x += y

let baz = true;

// Boolean + Number -> addition
baz += 1; // 2

// Number + Boolean -> addition
baz += false; // 2

let foo = "foo";

// String + Boolean -> concatenation
foo += false; // "foofalse"

// String + String -> concatenation
foo += "bar"; // "foofalsebar"

let bar = 5;

// Number + Number -> addition
bar += 2; // 7

// Number + String -> concatenation
bar += "foo"; // "7foo"

let x = 1n;

// BigInt + BigInt -> addition
x += 2n; // 3n

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