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 isMhairOldEnoughToDrive = true;
let kilometersMhairiDrivesToWork = 9.4;

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"hello Mhairi" === `hello ${mhairiName}`;
"${mhairiName} is 28" === `Mhairi is ${mhairiAge}`;
isMhairOldEnoughToDrive;
isMhairOldEnoughToDrive === 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 4, 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". We can add "am" to the time to 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 after midnight and before midday. In these cases, there is a pattern: take the current time and add "am" to the end.

Now we need to assert that the function behaves correctly when the time is later than midday. 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}`
);

🔄 Reusing variable names

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

SyntaxError: Identifier 'currentOutput' has already been declared

In this case, identifier means name of a variable, so in a variable declaration like

const currentOutput = formatAs12HourClock("08:23");

currentOutput is the identifier.

When an error is thrown, it means the execution of the program stops at this point and an error report is sent to the user. However, we want to do multiple assertions.

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}`
);

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

Now the second assertion fails with the following message:

Assertion failed: current output: 23:00 am, target output: 11:00 pm

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:

flowchart LR

A{Is the time before midday?} -- true --> B[Add am to time string]
A -- false --> C[???]

We need to work out what to do in the case when the time input is later than midday.

🧭 Describing the strategy

Let’s describe the strategy for dealing with an input that is after midday.

Earlier we observed that when the time goes beyond midday then we can 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 using a flowchart:

Starting with an input like "23:00":

flowchart LR

A[extract the hours, '23', from the time '23:00'] --> B[convert '23' to a number, 23]
B --> C{check: Are the hours greater than 12?}
C -- false --> D[add am to time string]
C -- true --> E[subtract 12 from the hours, to get 11]
E --> F[add 'pm' to the rest of the time, to get '11:00 pm']
F --> G[return the new time]

This approach involves running some code conditionally. In this case, we’re only going to continue doing Steps 4-6 if the condition hours are greater than 12 is true.

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
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:

flowchart LR
    IC{condition}
    IB[Execute code in body]
    EXIT([End of if statement])
    IC --> |true| IB
    IC -.-> |false| EXIT
    IB --> EXIT

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

Number(time.slice(0, 2)); // evaluates to 23

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:

flowchart TD

A{Check: Are the hours greater than 12?}

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

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

flowchart LR

D[Step 4: subtract 12 from the hours]
D --> E[Step 5: add 'pm' to the rest of the time]
E --> F[Step 6: return the new time]

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