How to Use JavaScript variables?

Modern variables

let should be used to create all mutable variables in modern JavaScript code. We use let when we want to be able to assign another value to a variable. In all other cases a const should be used instead.

let variables are block scoped, which means that they are valid within the scope they are defined in and will not mutate a shadowed (variable with same name in parent scope) variable.

Mutation

We can modify the value of a let variable by assigning it a new value using the = symbol.

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  let counter = 100 // initialized to 100
  counter = 99      // mutated to 99

Initialization

We can declare a let variable without immediately initializing it with a value. We have to be careful when doing this though, as we will get a runtime reference error if we use the variable before we have declared it.

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  let counter
  counter = 99
  console.log(counter) // 99

In the above example we declare a variable counter and use it only after we have given it a value.

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  console.log(counter) // ReferenceError
  let counter = 99

In this example we try to use the variable counter in a function call without declaring it first. This results in an error.

Scope

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let env = 'prod'                  // globally scoped

// new scope
{
  let country = "Norway"         // country is created here
  console.log(country)           // "Norway"

  // new nested scope
  {
    let capital = "Oslo"          // capital is created here
    console.log(country, capital) // "Norway Oslo"
  }                               // capital is destroyed here
  
  country = "Sweden"              // can mutate
  console.log(country)            // "Sweden"

  console.log(env)                // "prod"
  console.log(capital)            // ReferenceError
}                                 // country is destroyed here

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season

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winter