JavaScript Advanced: Scopes & Type Coercion

We will simplify 2 challenging advanced JavaScript topics: Scopes & Type Coercion.

Scopes

Scoping: How a program's variables are organized and accessed.

Lexical environment: the scope or environment the engine is currently reading code in.

A new lexical environment is created when curly brackets are used, even nested brackets {{...}} create a new lexical environment.

The execution context tells the engine which lexical environment it is currently working in and the lexical scope determines the available variables.

Scope: The environment in which a certain variable is declared. There is a Global scope, Function scope, and Block scope.

Global scope:

1. Available outside of any function or block.
2. Variables declared in Global scope are accessible everywhere.

const name = 'Dan';
const human = true;

Function scope:

1. Variables are accessible only inside a function or block, NOT outside.
2. It is also called Local scope.

function localScope() {
  const localVariable = 'Local';
  console.log(localVariable); // Accesses localVariable
}

localScope(); // Output: "Local"
console.log(localVariable); // Output: ReferenceError

Block scope:

1. Variables are accessible only inside a block.
2. #1 only applies to let and const variables.
3. Functions are also block scoped (only in Strict mode)

function localScope() {
  const localVariable = 'Local';
  console.log(localVariable);
}

localScope(); // Output: "Local"
console.log(localVariable); // Output: ReferenceError

Scope of a variable: The region of a code where a certain variable can be accessed.

Type Coercion

Type coercion is the process of converting one type of value into another. There are 3 types of conversion in JavaScript:

1. to string
2. to boolean
3. to number

let string = '1';
let number = 1;
number == string; // Output: true

// == is loose equality, === is strict equality
// double equals (==) will perform a type conversion
// one or both sides may undergo conversions
// in this case, 1 == 1 or '1' == '1' before checking equality

Strict equals: The triple equals (===) or strict equality compares two values without type coercion. If the values are not the same type, then the values are not equal.

let string = '1';
let number = 1;
number === string; // Output: false