Month: July 2017

JavaScript Corners – Part 8
References (Continued)

JavaScript Corners – Part 8
References (Continued)

Given an object o  with a member function f  that prints out what the this value is:

We know what the following prints:

And we know what the following prints1:

I always thought that the difference came down to the fact that o.f()  is actually invoking a different operator — something like a “member call operator”.

However, what do you think the following prints?

My guess, up until today, would have been that this prints “global”, since with the parentheses, this is no longer invoking the member call operator, but is instead invoking the call operator.

But I was wrong. There is no such thing as a “member call operator”. Rather, the “call” operator just behaves differently depending on whether the target of the call is a value or a reference2.

So this actually prints “o”.

But hang on. Why didn’t the parentheses coerce o.f to a value?

One might have expected the parentheses to automatically dereference  o.f, something like the following examples that use the logical OR and comma operators to coerce the target to a value instead of a reference:

Indeed, this could have been the case for bare parentheses as well, but the language designers chose not to do it that way, so that the  delete and  typeof operators still work when extraneous parentheses are provided:

 


  1. assuming the use strict directive isn’t provided in this case 

  2. To be more accurate, the target also behaves differently depending on whether the target reference refers to a property of an object vs a variable in an environment record 

JavaScript Corners – Part 7
Calls and With Statements

JavaScript Corners – Part 7
Calls and With Statements

Here’s a quick one. What does the following print? (Assuming not in strict mode)

In non-strict mode, the naked function call foo() gets a  this value that is the global object. So the first case prints “Global”.

In the second case, we’re invoking foo as a member of  bar, and so the this value is bar (it prints “Bar”).

The last case is the most interesting, and the most useless (since with statements are strongly discouraged, and cannot be used outside of non-strict mode). The this object in this case is actually bar. JavaScript recognizes that the function foo here is being invoked within the context of a with statement, and implicitly uses the bar object. This prints “Bar”.

JavaScript Corners – Part 6

JavaScript Corners – Part 6

In what order does the following evaluate?

TL;DR Answer


Step 1: Variable Access

First off, what does this code even mean? If you’re not intimate with JavaScript, this might seem like a very confusing line of code. In fact, even if you’re familiar with JavaScript, this can be confusing.

So let’s break it down, starting with:

The expression a loads the value a from the surrounding scope1. This is done by searching up the scope chain until a is found.

There are a number of different types of scopes in JavaScript, including those that refer to blocks (like the inside of a for-loop), functions (the contents of a function), objects (scopes that are created using a with statement, or the global scope).

For our purposes, let’s define a at the global scope. You’ll see why in a moment. Assuming we’re working in Node.js, the global object is called global, and properties of the global object are part of the global scope2.

But, since we’re interested in the order of evaluation, it would be useful to know when the value a is accessed. Luckily, in JavaScript, you can define properties that have a getter and/or setter, which we can use to log when the global variable is accessed:

Great! We can now see when the global variable “a” is accessed. There aren’t many languages where you can do that. Hooray for JavaScript.

We may want to define more globals this way, so lets refactor this to use a helper:

Step 2: Calling the function

Now let’s look at the following statement:

This is, unsurprisingly, a function call. It first evaluates a, as indicated above, by fetching a from the current scope. Then it calls a as a function. Nothing special going on here.

But to make this work with our a, we’re going to need to make sure that a is defined as a function, and not the value 42. So let’s change our getter to return a function:

To answer our original question, we’re going to need to create a whole bunch of functions. So let’s again refactor this into a helper:

Step 3: Member access

The expression x[y], in JavaScript, is a property lookup. It evaluates the expressions x and y, and then finds the property on the object x that has the name resulting from the expression y. Here’s a snippet that illustrates this:

If you’re not very familiar with JavaScript, it’s important to note here that the property name used here is "myProp", and not "y". The property name is the result of evaluating y.

Again, it will be useful to know exactly when the property is accessed, so let’s use a getter instead:

Here I’ve just used the ES6 getter syntax, rather than using defineProperty.

As before, we’re going to need to do this a few times, so let’s create a helper function:

Step 4: Assignment

The last piece of the puzzle is the assignment operator. Consider the following code:

The assignment operator, like the other operators so far, will evaluate the each operand, and then perform some operation on the results. In the above case, x is evaluated, and then y is evaluated, and then the result of y is assigned to the result of x.

But wait. What do you mean “the result” of x?

The model here that JavaScript uses internally, is that x actually evaluates to a reference. This is a type in JavaScript which you’ve probably never heard of. A reference value consists of two components:

  • A base value, that tells you what container the value is stored in
  • A name, that tells you which value in the container is being referred to

In this case, the expression x evaluates to a reference that has the following attributes:

  • A base value that is the global object
  • A name that is the string "x"

In other words, the reference value is something like the English description “the property x on the global object”. When you assign to x, you are assigning to “the property x on the global object”. When you delete x, you are deleting “the property x on the global object”.

The expression y also evaluates to a reference, but the assignment operator coerces that reference to the actual referenced value. The same thing is done in expressions such as x + y or x(y).

Here’s another example of an assignment:

In this case, the base value of the reference is the object x, and the name is y.  The assignment sets the value referred to as “the property ‘y’ of the object x”. Similarly, you can do delete x.y to delete “the property ‘y’ of the object x”.

In a more detailed consideration of the above example, x and z evaluate to references. Both x and z are then coerced to values (dereferenced, by fetching the property or variable), and then a third reference is created refers to the property y on the base object x.

But, what order does this occur in? To find out, let’s use our trusty helper functions:

This might come as a little bit of a surprise. The expression x is evaluated before the expression y, and then the assignment takes place. In some ways, one expects the opposite — one expects that the left hand side of an assignment is not considered until the right hand side.

This seems to be a general rule in JavaScript. Operands are evaluated from left to right, and then the operator is executed. Perhaps an exception to this rule-of-thumb, is that the short-circuiting operators such as && must necessarily execute part of the operation without all the operands fully evaluated.

Side note: in languages such as C++, the order of the left and right hand side of a most operators is not defined. The compiler can chose to evaluate them in whatever order it thinks is best, or even evaluate them simultaneously (e.g. if the CPU has multiple cores). JavaScript is different, in that the specification lays out a specific, unambiguous ordering.

We can follow this to its logical conclusion, and determine the order of execution of the whole of the original program in question:

Can we abuse it? (Advanced)

The reason I started looking into this at all, is that I was trying to discover a way to “see” references. They are objects that exist in the execution model, but are never shown explicitly to the user of the language, so do they really need to exist at all?

This is import to me, because I’m writing a JavaScript compiler, and need to know whether references are best left as just a description mechanism in the ECMAScript specification, or if they should be considered to be real entities with real allocated memory in the runtime.

So, can we design an example, that unequivocally proves that there must be a reference allocated in memory at some point?

Here’s my attempt:

What I’ve done here is break up the x[y] = z assignment using the await operator. The await operator will suspend the statement (and the rest of the async function), allowing us to swap out various things in the environment to see if we can mess with the operation while it is suspended. What we’re trying to prove here, is that the reference itself must be preserved in memory, from the time that the operation is suspended, to the time that it is resumed (when z is resolved).

To make it even more apparent, I’ve executed the async function multiple times, trying different ways to “mess” with the pending operations.

Conclusions

This experiment has proven to me that references are “almost” tangible objects. We can see that they must exist in memory under some circumstances, and that they are not simple “pointer” values — they must refer to both the object and the property name.

This leads to some interesting results when it comes to the order of evaluation of various expressions. While this knowledge isn’t needed for everyday programming scenarios, it helps to have a deeper understanding of what’s going on so that we know where the limit lies.

 

 

 

 


  1. Known in ECMAScript as a Lexical Environment 

  2. There is an interesting recursion here, since the value global here is also a globally scoped binding, which means the global property on the global object points to itself. You can see this if you have a statement like console.log(global.global.a) 

Coding on Paper in an Interview

Coding on Paper in an Interview

My company is in the process of hiring a back end or full stack software engineer. During the interview process, I often get complaints from candidates when I bring out an old fashioned pad of paper and pencil and ask them to write some code for me (or I offer for them to code on the whiteboard).

This is 2017, and we have syntax highlighters, auto-complete, Stack Overflow, etc. Why would I want to test a candidate’s ability to code on paper?? This seems barbaric and absurd. Like asking a racing car driver to ride around a track on a bicycle to see how well he handles the corners. Surely we’re past this antiquated form of testing in this day and age?

Well, no. Let me share my side of the story.

I need people who can code

The position is for a software engineer. I need to know that you can code1. It’s quite simple.

We can have a good chat about Azure, about how you parted the red sea, or how you helped get a man on the moon. But in the end, you’re here to code, so you need to show me that you can.

Hiring a coder without seeing them code, is like hiring a chef without tasting their food.

The number one best way to test your real-world coding ability would be to have you work with me for 3 months, but that’s not a practical way to screen people for a new position.

Another way for me to establish your level of skill would be for me to look at work you’ve done in the past. AKA your public contributions to GitHub, Stack Overflow, blogs, etc. I am perhaps one of the few managers who will actually clone your GitHub repos and look through your code, your commit history, your documentation, your blog posts2, your answers on forums, etc.

A quick side note on this: remember that I’m also a software engineer, like you. I understand perfectly well that there are times when you need to throw a heap of “poor” code together, just to get something done quickly, or because it’s just an experimental throw-away project that you were working on in your spare time. I won’t judge you on this. The signature of good style and craftsman ship will still shine through, even in your darkest code, so I still read it.

But, I can’t just look at past work you’ve already done. I also need to look inside your head. I need to see those cogwheels spinning when you’re working on a problem. I don’t just want to see the final outcome, I want to know what thoughts went through your mind on the journey to get there. The best way to do this is for us to work on a piece of code together, in an interview. Here you can explain to me what you’re thinking, and ask questions3

The code itself is the easy part

In the past, I used to ask two coding questions in an interview. I started with an easy one, and then would progress to the harder question. But I became tired of the emotional distress of watching people struggle through the harder question, and often eventually give up.

So, now I just ask the easy question4.

In fact, here it is. I’ll give it to you.

Write a function that adds up a collection of numbers.

That it.

That’s it?

Yes. That’s it.

In fact, I’ll give you the5 answer too6 :

Here’s another one:

Side note: One person has pointed out that they would just use the Sum function that .net provides. Well done. Ten brownie points for you7. But I then I will follow on by saying something like “Somebody on the .net development team had to write that builtin Sum function. Pretend it was you. How would you write it?”

There are obviously many different ways to write some code that satisfies the initial requirements statement of this test question. This is true in the real world as well — feature requests come in many different forms, to varying degrees of specification, and you need to be able to dig in and understand the problem that the user is trying to solve, and explore different solutions and the effectiveness of each.

Be resourceful. Dig in.

I don’t care whether or not you remember the syntax for generics or extension methods in C#. I don’t care if you remember what the C# equivalent to a fold/reduce function is. These are questions that Google or Stack Overflow can answer in the real world.

I don’t care about your spelling. This is something a spell checker would check in the real world.

I want to see how you engage a problem. When you’re actually working on the job, you’re going to have all sorts of weird and wonderful requests thrown your way. How do you deal with them? Do you blindly start pecking at the keyboard, or do you dig into what’s actually being asked?

If you need some tool to help you, do you say “this company is so stupid, they don’t even provide xyz!”, or do you say “how do we get xyz?”.

The same is true in an interview. Do what you need to do to get the job done. If you need to understand the context more, then ask me. If you need auto-complete and an IDE, or Google, ask for a laptop (actually I typically offer one).

At the very least, I expect you to ask me what kind of numbers we’re adding together.

If you need help remembering the way something works, or the syntax for something, ask me. Let’s work through it together. I want to know that you can work effectively with me and others on the team. I want to know that you feel comfortable asking for advice, that you’re not the kind of person to suffer in silence.

If you think that the whole thing is stupid, let me know. I want to know that the people on my team can challenge me and can stand up to being challenged themselves.

It’s about why

It’s not about what your answer looks like. It’s about why you made certain choices. Were there performance concerns you were worried about? What usage scenarios were you planning for — and did you ask? Is one language construct better than another from a maintainability standpoint? What approaches did you consider, and how did you weigh up each?

If you’re [un]lucky enough to interview with me, keep in mind that the main thing I’m looking for is your reasoning and approach when encountering a problem. Talk me through your thinking.

It’s a toy

Yes, this coding exercise is a toy example. And yes, I’ll make stuff up as I go. If you ask whether the numbers need to be integers or floats, I’ll make the answer up as a I go, and it might be different every time I do an interview.

But put yourself in my shoes for a moment. In an interview setting, we only have time to play with a toy example. We don’t have time for me to explain a real problem and have you craft a real solution for me. I need to know how you code, and a toy example is the only tool in the box that can get the job done effectively in a short span of time.

Hopefully by now you understand that it really won’t help you to use a laptop in my interview. The things I’m looking for are things where a laptop won’t help you. Sorry. But do whatever makes you feel comfortable.


  1. …and that you can communicate effectively, and work well with my team, and have the relevant experience. But all of this is pointless if you can’t code. 

  2. I don’t care what the blog posts are about actually — I will evaluate you based on passion, ability to communicate clearly, etc. Bonus points if your blog is about coding, and I can get a two-for-one evaluation on communication and coding. 

  3. Like maybe “WTF are we doing this for?” 

  4. [Edit:] Since writing this post, I’ve changed my mind about the second, “hard” question. I now ask the hard question as well, depending on how much time is available. I might also ask a “time-boxed” version of the hard question — saying something like “do as much as you can of this in 10 minutes”. Part of the reason is that I want to give you the opportunity to present your skills on structurally different questions. 

  5. *an 

  6. Since the position is for an Azure developer, the most common language of choice for this question is C# 

  7. Even if you didn’t know that it existed, I’d give you points for saying “I’m sure this function already exists in some library somewhere. I would probably first work out if I can use that, rather than reinventing the wheel”