson 7 – Fix typos

Typos happen in every corner of the world (see pictures below) and it is your responsibility to fix them. It is quite common nowadays to have non-native speakers in the development team. So, besides typos, people can simply write wrong words because they don’t master the UI language.


Lesson 8 – Sentences with the right meaning

churchA sentence with a wrong meaning is worse than a typo. That’s the case of our next sign: If your worries aren’t strong enough to kill you, the church can give *them* a hand.

Sentences in the UI can also be misleading, mainly when the system tries to explain more than needed. Your UI – and its sentences – should be simple, small and clear enough to guide the users.

Lesson 9 – Alignment

alignmentAlignment is important because users don’t read text on the UI. They scan it. So, well aligned text and controls can help dividing the screen into digestible pieces of information. Unfortunately this isn’t the case of our next picture. After reading the first sign, I wonder if people hire cats to make burgers with their flesh.

Lesson 10 – Creativity saves the day

creativityAt last, remember that your creativity can connect the user with a whole new experience. Let your imagination fly when you build user interfaces, as it happens with the guys from the last picture. Good interfaces make users forget their computers pretty much like readers forget their books when they read good stories.



This article was an attempt to show my different point of view on user interfaces. The idea of showing real signs is just a funny way to teach concepts that can make applications more usable. Remember that everything around us can teach lessons, but only those that really pay attention can take advantage of that. This is a daily exercise that you should execute when you drive or walk on the streets. It is worth trying, trust me.