We usually write the time in one of two ways: the analogue 12 hour clock or the digital 24 hour clock. The 12 hour clock counts up to 12: it resets at midday. The 24 hour clock counts up to 24: it resets at midnight.

We use the notation “HH:MM”. HH is our stand-in for the hours value. MM is our stand-in for the minutes value.

🧩 Stating the problem

Let’s pose a problem: given any time in 24 hour clock, we want to format it as a 12 hour clock time. To achieve this goal, we’re going to implement a function formatAs12HourClock.

Given a time in 24 hour clock
When we call formatAs12HourClock
Then we get back a string representing the same time in 12 hour clock.

🧪 Our tests:

I expect formatAs12HourClock("09:00") to be "09:00 am"
I expect formatAs12HourClock("14:19") to be "2:19 pm"

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function formatAs12HourClock() {}

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function formatAs12HourClock() {}

formatAs12HourClock("05:30");

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function formatAs12HourClock() {}

formatAs12HourClock("20:10");

🧩 Comparing values

We have learned how to log values to the console. We can also compare two values. We check that a function produces some target output with a comparison.

We compare the current output of formatAs12HourClock("08:00") with the target output of "08:00 am" and ask: are these two values the same? We use a comparison operator to compare two expressions and check if they evaluate to the same value. We use the strict equality operator === to check if two values are the same.

Left equals Right

formatAs12HourClock("08:00") === "8:00 am";

=== checks if the values on the left and right of the operator are the same. We can think of formatAs12HourClock("08:00") === "8:00 am" as the question: “Is the return value of formatAs12HourClock("08:00") equal to "8:00 am" ?” This leads to the question:

What will the expression formatAs12HourClock("08:00") === "8:00 am" evaluate to?

✅ ❌ Boolean values

// using the strict equality comparison expression

console.log(42 === 10 + 32);
// logs true

console.log(10 * 5 === 60);
// logs false

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true === false;
5 == 2 + 4;
4 * 5 == "20";
3 * 2 === 6;
Math.min(3, 4, 5) === 4;
let mhairiName = "Mhairi";
typeof mhairiName === "string";
let mhairiAge = 28;
let isMhairiOldEnoughToDrive = true;
let kilometersMhairiDrivesToWork = 9.4;

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"hello Mhairi" === `hello ${mhairiName}`;
"${mhairiName} is 28" === `Mhairi is ${mhairiAge}`;
isMhairiOldEnoughToDrive;
isMhairiOldEnoughToDrive === true;
mhairiAge >= 18;
29 <= mhairiAge;
Math.round(kilometersMhairiDrivesToWork) === 9;

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console.log(10 + 32) === 42;

const calculation = 10 + 32;
const result = 42;
console.assert(calculation === result);

const calculation = 10 + 32;
const result = 42;
console.assert(calculation === result);

function formatAs12HourClock() {}
console.assert(formatAs12HourClock("08:00") === "08:00 am");

Clarity with arguments

It would be useful to have more information as to why this assertion failed. We can pass an additional argument to console.assert:

function formatAs12HourClock() {}

console.assert(
  formatAs12HourClock("08:00") === "08:00 am",
  `current output: ${formatAs12HourClock("08:00")}, target output: 08:00 am`
);

Let’s break down these arguments to make sense of what’s going on:

1. first argument - formatAs12HourClock("08:00") === "08:00 am" - the condition we’re checking
2. second argument - `current output: ${formatAs12HourClock("08:00")}, target output: 08:00 am` - a message string that will be logged to the console if the condition is false.

🧹 Refactor

We can tidy up the assertion even further. As we’re reusing the same expressions, we can store their result in variables with meaningful names so we can reuse them:

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function formatAs12HourClock() {}

const currentOutput = formatAs12HourClock("08:00");
const targetOutput = "08:00 am";
console.assert(
  currentOutput === targetOutput,
  `current output: ${currentOutput}, target output: ${targetOutput}`
);

Execute this code; we now get a log in the console:

Assertion failed: current output: undefined, target output: 08:00 am

🧰 Implementing the functionality

On line 3, the function is being passed a single argument "08:00". But our function ignores it: it doesn’t declare any parameters. We can parameterise the function and label the input as time:

function formatAs12HourClock(time) {}

According to our assertion, when we call our function with an input of "08:00" we need to create an output of "08:00 am". If we add "am" to the time, we’ll get the target output. We can update our function with a template literal, set the return value and then re-run our code including our assertion to check the function is returning the correct value.

📓 We can and should continually check our assertions to see if our function’s current output meets our target output.

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function formatAs12HourClock(time) {
  return `${time} am`;
}

const currentOutput = formatAs12HourClock("08:00");
const targetOutput = "08:00 am";
console.assert(
  currentOutput === targetOutput,
  `current output: ${currentOutput}, target output: ${targetOutput}`
);

✅ Nothing is printed to the console, so this assertion is passing 😎

💼 Checking different cases

So far we’ve only created assertions that check the function’s behaviour for times between midnight and midday. In these cases, there is a pattern: take the current time and add " am" to the end.

But this isn’t the pattern we need to follow for all times. To make sure our function works for all times, we need to write more assertions.

We need to assert that the function behaves correctly when the time is later than midday.

Before we think about any code, we should think about our problem. Separating problem and code lets us focus better. First we can focus on the data. Then we can focus on the code.

First, let’s think of an example time in 24 hour clock - we’ll pick 23:00.

Next, let’s work out what we expect our example time to be in 12 hour clock: 11:00 pm.

Now that we’ve thought about the problem, we can write the code. Let’s create an assertion for our function when passed an input of "23:00":

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function formatAs12HourClock(time) {
  return `${time} am`;
}

const currentOutput = formatAs12HourClock("08:00");
const targetOutput = "08:00 am";
console.assert(
  currentOutput === targetOutput,
  `current output: ${currentOutput}, target output: ${targetOutput}`
);

const currentOutput = formatAs12HourClock("23:00");
const targetOutput = "11:00 pm";
console.assert(
  currentOutput === targetOutput,
  `current output: ${currentOutput}, target output: ${targetOutput}`
);

Save this code to a file. Before you run it in Node, write down what you think will happen. Then run it with Node - compare what you saw with what you predicted.

An error is thrown

When we run the file with Node, we get an error in the console:

% node clock-example.js
/Users/dwh/CYF/clock-example.js:12
const currentOutput = formatAs12HourClock("23:00");
      ^

SyntaxError: Identifier 'currentOutput' has already been declared
    at wrapSafe (node:internal/modules/cjs/loader:1383:18)
    at Module._compile (node:internal/modules/cjs/loader:1412:20)
    at Module._extensions..js (node:internal/modules/cjs/loader:1551:10)
    at Module.load (node:internal/modules/cjs/loader:1282:32)
    at Module._load (node:internal/modules/cjs/loader:1098:12)
    at TracingChannel.traceSync (node:diagnostics_channel:315:14)
    at wrapModuleLoad (node:internal/modules/cjs/loader:215:24)
    at Function.executeUserEntryPoint [as runMain] (node:internal/modules/run_main:158:5)
    at node:internal/main/run_main_module:30:49

Node.js v22.4.1

When an error is thrown, the program stops and an error report is sent to the user.

As programmers, we will see a lot of errors. It’s useful for us to be able to read them.

Interpreting the output

Each line of output here tells us something useful.

The first line is:

/Users/dwh/CYF/clock-example.js:12

Your output was probably different. But it will have the same parts. Some text, then a colon (:), then a number.

Often, looking at one line of a file is enough to understand what’s wrong. So the message also shows us a copy of the line that caused the problem:

const currentOutput = formatAs12HourClock("23:00");

Then the output tells us the error message:

SyntaxError: Identifier 'currentOutput' has already been declared

We may not know what this means yet, but it’s something we can learn about.

Each line starting with “at” is showing us a “Stack trace”. We’ll skip over this for now. In the future we’ll see how it can be useful to us.

Finally, we have this line:

Node.js v22.4.1

We saw this error - let’s try to understand it:

SyntaxError: Identifier 'currentOutput' has already been declared

Knowing what we changed

It can be useful to remember when our code last worked, and what we changed since then.

When we just had the first 10 lines of code here, everything worked. When we added the rest, we got this error:

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function formatAs12HourClock(time) {
  return `${time} am`;
}

const currentOutput = formatAs12HourClock("08:00");
const targetOutput = "08:00 am";
console.assert(
  currentOutput === targetOutput,
  `current output: ${currentOutput}, target output: ${targetOutput}`
);

const currentOutput = formatAs12HourClock("23:00");
const targetOutput = "11:00 pm";
console.assert(
  currentOutput === targetOutput,
  `current output: ${currentOutput}, target output: ${targetOutput}`
);

Interpreting the error message

The error message tries to tell us useful information:

SyntaxError: Identifier 'currentOutput' has already been declared

When we get an error, we should make sure we understand all of the words in the error message. If we don’t, we should look them up or ask someone.

We saw this error:

SyntaxError: Identifier 'currentOutput' has already been declared

Now that we understand it, let’s fix it.

We’re not allowed to declare a new variable with the same name as an old one. Both lines 5 and 12 here try to declare a new variable named currentOutput:

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function formatAs12HourClock(time) {
  return `${time} am`;
}

const currentOutput = formatAs12HourClock("08:00");
const targetOutput = "08:00 am";
console.assert(
  currentOutput === targetOutput,
  `current output: ${currentOutput}, target output: ${targetOutput}`
);

const currentOutput = formatAs12HourClock("23:00");
const targetOutput = "11:00 pm";
console.assert(
  currentOutput === targetOutput,
  `current output: ${currentOutput}, target output: ${targetOutput}`
);

Fixing the error

We want to do multiple assertions. And we’re using variables in our assertions. But we’re not allowed to use the same name twice. The simplest way we can fix this problem is by changing the name of the second variable. Remember to also change where we use the variable, not just where we declare it!

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function formatAs12HourClock(time) {
  return `${time} am`;
}

const currentOutput = formatAs12HourClock("08:00");
const targetOutput = "08:00 am";
console.assert(
  currentOutput === targetOutput,
  `current output: ${currentOutput}, target output: ${targetOutput}`
);

const currentOutput2 = formatAs12HourClock("23:00");
const targetOutput = "11:00 pm";
console.assert(
  currentOutput2 === targetOutput,
  `current output: ${currentOutput2}, target output: ${targetOutput}`
);

Trying again

After making this change, let’s try running our code again. We get this output:

% node clock-example.js
/Users/dwh/CYF/clock-example.js:13
const targetOutput = "11:00 pm";
      ^

SyntaxError: Identifier 'targetOutput' has already been declared
    at wrapSafe (node:internal/modules/cjs/loader:1383:18)
    at Module._compile (node:internal/modules/cjs/loader:1412:20)
    at Module._extensions..js (node:internal/modules/cjs/loader:1551:10)
    at Module.load (node:internal/modules/cjs/loader:1282:32)
    at Module._load (node:internal/modules/cjs/loader:1098:12)
    at TracingChannel.traceSync (node:diagnostics_channel:315:14)
    at wrapModuleLoad (node:internal/modules/cjs/loader:215:24)
    at Function.executeUserEntryPoint [as runMain] (node:internal/modules/run_main:158:5)
    at node:internal/main/run_main_module:30:49

Node.js v22.4.1

Fortunately, we’ve seen this kind of error before. It’s exactly the same as the last one, but about the targetOutput variable on line 13, instead of the currentOutput variable on line 12.

Now the second assertion fails with the following message:

% node clock-example.js
Assertion failed: current output: 23:00 am, target output: 11:00 pm

We expect this. Our function just adds " am" to the end of the time, which only works for times in the morning. We have proven our code isn’t complete yet. Next we can fix it so that this test passes.

Our function works for morning inputs like "08:00". In this case, the function returns the target output of "08:00 am" as required. However, at the moment, the output of formatAs12HourClock("23:00") is "23:00 am".

💡 We need to execute some different logic when the time is beyond midday

We can interpret this behaviour as a question:

We need to make two changes to our code.

1. We need to do something different depending on whether the time is before midday. This is called running code conditionally.
2. And then we need to know what we do if the time is after midday.

We don’t need to solve the whole problem at once. First let’s work out how to do something different depending on the time. We can worry about what we need to do differently once we’ve solved this problem.

Recall: JavaScript programs are built up from sequences of declarations and statements.

In programming, we can use an if statement to execute some code when a given condition is true. In JavaScript, we can write an if statement as follows:

if (condition) {
  // do some code in here
}

The if statement consists of:

1. if keyword: this is the start of the if statement
2. condition: condition is an expression that evaluates to true or false. The condition must be in a pair of parentheses: ()
3. {}: a block statement: any code we want to execute if the condition is true goes inside the curly braces here

We can represent this with a diagram too:

function checkDivisibility(a, b) {
  if (a % b === 0) {
    return `${a} is divisible by ${b}`;
  }

  return `${a} is not divisible by ${b}`;
}

console.log(checkDivisibility(10, 2));
console.log(checkDivisibility(50, 3));

function getCountryCode(phoneNumber) {
  if (phoneNumber.startsWith("+44")) {
    return "UK";
  }
}

getCountryCode("+447831620328");
getCountryCode("+989871783972");

For formatAs12HourClock our strategy for inputs like "23:00" involves checking if the hours value is less than 12. For this purpose, we can use the greater than comparison operator >.

> will check if the value on the operator’s left side is less than the value on the operator’s right side.

So 3 > 12 evaluates to false, as 3 is not greater than 12.

So provided we have an expression for hours, we can write an if statement as follows:

if (/* here goes an expression here that evaluates to the hours */ < 12) {
  // do code to format the 12 hours
}

To complete the logic, we can form a sub-goal 🧶 🧶 sub-goal A sub-goal is a goal for a smaller problem that makes up some bigger problem .

Any time we’re solving a problem, we can define a goal - a thing we need to achieve to consider the problem solved. We can break a problem into smaller problems, each with its own sub-goal. The problem-solving process involves continually breaking down problems into smaller manageable problems, each with its own sub-goal.

For the implementation of formatAs12HourClock, we can form a sub-goal as follows:

🎯 Sub-goal: Find the hours value from the time input

Given a time string we need to access the first 2 characters of the string which represent the hours.

Strings are zero-indexed. Index means position, so zero-indexed means we start counting character positions from 0 onwards.

Here are the positions/indexes for "23:00"

In JavaScript, we can use square bracket notation to access specific characters in the string using the index.

time[0]; // evaluates to "2"
time[1]; // evaluates to "3"
time[2]; // evaluates to ":"
// etc

Square bracket access will only give a single character. We must use another method to extract multiple characters from the given string.

✂️ Extracting a slice

To extract 1 or more characters from a string, we can use a function called slice 🧶 🧶 slice slice is a function that can take 2 arguments: a start index and an end index. slice will return a section of the string from the start index up to but not including the end index.

time; // holds the value "23:00"

time.slice(0, 2); // will access the characters below

So time.slice(0,2) will evaluate to "23" when the time is "23:00".

Finally we must convert "23" to the number 23, otherwise we can’t compare this value properly.

JavaScript uses different ways to compare values depending on their types. If you compare two strings (which may contain numbers), it will do something different than if you compare two numbers.

We can use the Number function to convert the string into a number.

Earlier we defined a sub-goal to find a value for the hours from the time input. We’ve found that Number(time.slice(0,2)) is an expression that evaluates to the hours from time. So we can write an if statement using this expression:

if (Number(time.slice(0, 2)) > 12) {
}

If the time is "23:00" then the expression Number(time.slice(0, 2)) > 12 will evaluate to true and the body of the if statement will be executed.

This if statement is implementing the following part of the diagram from earlier:

Before we worry about how we handle times in the afternoon, we can check that we’ve solved this sub-goal.

We can check that we are correctly identifying times in the afternoon by adding in our if statement (which we think is correct), with a placeholder body:

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function formatAs12HourClock(time) {
  if (Number(time.slice(0, 2)) > 12) {
    return "Don't know how to handle times in the afternoon yet";
  }
  return `${time} am`;
}

const currentOutput = formatAs12HourClock("08:00");
const targetOutput = "08:00 am";
console.assert(
  currentOutput === targetOutput,
  `current output: ${currentOutput}, target output: ${targetOutput}`
);

const currentOutput2 = formatAs12HourClock("23:00");
const targetOutput2 = "11:00 pm";
console.assert(
  currentOutput2 === targetOutput2,
  `current output: ${currentOutput2}, target output: ${targetOutput2}`
);

If we run our code, we expect the current output of the 23:00 test-case to have changed. It won’t be correct - the assertion will still fail. But if it hasn’t changed, we know our if statement is wrong.

% node clock-example.js
Assertion failed: current output: Don't know how to handle times in the afternoon yet, target output: 11:00 pm

Even though we know the code on line 3 is incorrect, this was a useful step. It allowed us to run our code more often, and check that we’re on the right track.

Now we can focus on just one problem: how to handle times after midday (i.e. fixing line 3). We don’t need to worry about both detecting the time and handling it.

If the output of this assert still printed "23:00 am" we would have stopped here and debugged that. Again, we could focus on just one problem.

Now we can think about what we do when we’ve identified a time is after midday.

Earlier, we observed what to do when the time goes beyond midday: subtract 12 from the hours time to get the new hours for the 12 hour clock time.

Before writing code, we can define our approach in steps:

Starting with an input like "23:00":

Now we can format the string using our approach from earlier: we’ll need to append "pm" to the string expression and subtract 12 from the hours. So we get the following:

if (Number(time.slice(0, 2)) > 12) {
  return `${Number(time.slice(0, 2)) - 12}:00 pm`;
}

The return statement above implements the following steps we set out earlier:

Now we can re-run our assertions from earlier to check our function behaves as target.

Let’s look at our code, which passes all of the tests we’ve written:

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function formatAs12HourClock(time) {
  if (Number(time.slice(0, 2)) > 12) {
    return `${Number(time.slice(0, 2)) - 12}:00 pm`;
  }
  return `${time} am`;
}

const currentOutput = formatAs12HourClock("08:00");
const targetOutput = "08:00 am";
console.assert(
  currentOutput === targetOutput,
  `current output: ${currentOutput}, target output: ${targetOutput}`
);

const currentOutput2 = formatAs12HourClock("23:00");
const targetOutput2 = "11:00 pm";
console.assert(
  currentOutput2 === targetOutput2,
  `current output: ${currentOutput2}, target output: ${targetOutput2}`
);

Inside the formatAs12HourClock function we do exactly the same thing twice.

There are a few reasons this isn’t ideal.

1. It’s not clear what this value represents. You can read it and work it out, but that takes some time.
2. Doing the same thing twice is slower than doing it once.
3. In the future if we need to change this code’s implementation, we would need to change it twice.
   Right now our code assumes the hours in a time are always two digits (like 05:00). What if we wanted to support single-digit hours, like 5:00? We would need to make the same change to both lines. It would be easy to change one line and forget the other, which would lead to a bug.

Refactor

Once your code passes your test, look for ways you could make your code better. This doesn’t mean changing what it does - the code works. It means changing how it’s written.

This is called refactoring 🧶 🧶 refactoring To refactor means to update our code quality without changing the implementation. Changing how it does something, not changing what it does. .

We can refactor our code to remove this duplication by introducing a variable:

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function formatAs12HourClock(time) {
  const hours = Number(time.slice(0, 2));
  if (hours > 12) {
    return `${hours - 12}:00 pm`;
  }
  return `${time} am`;
}

const currentOutput = formatAs12HourClock("08:00");
const targetOutput = "08:00 am";
console.assert(
  currentOutput === targetOutput,
  `current output: ${currentOutput}, target output: ${targetOutput}`
);

const currentOutput2 = formatAs12HourClock("23:00");
const targetOutput2 = "11:00 pm";
console.assert(
  currentOutput2 === targetOutput2,
  `current output: ${currentOutput2}, target output: ${targetOutput2}`
);

This code does exactly the same thing as the previous code. But it is better in a few ways:

1. We can now tell more easily what this expression represents. The variable name conveys: it’s the hours from the time.
2. We only compute the hours once, not twice, which will be a little bit faster.
3. If we need to change how we identify the hours (e.g. to support single-digit hours), we only need to update one bit of code. Both lines 3 and 4 will automatically use the same value, because they’re referring to the same variable.

We started off writing one test for our code - checking that it correctly handled the input 08:00. We wrote an implementation that passed all our (1) tests!

Then we realised there was a bug - it didn’t handle times after midday correctly. So we wrote another test - for the input 23:00. We saw our implementation failed that test. And we fixed it. And we had an implementation that passed all our (2) tests!

When will we be happy that our implementation works for all possible inputs? When do we have enough tests?

Groups of input

One way to approach this is to think about what groups of input our problem may have.

We’ve already identified two examples of groups of input to the problem of converting 24 hour clocks to 12 hour clocks: Times before midday and times after midday.

One way to find extra cases to consider (and extra tests to write) is to try to think of different groups of input.

For example, some times are exactly on the hour (end in :00) and other times have a non-zero number of minutes.

Edge cases

Another way to consider this question is to think about what edge cases there are in the problem.

Some example edge cases for this problem are:

00:00

The minimum time, which is 12:00 am in 12 hour clock.
This is also the only hour that is bigger in 12 hour clock than 24 hour clock.

24:00

The maximum time.

12:00

Where time changes from am to pm. The edge between morning times and afternoon times.

Often these edge cases are where bugs happen.