Learning web development: strings and methods in JavaScript

[2025-08-17] dev, javascript, learning web dev

In the last chapter, we worked with numbers. In this chapter, we’ll work with text and write our first applications.

You can download a repository with the examples

Go to the GitHub repository learning-web-dev-code and follow the instructions there.

Strings: a data type for text

A data type is the set of all values that are “similar” (of the same kind). So far, we have encountered two data types:

Numbers: 8, 1.5, etc.
Functions: (x) => {return x} etc.

Strings are yet another data type: A string is text – a sequence of zero or more characters (textual symbols). These characters are mentioned in quotes – we can use either single quotes or double quotes:

'A string'
"Also a string"

Strings can be empty (contain zero characters):

''
""

It is interesting to observe that the following two expressions have different syntax but produce the same result:

'abc'
"abc"

That is similar to something we have encountered before: The following two numbers are also syntactically different but create the same value:

123.0
123

String concatenation: joining strings together via the plus operator `+`

We can store strings in variables and join them together via the plus operator +:

> let str = '';
> str = str + 'How are';
> str = str + ' you?';
> str
'How are you?'

A function that returns strings

Functions can also return strings:

const sayHello = (name) => {
  return 'Hello ' + name + '!';
};

We can copy the previous multi-line code to a console and try out how the function works:

> sayHello('Terry')
'Hello Terry!'

Nesting variables: objects and properties

In JavaScript, variables can be nested: A variable can itself contain variables. We’ll learn how to do that ourselves in a future chapter. But JavaScript already has many predefined nested variables – e.g. variable Math contains a variable PI whose value is a number:

> Math.PI
3.141592653589793

The dot (.) is used as a separator when accessing nested variables. That is similar to how slashes (/) or backslashes (\) separate directory names in file paths. JavaScript uses the following terms:

Math is a container for variables. Such a container is called an object.
PI is called a property of Math.

Methods: functions stored in properties

Properties can also contain functions and we can call those functions – e.g.:

> Math.sqrt(9)
3

The property Math.sqrt contains a function that computes the square root of a number. A property whose value is a function is also called a method.

Some methods are aware of their container and derive a result from its contents – e.g.:

> 'hello'.toUpperCase()
'HELLO'

Project: `log-hello.html`

Let’s move on from the browser console to an actual web app: Roughly, a web app is a web page with an embedded program. (There are many other valid definitions of what a web app is; but this one will be good enough for us – for now.)

The following HTML code contains JavaScript code:

<!doctype html>
<html>
<head>
  <meta charset="UTF-8">
  <title>Log hello</title>
</head>
<body>
  <script type="module">
    ···
  </script>
</body>
</html>

What do all these HTML tags mean?

Except for <script>, all of these HTML elements are a normal part of a minimal HTML “skeleton”.
<script> contains the JavaScript code.
- type="module" means that this is actually a small JavaScript module. What exactly that means is not important yet. You can think of it as a modern unit/chunk of code: Modules were added to JavaScript relatively late and have features that were not built into the language before that. They are often stored in extra files (one module per file).

JavaScript code

<script> contains the following JavaScript code:

const sayHello = (name) => {
  return 'Hello ' + name + '!';
};
const message = sayHelloTo('Terry');
// Log value of `message` to console:
console.log(message);

Only the last two lines do something new.

Comments

The second-to-last line is a comment:

// Log value of `message` to console:

Whenever a double slash (//) appears in code, JavaScript ignores it and everything that comes after it – until the end of the line. Why is that useful? It lets us add explanations for humans so that they understand the code better: Code is written for humans first, computers second.

`console.log()`

In the last line, we log the value of the variable message to the console:

console.log(message);

The global object console has a method .log() that shows values in the console. If you open the file log-hello.html in a web browser, you can see this message in the console:

Hello Terry!

Project: `log-clicks.html`

Let’s say we have an HTML page with the following <body>:

<a id="link" href="">Click here</a>

This is an empty link: href="" is only necessary because the link isn’t clickable otherwise. We’d like to run JavaScript code every time the user clicks on this link. We can achieve that as follows:

let count = 1;
const link = document.querySelector('#link');
link.addEventListener(
  'click',
  (event) => {
    event.preventDefault(); // (A)
    console.log('Click ' + count); // (B)
    count = count + 1;
  }
);

Note: // (A) etc. give “names” to lines. Since they are comments, JavaScript ignores them. Once again, this is information for humans, not for the computer.

In line 2, we retrieve the JavaScript object for the HTML element whose HTML ID is link. The hash symbol (#) before link is CSS syntax for HTML IDs. Now that we have that link object, we can do stuff with it: In line 3, we add an event listener to link:

An event is something that happens once, regularly or irregularly. There are many kinds of events: the user clicks, the user presses a key, etc. In this case, we are interested in mouse clicks (and finger taps) which is why the first argument of the method call link.addEventListener() is 'click'. The second argument is a function that is called every time the user clicks on link. That function is called an event listener

If an <a> element has an href attribute, clicking on it jumps to the address specified by href. We don’t want that to happen, so we prevent it in line A, via event.preventDefault() – which, as the name implies, prevents the web browser from performing the default action.

Note that the variable count is created in line 1 – outside the event listener. That’s why its value is preserved between invocations of that function: Variables declared inside a function are created fresh every time it is called.

You can now open log-clicks.html in your browser and see how it works. You need to open the browser console to see the output produced in line B.

What is the difference between `123 vs. '123'`?

Now that we know both numbers and strings: What is the difference between the following two values?

123
'123'

123 is a number: We can perform arithmetic with it etc.
'123' is a string: We can concatenate it with other strings etc.

The difference is similar to the difference between a dog and a photo of a dog: The photo looks like a dog, but you can’t play fetch with it etc. Similarly, the string '123' looks like a number but is not a number.

Converting between numbers and strings

JavaScript has a function String() that converts numbers to strings:

> String(123)
'123'

It also has a function Number() that converts strings to numbers:

> Number('123')
123

Additionally, the plus operator (+) converts a number to a string if the other operand is a string – e.g.:

> 1 + 'X'
'1X'
> 'X' + 1
'X1'
> 'I bought ' + 3 + ' apples'
'I bought 3 apples'

Project: `display-clicks.html`

So far, we have used the console to display status information. However, we can’t expect normal users to open the console to see that information – the console is only meant for developers. Let’s display the status information right on the web page.

The HTML specifies how the user interface looks at the beginning (its initial state):

<div>
  <a id="link" href="">Click here</a>
</div>
<div>
  Number of clicks: <span id="clickCount">0</span>
</div>

We once again react to clicks via an event listener:

const link = document.querySelector('#link');
const clickCount = document.querySelector('#clickCount'); // (A)
let count = 1;
link.addEventListener(
  'click',
  (event) => {
    event.preventDefault();
    clickCount.innerText = String(count); // (B)
    count = count + 1;
  }
);

Explanation:

In line A, we retrieved the HTML element whose ID is 'clickCount'.
In line B, we don’t log count to the console, we convert it to a string and store it in the property .innerText of the HTML element clickCount. This special property changes the content of the HTML element.

You can open display-clicks.html to see this web app in action.

Project: `temperature-converter.html`

temperature-converter.html is an app that converts temperatures from degree Celsius to degree Fahrenheit: It has two text fields. If we enter a number in one of them, it shows the correct analog in the other one. Let’s see how that works for the Celsius text field – the code for Fahrenheit is similar.

This is the HTML for the text field:

<label>
  Celsius: <input id="celsius" type="text" size="10">
</label>

This is the JavaScript code:

const celsius = document.querySelector('#celsius');
const fahrenheit = document.querySelector('#fahrenheit');

const celsiusToFahrenheit = (c) => {
  return (c * (9/5)) + 32;
};
celsius.addEventListener( // (A)
  'change',
  () => {
    const c = Number(celsius.value); // (B)
    fahrenheit.value = String(celsiusToFahrenheit(c)); // (C)
  }
);

In line A, we once again use an event listener. This time, we listen to change events: They are triggered by pressing Return while inside a text field and by clicking outside a text field – so that the text cursor inside it disappears.

Whenever there is a change (line B):

We use the special property celsius.value to retrieve the content of the text field celsius as a string.
We convert that string to a number via the function Number() and
store the result in the variable c.

In line C:

We convert c to Fahrenheit.
We convert the result to a string.
We store the Fahrenheit string in the fahrenheit text field so that it is displayed in the user interface.

So, the special property .value of text field HTML elements can be used in two ways:

We can read it to retrieve the current content of the field.
We can write to it (assign to it) to change what’s inside the field.

Exercise (without solution)

Change temperature-converter.html so that it converts currencies (e.g. between USD and EUR).

Learning web development: strings and methods in JavaScript

You can download a repository with the examples #

Strings: a data type for text #

String concatenation: joining strings together via the plus operator + #

A function that returns strings #

Nesting variables: objects and properties #

Methods: functions stored in properties #

Project: log-hello.html #

JavaScript code #

Comments #

console.log() #

Project: log-clicks.html #

What is the difference between 123 vs. '123'? #

Converting between numbers and strings #

Project: display-clicks.html #

Project: temperature-converter.html #

Exercise (without solution) #