What is the difference between global and local variables in JavaScript?

Global variables, declared outside any function, are visible everywhere; local variables, declared inside a function or block, only exist within it.

Where you declare a variable decides who can see it — this is called its scope.

• A global variable is defined outside all functions and can be read or changed anywhere in the program.
• A local variable is defined inside a function (or a { } block) and only exists there; outside, it's as if it never existed.

let global = 2;          // global: visible everywhere

function myFunc() {
    let local = 1;       // local: only inside myFunc
    // here, both global and local are accessible
}

// here, global is accessible
// but referring to local throws a ReferenceError — it doesn't exist out here

A dangerous gotcha: if you assign to a variable without let/const, JavaScript doesn't error — it silently creates a global variable instead. This "accidental global" is a classic source of bugs:

function bad() {
    oops = 5;   // no 'let' — accidentally creates a global!
}

Best practice: always declare with let or const, and reach for const whenever the value shouldn't change. This keeps variables tightly scoped and prevents accidental globals.

Go deeper:

• MDN: Closures — how lexical scope works and why an inner function keeps access to its outer variables.