The answer
Use spread to create a new object with the added property: users.map((u, i) => ({ ...u, id: i + 1 })). Do not mutate the original object.
map in JavaScript: Examples and Use Cases
How do you add a property to each object in an array with map in JavaScript?
More FAQs in map in JavaScript: Examples and Use Cases
It calls a callback on each element of the array and returns a new array with the results. The new array has the same length as the original. The original array is not mutated.
Use map with a callback that returns the property: users.map(u => u.name). This returns a new array of just the names.
Because the arrow function has a block body (braces) with no return statement. Block bodies need an explicit return. Use an expression body (n => n * 2) or add return (n => { return n * 2; }).
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