Nullish coalescing operator

const foo = null ?? "default string";
console.log(foo);
// Expected output: "default string"

const baz = 0 ?? 42;
console.log(baz);
// Expected output: 0

leftExpr ?? rightExpr;

let foo;
...
//  foo is never assigned any value so it is still undefined
let someDummyText = foo || 'Hello!';

let count;
let text;
...
count = 0;
text = "";
...
let qty = count || 42;
let message = text || "hi!";
console.log(qty);     // 42 and not 0
console.log(message); // "hi!" and not ""

let myText = ""; // An empty string (which is also a falsy value)

let notFalsyText = myText || "Hello world";
console.log(notFalsyText); // Hello world

let preservingFalsy = myText ?? "Hi neighborhood";
console.log(preservingFalsy); // '' (as myText is neither undefined nor null)

function A() {
  console.log("A was called");
  return undefined;
}
function B() {
  console.log("B was called");
  return false;
}
function C() {
  console.log("C was called");
  return "foo";
}

console.log(A() ?? C());
// logs "A was called" then "C was called" and then "foo"
// as A() returned undefined so both expressions are evaluated

console.log(B() ?? C());
// logs "B was called" then "false"
// as B() returned false (and not null or undefined), the right
// hand side expression was not evaluated

null || undefined ?? "foo"; // raises a SyntaxError
true || undefined ?? "foo"; // raises a SyntaxError

(null || undefined) ?? "foo"; // returns "foo"

let foo = { someFooProp: "hi" };

console.log(foo.someFooProp?.toUpperCase()); // "HI"
console.log(foo.someBarProp?.toUpperCase()); // undefined

function getMiscObj() {
  return {
    aNullProperty: null,
    emptyText: "", // this is not falsy
    someNumber: 42,
  };
}

const miscObj = getMiscObj();

const newObj = {};
newObj.propA = miscObj.aNullProperty ?? "default for A";
newObj.propB = miscObj.emptyText ?? "default for B";
newObj.propC = miscObj.someNumber ?? 0;

console.log(newObj.propA); // "default for A"
console.log(newObj.propB); // "" (as the empty string is not null or undefined)
console.log(newObj.propC); // 42