Objects

Assign a new property to an existing object instance

const obj = { a: 1, b: 2 };
// Direct assignment
obj.c = obj.c ?? 3; // only sets if not already present

// Or simply
obj.c = 3;

Assign a default property to many existing objects (an array of objects)

const items = [{ a: 1 }, { a: 2, c: 5 }];

const withDefaults = items.map(item => ({
  c: 3,        // default value
  ...item,     // existing properties override the default
}));

Put the default before the spread so any existing value wins. If you want the new property to always overwrite, put the spread first:

const withDefault = { c: 3, ...obj };

or

const withDefault = { ...obj, c: obj.c ?? 3 };

Bracket notation

Useful when the key is dynamic (a variable) or not a valid identifier.

const obj = { a: 1, b: 2 };

const newKey = 'c';
const value = 3;

obj[newKey] = value; // obj is now { a: 1, b: 2, c: 3 }

Within a condition

if (!(newKey in obj)) {
  obj[newKey] = value;
}

Or more concisely:

obj[newKey] ??= value;

That last one (??=, logical nullish assignment) is the modern shorthand. It only assigns if obj[newKey] is null or undefined.