We want to use computers without understanding exactly how they are built. Every day we ask machines to do things, and usually we have no idea how these machines work. We could not use modern technology if we had to understand it completely before we could use it; it would take too long! Instead we use interfaces 🧶 🧶 interfaces Think of an interface as a gate that allows communication between a user and a machine.
The user asks the machine to do things via the interface.

Think about a cash machine (ATM). We go to a hole in the wall with a screen and a keypad. The screen and the keypad are the user interface. We press the buttons and ask the machine to do things - like giving our balance, or withdrawing some money from an account.

Programmers need interfaces to ask computers to do things. A computer terminal is an interface where programmers can issue commands to a computer. Because users enter text instructions and receive text output, we say that the terminal is a text-based interface.

LS Activity The terminal on Mac

Interface via the terminal

We can input a command into the prompt and hit enter. The terminal then passes this command to the computer to execute. Find your own terminal and input the ls command:

ls

🖊️ Writing computer instructions

We can issue commands to the computer using the terminal. These commands are instructions that the computer knows how to interpret.

The computer knows ls means “list the files and directories in the current directory”.

During the execution of a computer program, a computer will store and modify data 🧶 🧶 data Data is information. Text, images, numbers are all forms of data. The data in a executing program is sometimes called the state. A computer program will modify data with operations 🧶 🧶 operations Operations modify or create data, from the current data in the program. Adding numbers, joining words, changing text to ALLCAPS, are all operations.

ls is a shell command. Shell is a programming language we use to interact with the files and folders on our computer. You already know at least two more programming languages. Can you name them?

We’re going to focus on the JavaScript programming language.

A programming language organises data with rules so we understand what we can and cannot do with it. Languages split data up into different categories called data types 🧶 🧶 data types A data type is a grouping of data with some particular properties . In JavaScript, we have five data types. We will look first at numbers and strings.

Number data type

10 is an example of the number data type.
3.14 is also part of the number data type; non-integers are a type of number.

String data type

A string is a sequence of characters demarcated by quotes.

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"Code Your Future";

🧮 Creating expressions

Think of the numbers 10 and 32. We could ask questions about these numbers, like: What is the sum of 10 and 32?

Another way to say this is what do 10 and 32 add up to?. In English we can say this in many ways, but in JavaScript we can say this using numbers and an operator. Just like in mathematics, “the sum of 10 and 32” can be written as 10 + 32:

10 + 32

In JavaScript, + is an operator 🧶 🧶 operator An operator represents an operation, or act. It’s a symbol. In this example, + represents the operation “make the sum of the numbers”. It symbolises addition.

The combination of symbols 10 + 32 is an expression 🧶 🧶 expression An expression is a value or any valid combination of values and symbols that results in a single value. We say that expressions evaluate to a single value. So we say that 10 + 32 evaluates to the value 42.

10 is also an expression. It evaluates to the value 10.

Computer programs are built from many expressions. We must understand how expressions are evaluated to understand how computer programs are executed.

We can take an expression like 36 * 45 and ask what it evaluates to. If we know what the * operator represents (multiplication) and if we understand the arithmetic rules represented by the operation we can evaluate this expression ourselves.

Happily, computers can evaluate expressions for us.

NodeJS is an application that runs JavaScript programs. In other words, NodeJS can understand and execute programs written in JavaScript. One feature of Node is the REPL.

We input JavaScript instructions that are then executed by NodeJS. The REPL replies with, or prints out, the result of this execution.

10 + 32

32 / 10

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:

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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 is used to refer to a variable.
• = this 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.

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`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:

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const greetAlicia = "Hello there, Alicia";

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

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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.

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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 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.

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.

📲 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:

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const roundedValue = Math.round(10.3);

or we can write:

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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:

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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, 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.

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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:

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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, same as the math expressions.

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

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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.