How to Use JavaScript variables?

Constants

const is similar to let with the only difference being that we cannot change the value the constants points to. We should use const to define values and functions that we have no intention of mutating. This, of course, means that we must initialize a const immediately with a value since we cannot bind it to another value later on.

const constants are block scoped, which means that they are valid within the scope they are defined in.

Mutation

We cannot modify the value a constant is bound to. The code in the example below will give us an error telling us that we can't assign to a constant.

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  const age = 23 // initialized to 100
  age = 24       // invalid assignment to const

const essentially creates immutable bindings, which means that we cannot change what value the constant is bound to. But it is important to remember that, in JavaScript, complex data types (arrays, functions and objects) are passed by reference. So we can modify properties of these complex data types while keeping the original reference. The code below will work without errors.

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  const person = {
    name: "Hadrian",
    occupation: "Warrior"
  }
  
  console.log(person.name) // "Hadrian"
  person.name = "Royce"    // mutates object
  console.log(person.name) // "Royce"

When we initialize person with a complex data type (object) we bind the constant to the reference of this new object. We can mutate the internals of the object without causing an error.

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person = {} // Error

The code above will not work however, since it tries to bind the constant to a value with a completely different reference.

Initialization

Constants must be initialized with a value.

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

In the above example we declare a constant counter and initialize it with a value.

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  const counter // SyntaxError

This example will result in a syntax error telling us we are missing = in the constant declaration.

Scope

let and const behaves the same way when it comes to scope as they are both block scoped.

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const env = 'prod'                // globally scoped

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

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

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

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planet

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Tellus