(+=)

let a = 2;
let b = "hello";

console.log((a += 3)); // 
// : 5

console.log((b += " world")); // 
// : "hello world"

x += y

let bar = 5;
bar += 2; // 7

let baz = true;
baz += 1; // 2
baz += false; // 2

let x = 1n;
x += 2n; // 3n

x += 1; // TypeError: Cannot mix BigInt and other types, use explicit conversions

let foo = "foo";
foo += false; // "foofalse"
foo += "bar"; // "foofalsebar"

let bar = 5;
bar += "foo"; // "5foo"