Writing your first JavaScript programme

By the end of this article, you will have written a JavaScript programme that outputs a sentence based on a number of variables.

If you've made it to this article you should have read the JavaScript Introduction, and be familiar with variables and data-types.

JavaScript is a weakly-typed language, so its types are inferred. To declare a variable in JavaScript we can either use the var, let or const keywords.

The var declaration pre-dates let and const, but it is generally accepted that the latter should be used.

When to use let or const depends on whether you are likely to change your variable at a later stage. If the answer to that question is no, use const (short for constant) otherwise use let.

In the following example:

  • x is assigned the value "Hello World" and has an inferred type of string.
  • y is assigned the value true and has an inferred type of boolean.
  • a is assigned the value 1 and has an inferred type of number.
var x = "Hello World"

const y = true

let a = 1

As JavaScript is a weakly-typed language, there is no way to declare a variable as a specific data-type.


Your first JavaScript programme

It's finally time to write your first JavaScript programme. You can write this anywhere on your machine, however, it is best practice to keep your development code well organised.

Create a new directory, in a location of your choice, called firstApplication, and open it in your code editor of choice.

In your new firstApplication directory, create a new file called index.js, add the below code into it, and save the file.

console.log("Welcome to my first JavaScript programme")

Now, open a terminal, navigate to your firstApplication folder, and run the following command:

node index.js

Your output should look very similar to the following:

$ node index.js         
Welcome to my first JavaScript programme

As you can see, the console.log function outputs text to the terminal. Unfortunately, as a first programme goes, this is pretty boring. Let's make this more personal with variables!

Replace the contents of index.js with the following code:

const name = "Simon"
const age = 29
const profession = "Developer"

console.log(
  "Welcome to my first JavaScript programme. My name is %s. I am %d years old. I work as a %s", 
  name, age, profession
)

We’ve extended our programme to output a longer sentence with some specific information about us.

Notice in the first parameter, to console.log (the text beginning “Welcome to my first”), we have string substitution (%s and %d). These are effectively placeholders that we then pass in as additional arguments to console.log (name, age and profession).

The significance of the letter after the percent (%) sign, is it defines the type of the argument that will be passed in (%s denotes a string and %d an int).

Another way to write this is to use something called a template literal:

console.log(`Welcome to my first JavaScript programme. My name is ${name}. I am ${age} years old. I work as a ${profession}.`)

See how we’ve used backticks (`) instead of double quotation marks ("), and our placeholders are now declared using a dollar sign and curly braces (${}) instead of string substitution.

Well done for finishing your first JavaScript programme! Now see if you can complete the following challenges:

  • Change the variables so the programme prints out your name, age and profession, instead of mine.
  • Add a new variable, favoriteAnimal, and incorporate that into your outputted sentence.

Remember you can always reach out to us with any questions or feedback - we’d love to hear from you!

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