Summary - Scope

Build a counter module using global, function and block scope.

Summary - Scope

In this group you learned JavaScript's four scope ideas: global (outermost, accessible everywhere), function (inside a function, closed from outside), block (let/const inside { }) and hoisting (var/function are lifted; let/const stay in the TDZ).

Now you will use them together: you will write a counter module that keeps state with global and function scope, then compare inner and outer variables with shadowing.

Exercise Write a counter module

Declare a global counter with let counter = 0 outside. Write a function called increment() that increases counter by one and returns the new value. Outside, call console.log(increment()) twice.

Show example solution

let counter = 0;

function increment() {
  counter = counter + 1;
  return counter;
}

console.log(increment());
console.log(increment());

Exercise Compare with shadowing

Declare let counter = 10 outside. Write a snapshot() function that declares let counter = 50 inside, shadowing the outer counter, and returns counter. Outside, first log the inner result with console.log(snapshot()), then log the outer counter with console.log(counter).

Show example solution

let counter = 10;

function snapshot() {
  let counter = 50;
  return counter;
}

console.log(snapshot());
console.log(counter);

Test yourself

Match the scope type

Global

Accessible everywhere
Function

Only inside the function
Block

let/const inside { }
Hoisting

var is lifted up

Complete the block scope

if (active) {
  ____ status = 'active';
  console.log(status);
}

Show answer

if (active) {
  let status = 'active';
  console.log(status);
}

Next lesson: What is a closure?

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