2.6.1 Let, Const, and Var

Overview

What you'll learn:

• Creating different types of variables
• Why use constants
• Differences between let, const, and var
• Parameter variable

Variables and Datatypes

let

In the last post, we learned that we can create a variable with the let keyword and how a variable is like a labeled bucket which we can put things in.

There are also two other keywords we can use.

const

If you read other people’s code, you might see const instead of let.

function sandwichMaker(meat){
  const sandwich = "🍞"
  sandwich = sandwich+meat
  sandwich = sandwich+"🍀"
  sandwich = sandwich+"🍞"
  return sandwich
}

This defines a constant instead of a reassignable variable.

Similar to variables, a constant is a labeled bucket too. We can use its value by the name. However, unlike a variable, once we put a value into a constant, we cannot change it. That’s why it's called a constant, and why we get errors in these lines.

function sandwichMaker(meat){
  const sandwich = "🍞"
  // Error "sandwich is read-only and cannot be reassigned
  sandwich = sandwich+meat
  sandwich = sandwich+"🍀"
  sandwich = sandwich+"🍞"
  return sandwich
}

What's so good about a constant?

You might ask what’s good about a constant if we cannot change its value. Well, just like many things in life, adding some limitations can bring benefits. This barrier can be exactly what we want. It can make the code easier to read and help us avoid mistakes because we would not need to worry about accidental changes to the values.

For example, if you are dealing with a physics game, you might need to add some physical constants such as gravity that we don't want to accidentally change.

var

If you have studied JavaScript before, you probably know there’s another keyword to define variables. It’s var instead of let

function sandwichMaker(meat){
  var sandwich = "🍞"
  sandwich = sandwich+meat
  sandwich = sandwich+"🍀"
  sandwich = sandwich+"🍞"
  return sandwich
}

var vs let

Similar to let, this defines a variable sandwich. We can use its value by the same name. We can also change the value of the variable with an equal sign. However, there are some subtle differences between the variables defined by let and the ones by var. But let’s not worry about it at the moment.

You can consider let as a better way to define variables than var. We’ll always use let instead of var.

Parameter

In this function, the parameter meat is, in fact, a variable too.

function sandwichMaker(meat){
  let sandwich = "🍞"
  sandwich = sandwich+meat
  sandwich = sandwich+"🍀"
  sandwich = sandwich+"🍞"
  return sandwich
}

You can think of it as if there’s a hidden let statement at the beginning of the function. It initializes the variable with the value we pass in when calling the function.

function sandwichMaker(){
  let meat = "🥓"
  let sandwich = "🍞"
  sandwich = sandwich+meat
  sandwich = sandwich+"🍀"
  sandwich = sandwich+"🍞"
  return sandwich
}

let someVariable = sandwichMaker()

This would essentially be equivalent to

function sandwichMaker(meat) {
  let sandwich = '🍞'
  sandwich = sandwich + meat
  sandwich = sandwich + '🍀'
  sandwich = sandwich + '🍞'
  return sandwich
}

let someVariable = sandwichMaker('🥓')

However, the first method can't change because it doesn't have any input anymore!

Conclusion

That is let, const, and var! Remember we'll be using let instead of var! We'll learn about a special function called onTap next!