Scope and storage

Scope decides where a name can be used. JavaScript has function scope and block scope; names declared with let or const exist only inside their surrounding brace block.

Local variables and parameters

Function parameters and variables declared inside a function belong to that call. Another function can use the same names without referring to the same variables.

function swap(a, b) {
    const temporary = a;
    a = b;
    b = temporary;
    console.log(a, b);
}

let a = 3;
let b = 8;
swap(a, b);       // prints 8 3 inside the function
console.log(a, b); // still 3 8 outside

Numbers are passed by value, so swapping local parameters does not change the outer variables.

Block scope and outer variables

A name declared with let or const inside an if, for, or standalone brace block cannot be accessed after that block. A variable declared outside all functions has module or global scope and can be read by several functions. Shared mutable state can make a program harder to follow, so keep it limited.

let score = 0;

function addScore(points) {
    score += points;
}

function getScore() {
    return score;
}

addScore(10);
console.log(getScore()); // 10