JavaScript Fundamentals

Notes from javascript.info

code
javascript
javascript.info
webdev
Published

March 28, 2026

Modified

March 28, 2026

Code Structure

Semicolons

We use a semicolon at the end of statements, but they can usually be omitted in cases where a line break exists:

// These are equivalent 
alert('Hello!');
alert('world');

alert('Hello!')
alert('world')

  • The second works because a line break is an ‘implicit’ semicolon, which is called an automated semicolon insertion

    • This doesn’t always work, so probably best to keep using a ; at the end of a statement

Comments

Single line comments use // and comment blocks use /* */

// Single line comment
/* 
Block
Comment
*/

use strict

The use strict directive switches the engine to “modern” mode, which changes some of the built-in features.

  • Is enabled by placing use strict at the top of a script or function

  • It is supported by all browsers

  • Generally recommended to always start scripts with use strict

Variables

Variables are declared (created) using the let keyword:

let message

Now that the variable has been declared, we can assign it a value with the assignment operator =:

let message
message = 'Hello' //stores a string

We can declare multiple variables in one line:

let user = 'Jon', age = '37', message = 'Hello'
  • It’s generally easier to read when they are vertically spaced

Variable naming

There are two requirements for variable names in JavaScript:

  1. The name can only be made up of letters, digits, or the symbols $ and _
  2. The first character can’t be a digit

camelCase is common for variables

Note: Assignment without strict

We generally need to define a variable before using it, but that was not always the case with JavaScript. We could create a variable without using let. This still works if we don’t include use strict

Constants

Use const instead of let for declaring constant variables:

const myBirthday = '2020-02-02'

  • Constants can’t be changed

    • An error will be thrown if we try to alter a constant.

  • Capitalized constants are generally used for hard-coded values

Tasks

1) Declare two variables: admin and name

2) Assign the value Jon to name

3) Copy the value from name to admin

4) Show the value of admin using alert

Answer:

let admin name // 1

name = 'Jon'; // 2
admin = name; // 3
alert(admin); // 4

Data types