In programming we often want to reuse our work. Consider the string: "Hello there"

Suppose we want to create different greetings for different people, like: "Hello there, Alicia" "Hello there, Barny"

We can use a variable to store this string and reuse it. How can we create a variable 🏷️ 🏷️ variable A variable is a label for a piece of data. We assign a piece of data to a label and then refer back to this label, in place of the data.

We can create a variable in our program by writing a variable declaration 🧶 🧶 declaration A declaration is an instruction that binds an identifier to a value. , like this:

const greeting = "Hello there";

Break down the different syntactic elements of this variable declaration:

• const is a keyword used to indicate we’re creating a variable.
• greeting is the identifier - it can be used to refer to a variable after it has been declared.
• = is the assignment operator. It means assign to the label greeting the value of the expression on the right hand side.
• "Hello there" - this is the expression whose value we’re assigning to the label greeting.

Try it yourself Watch and follow along

Type this variable declaration into the REPL:

const greeting = "Hello there";

Now refer to the label greeting in the REPL:

`${greeting}, Alicia`

Our greeting variable is stored in memory. We can reuse it to build more expressions:

`${greeting}, Barny`

We just used backticks to create a template literal.

`A template literal places ${expressions} inside strings;

With template literals, we can insert expressions into strings to produce new strings. Any time we want to reference a variable inside a template literal we use a dollar sign $ and a set of curly braces {}. We can put any expression (e.g. a variable name) inside the curly braces. The value that expression evaluates to is then placed inside the string.

When an operation uses an expression, that expression is immediately evaluated, and how it was written is forgotten about. That means that the greetAlicia variable is the same in all three of these cases:

const greetAlicia = "Hello there, Alicia";

const name = "Alicia";
const greetAlicia = `Hello there, ${name}`;

const greeting = "Hello there";
const name = "Alicia";
const greetAlicia = `${greeting}, ${name}`;

The greetAlicia variable doesn’t remember whether you used variables to make it or not - in all three cases, greetAlicia contains the string "Hello there, Alicia". Once a value is made, it doesn’t matter how it was made.

A variable declaration is an example of a declaration 🧶 🧶 declaration A declaration is an instruction that binds an identifier to a value . It has the effect of creating a variable.

let versionNumber = "2.0.0"; // declaration
versionNumber = "2.0.1"; // statement

The code above has one variable declaration and one statement.

1. The first line is a declaration - creating a variable versionNumber with a value of "2.0.0"
2. The second line is a statement - reassignment 🧶 🧶 reassignment Reassignment means changing the value associated with an identifier. of the value of versionNumber to "2.0.1"

In this example, we’ve used the let keyword to declare a new variable. The let keyword allows us to create new variables like the const keyword.

However, we can reassign the value of a variable that is declared with the let keyword.

If we’d used const to declare versionNumber, we wouldn’t be allowed to reassign it a new value.

In JavaScript, we build up programs by combining declarations and statements.

Now, instead of adding or multiplying numbers, we’ll consider 10.3.

🤔 “What is the nearest whole number to 10.3?”

The process of finding the nearest whole number to a decimal number is called rounding. So we could rephrase our question as:

🤔 “What does the number 10.3 round to?”

♻️ Reusing instructions

There is no operator for rounding the number 10.3 in JavaScript. But we will want to round numbers again and again. We should use a function 🧶 🧶 function A function is a reusable set of instructions. .

Math.round is a function. Because a function is a reusable set of instructions, Math.round rounds any number.

Functions usually take inputs and then apply their set of instructions to the inputs to produce an output.

Try it yourself Watch and follow along

1. Write Math.round in the Node REPL
2. Hit enter to evaluate our expression

The REPL output [Function: round] is telling us Math.round is a function.

📲 Calling a function

For our function to work, we need Node to read the instructions and execute 🧶 🧶 execute Execution means the computer reads and follows instructions. them. Write the following in the REPL:

Math.round(10.3);

Notice the ( and ) brackets after the name of the function and a number inside the brackets. These brackets mean we are calling the function. The number inside the brackets is the input we’re passing to the function.

Math.round(10.3) is a call expression; read this as:

“apply the set of instructions for Math.round to the number 10.3.”

If we type Math.round(10.3) then we get the result 10. So we say that Math.round(10.3) returns 10.

A call expression is an expression which evaluates to the value returned by the function when it is called. So the expression Math.round(10.3) evaluates to the value 10.

If we assign that expression to a variable, or use it in a string, we’ll get the value 10. So we can write:

const roundedValue = Math.round(10.3);

or we can write:

const roundedValueInAString = `10.3 rounds to ${Math.round(10.3)}`;

Both of these instructions evaluate the call expression Math.round(10.3) to the returned value 10 as soon as the call expression appears. The variable roundedValue will have a numeric value 10 (just like if we’d written const roundedValue = 10;), and the variable roundedValueInAString will have a string value "10.3 rounds to 10".

So far we’ve seen how expressions can be evaluated using the Node REPL. The Node REPL is useful for evaluating expressions quickly.

But usually, our programs have many instructions, and we want to keep and re-run them instead of typing them out each time. So we save our instructions in files. Node can also execute instructions written in a file.

We use the node command to run a JavaScript file in the terminal. A JavaScript file ends with .js - this is the “file extension”.

Let’s suppose we have a file age.js. We run the command node age.js. This terminal command is an instruction to execute the program written inside age.js. Our program has five lines. So the computer will read and execute the program one line at a time:

const yearOfBirth = 1990; // declaration
const currentYear = 2023; // declaration

currentYear++; // statement
`I am ${currentYear - yearOfBirth} years old`; // statement

Once the computer executes these statements, the execution of the program is complete. But we’re left with a problem. With the REPL, when the user inputs an expression statement or declaration, the computer reads and executes the line and immediately prints feedback to the terminal. With a file, the computer will execute each line sequentially until completion without printing the values of each expression it evaluates.

So this new problem can be expressed as a question:

❓ Problem

“How can we check what the values evaluated to in our program during execution?”

Printing to the terminal

To look at values when our program runs, we can use a function called console.log.

console.log logs the result of expressions while our program is executing. This is very useful for complex programs when we need to check what values expressions evaluate to at specific moments of our program execution.

Let’s see how to use console.log . In a file called example.js, write the name of the function console.log.

console.log;

If we run this file with Node, we won’t be able to see anything in the terminal. As with Math.round we need to use the syntax for calling a function. Add brackets after the function name:

console.log("hello there!");

We should see the string "hello there!" logged out in the terminal.

🗣️ Recall: A programming language is a set of rules for writing computer instructions.

So we need to understand what happens when we break those rules.

Let’s take an example:

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const firstName = "Francesco;
const age = 33;
const nationality = "Italian";

On line 1, we have a variable declaration, but the string has a missing " We’re not obeying the syntactic rules for JavaScript: the rules for writing expressions, statements and other parts of the language.

When we execute the code above, we get this:

const firstName = "Francesco;
                  ^^^^^^^^^^^

Uncaught SyntaxError: Invalid or unexpected token

We get a SyntaxError message. This error message is telling us that we’ve broken the rules of the language.

const volunteer = "Shadi";
const volunteer = "Abdi";

const volunteer = "Shadi";
volunteer = "Hinde";

console.log(Math.round(10.3);

Saving return values

We can store the return value of a function in a variable. Function calls are also expressions. This means their value can also be stored in variables, just like with operations on numbers or strings.

Suppose we have a file arithmetic.js containing this code:

const result = Math.round(10.3);

When this program is executed, it creates a variable called result and assigns to it the return value of the function, in this case the rounded number.

So result will have a value of 10.

🔭 Logging and returning

Most functions return values we can use in our program.

Math.round takes a single input, does a calculation and then returns a value that we can use when our program is running.

Some functions don’t produce useful return values in our running program; but they can still cause effects.

const result = console.log("hello world");

When this program runs, the variable result will evaluate to undefined. undefined is a data type in JavaScript which usually means no value has been assigned. Unlike the number data type, which contains many possible values (1, 2, 10.3, etc), the undefined data type has exactly one value, undefined.

This can feel confusing as console.log is a function with a set of instructions. console.log does have an effect: it logs values to the console. However, console.log doesn’t produce an output that we can use inside the rest of our running program.

Let’s begin with this problem:

Given a decimal number I want to convert it into a percentage format.

For example, given the decimal number 0.5 we return the string "50%". Given the decimal number 0.231 we return the string "23.1%".

Restating the problem

Our function must convert any decimal to a percentage. We have used functions already. Here are some functions we’ve used:

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console.log("hello world"); // logs "hello world" to the console
Math.round(3.141); // evaluates to 3

All these expressions are function calls: we’re passing input ("hello world" or 3.141) to the functions (console.log or Math.round) to use their functionality. Math.round and console.log are functions that the JavaScript language designers have written and stored inside the language, because everyone needs them all the time.

No such pre-built function converts any number to a percentage, so we must write our own. We’re going to create a function called convertToPercentage with the following requirements:

Given a number input

When we call convertToPercentage with the number input

Then we get back a string representing the percentage equivalent of that number.

Here are some examples:

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convertToPercentage(0.5); // should return "50%"

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convertToPercentage(0.231); // should return "23.1%"

Useful expressions

It is often helpful to solve a problem in one specific instance before doing it for all cases.

We’re not going to define our function yet. Instead we will work out what our function should do. Then we’ll define a function which does the same thing.

In programming, we always try the simplest thing first. Let’s consider how to convert just one number to a percentage. Look at this variable declaration:

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const decimalNumber = 0.5;

We want to create an expression for the percentage using the value of decimalNumber. To convert to a percentage, we will multiply the number by 100 and then add a "%" sign on the end.

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const decimalNumber = 0.5;
const percentage = `${decimalNumber * 100}%`;

Recalling template literals, the expression in the curly braces will be evaluated first and then inserted into the string, giving us the percentage string.

const height = 10; // 10 is just an example of a value here - your code should still work if you change this to another value.
const width = 30; // Also just an example - your code should still work if this changes.

const area = FILL_ME_IN;
const perimeter = FILL_ME_IN;

const price = 130; // Just an example value. Try changing this value to 0, 10, or 1521, and make sure you still get the right answer from your code.

To create a function, we can use a function declaration. A function declaration looks like this:

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function convertToPercentage() {}

The function declaration consists of the following syntactic elements:

• function keyword, begins the function declaration
• convertToPercentage - names the function
• () - any input to the function will go between these round braces (our function above doesn’t take any input (yet), but it still needs the ()s)
• {} - the body of the function is written inside the curly braces (our function above doesn’t do anything yet, but it still needs the {}s)

We can create a function declaration by wrapping up the percentage variable and the expression for the percentage inside the function.

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const decimalNumber = 0.5;

function convertToPercentage() {
  const percentage = `${decimalNumber * 100}%`;
}

At the moment decimalNumber is not wrapped up inside the body of the function. In the following sections, we will explore what happens when this is the case.

To understand how convertToPercentage works we must build a mental model of how the computer executes our code. To build this model, we use a method called playing computer 🧶 🧶 playing computer .Playing computer means simulating how the computer executes our code. We “step through” the code, line by line, and work out what the computer does when it follows each instruction.

We will use an interactive code visualiser to play computer.

🖼️ Global frame

As we step through the program, we keep track of two things: memory and the line that is being currently executed. We keep track of this information using a frame 🧶 🧶 frame Think of a frame as the context in which some code gets executed. We use frames to keep track of memory and the line of code that is being currently executed. .

The global frame is always the first frame that gets created when our program starts executing. It is like the starting point for our program, the place where code gets executed first. When we run the code above, decimalNumber and convertToPercentage are both stored in the global frame.

🖼️  Local frame

Whenever we call a function a new frame is created for executing the code inside that function. In the example above, we call the function convertToPercentage on line 7 and then a new frame is created for convertToPercentage. Inside the convertToPercentage frame, the computer executes the instructions inside convertToPercentage, storing new variables in memory and keeping track of the current line that is being executed.

The function convertToPercentage will only be useful if we can access the percentage string that it creates. Otherwise, we won’t be able to use the result of convertToPercentage in other parts of our code. We can try accessing the percentage variable outside the function body like this:

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const decimalNumber = 0.5;

function convertToPercentage() {
  const percentage = `${decimalNumber * 100}%`;
}

convertToPercentage(0.5);
console.log(percentage);

However if we run the code above, we get an error:

ReferenceError: percentage is not defined

We get an error because of scope 🧶 🧶 scope Scope means where variables are and what you can access. . When we define convertToPercentage we also define a local scope - the region of code enclosed inside convertToPercentage’s function body. This region is convertToPercentage’s local scope. This means any variables we declare inside convertToPercentage’s local scope can only be accessed within this region. If we attempt to reference a variable outside the scope where it was declared, then get a ReferenceError.

We need a way to access the percentage string that is created inside convertToPercentage. To access values created inside functions, we write return statements 🧶 🧶 return statements We write a return statement to specify a function’s return value. If your function call is like a question, the return value is the answer. It’s what comes back. .

We can add a return statement to convertToPercentage like this:

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const decimalNumber = 0.5;

function convertToPercentage() {
  const percentage = `${decimalNumber * 100}%`;
  return percentage;
}

If we want, we could also remove the variable percentage, since we can return the value of the expression directly:

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const decimalNumber = 0.5;

function convertToPercentage() {
  return `${decimalNumber * 100}%`;
}

🔎 Checking the output

We can store a function’s return value in a variable.

const result = Math.round(10.3);
console.log(result); // logs 10 to the console

We call Math.round which takes the input 10.3 and then returns the rounded number 10. So result stores a value of 10.

Math.round is a function implemented by other developers and convertToPercentage is a function we’re implementing, but calling convertToPercentage is just like calling Math.round.

Now we want to call the function convertToPercentage and store the return value in a variable.

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const decimalNumber = 0.5;

function convertToPercentage() {
  const percentage = `${decimalNumber * 100}%`;
  return percentage;
}

const result = convertToPercentage(0.5);

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const decimalNumber = 0.5;

function convertToPercentage() {
  const percentage = `${decimalNumber * 100}%`;
  return percentage;
}

const result = convertToPercentage(0.5);
console.log(result);

50%

Our goal is for convertToPercentage to be reusable for any number. To check this goal, let’s call convertToPercentage with different arguments and check the return value each time:

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const decimalNumber = 0.5;

function convertToPercentage() {
  const percentage = `${decimalNumber * 100}%`;
  return percentage;
}

const output1 = convertToPercentage(0.5);
const output2 = convertToPercentage(0.231);

console.log(output1);
console.log(output2);

When we execute this code we want to log the target output for each input: 0.5 and 0.231:

50%
23.1%

However, given the function’s current implementation, we get the following logs:

50%
50%

🌍 Global scope

At the moment, decimalNumber is in the global scope 🧶 🧶 global scope Variables declared in the global scope are available everywhere in your program. Variables declared in a { block scope } are only available within that block. Any block within your program can access variables that are defined within the global scope. . Any functions we declare can reference variables in the global scope. If a variable is in the global scope, we say that variable is a global variable.

50%
50%

At the moment, decimalNumber is a variable in the global scope of our program:

const decimalNumber = 0.5; // defined in the global scope of our program

function convertToPercentage() {
  const percentage = `${decimalNumber * 100}%`;
  return percentage;
}

const output1 = convertToPercentage(0.5);
const output2 = convertToPercentage(0.231);

So long as decimalNumber is always in the global scope, convertToPercentage will always go to the global scope to get the value of decimalNumber.

However, we want convertToPercentage to work for any input we pass to it.

To make a function work for any number, we need to handle inputs. We do this using a parameter 🧶 🧶 parameter A parameter is a special kind of variable: its value is defined by the caller. .

decimalNumber is still a variable - but as a parameter we don’t assign decimalNumber a value inside the function’s body. It is a placeholder. When we call the function, we pass an input to the function, and the value of that input is assigned to the decimalNumber parameter when the function is called. This happens automatically.

We can add a parameter decimalNumber to our function:

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function convertToPercentage(decimalNumber) {
  // now decimalNumber is a parameter of convertToPercentage
  const percentage = `${decimalNumber * 100}%`;
  return percentage;
}

const output1 = convertToPercentage(0.5);
const output2 = convertToPercentage(0.231);

In the example above, we’re calling convertToPercentage twice: first with an input of 0.5 and second with an input of 0.231. In JavaScript instead of input we use the word argument 🧶 🧶 argument Arguments are inputs given to a function inside (). An argument means an input. .

We’re calling convertToPercentage twice: first with an argument of 0.5 and next with an argument of 0.231.

Think of a function as a box. We put data in and then act on it using the rules in the box; at the end, the box gives us new data back. In programming we say that we pass arguments into a function, the function’s code is executed and we get a return value after the function has finished executing. Here’s a diagram:

Here’s a diagram of what happens when convertToPercentage is passed a specific argument:

In this interactive widget we have defined a parameter decimalNumber in the function declaration inside parentheses after the function name convertToPercentage. In our mental model, a function call means going to convertToPercentage and running the code inside the function.