Variables are containers for information -- we can store any value in them.
Think of a variable like a box. You can put lots of things into the box, like a doll, rug, or cat. The box still contains something.
We use variables to help repeat, change, store, or edit our data.
In order to declare a variable in JavaScript we use the keywords var,
const, or let. The original declaration keyword was var, but in newer
versions of JavaScript const and let were implemented. const is used on
variables that will not be changed in your JavaScript code. let is used for
variables that do change.
Most of the time, we want our variables to immediately store a value (like a box with a cat in it). Therefore, we assign values to our variables when we declare them.
We can create a variable without assigning it a value, if we want. This would be like having an empty box.
let noValue
console.log(noValue)
// => undefinedHere are three examples of variable assignment and declaration at the same time.
const myVar = 42
console.log(myVar)
const sum = myVar + 8
console.log(sum)
let doubleSum = 2 * sum
console.log(doubleSum)When we declare a variable using let or with the old keyword var we
can update our variables:
let noValue
console.log(noValue)
// => undefined
noValue = "assigned now"
console.log(noValue)
// => 'assigned now'
var num = 0
console.log(num)
// => 0
num = 1
console.log(num)
// => 1But you can NOT reassign a variable declared with const
const mobilePhone = 'iPhone'
mobilePhone = 'samsung'
console.log(mobilePhone)
// => Uncaught TypeError: Assignment to constant variable.JavaScript is a "dynamically-typed" language, meaning a variable can switch between data types. The following change is valid.
var myFavoriteNumber = 5
console.log(myFavoriteNumber)
// => 5
myFavoriteNumber = "five"
console.log(myFavoriteNumber)
// => "five"- What is the difference between
varandlet? - Will the following code run without throwing an error?
const person = {
name: 'Frank',
age: 23,
favFood: 'tacos'
}
person.name = 'Frankie'- Semicolons can be put at the end of each line.
- We recommend using them consistently throughout this course!
- References:
JavaScript variables and function names are case sensitive. You can write them however you want, but they're conventionally written using camel case syntax.
- First letter of first word lowercase
- First letter of remaining words uppercase
- No spaces or punctuation between words
const pizzaTopping = "pepperoni";
const isThisVariableCamelCase = true;
const hasURLChanged = false;Because Javascript is case sensitive, it helps to be consistent in your naming conventions.
const pizzaTopping = "pepperoni";
console.log(pizzatopping);
// Uncaught ReferenceError: pizzatopping is not definedNote: Other types: kebab-case, which separates words with dashes, and snake case, which uses underscores.
Use kebab case for CSS classes. CSS is essentially case-sensitive, so this-class-name is much easier to read (and type!) than thisClassName.
snake_case is used in programming languages like Python.
You don't have to follow camelCase standards, you could just UPPERCASE EVERYTHING. But it's a good idea to be consistent everywhere, especially when working with other developers.
Comments are an extremely important part of writing code. They help us make sense of our code, especially for other people reading our code, or when we have walked away from some code and have completely forgotten what certain sections of it do. This happens much more quickly than you may imagine.
If you are working on a team, your documentation and commenting practices often translate to how easy you are to work with!
// Single line comment
/*
Multiple
line
comments
*/There is a short cut for commenting something out. To comment out a single line,
click anywhere on the desired line and hold command and press /. The same is goes
for commenting a line back in.
This short cut also works for multiline comments. Simply highlight all the lines you'd
like to comment out and do the same: command + /.