Variables created inside a function (including the function's parameters) can only be seen inside that function. They're created fresh every time the function is called, and they disappear when the function finishes:
function calculateTip(bill) {
const tipRate = 0.1;
const tip = bill * tipRate;
return tip;
}
console.log(calculateTip(200));
console.log(typeof tipRate);
console.log(typeof bill);It prints
20 undefined undefined
Outside the function, tipRate, tip and bill don't exist. (typeof is a safe way to check this: it gives "undefined" for a missing variable instead of throwing an error.)
This is actually great news. It means two different functions can use the same variable names without stepping on each other's toes:
function cookRice() {
const minutes = 20;
console.log(`Rice takes ${minutes} minutes`);
}
function boilEggs() {
const minutes = 8;
console.log(`Eggs take ${minutes} minutes`);
}
cookRice();
boilEggs();It prints
Rice takes 20 minutes Eggs take 8 minutes
Two different minutes variables, in two different rooms. No problem at all. They're like two people both called Lucas living in different houses. Nobody gets confused, because they never meet.