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Exercise 4

Create an array from the keys of the object obj below, with all of the keys converted to uppercase. Your implementation must not mutate obj.

let obj = {
  b: 2,
  a: 1,
  c: 3,
};

The order of the array does not matter.

Solution

let objKeys = Object.keys(obj);
let upperKeys = objKeys.map((key) => key.toUpperCase());
console.log(upperKeys); // => [ 'B', 'A', 'C' ]
console.log(obj); // => { b: 2, a: 1, c: 3 }

The challenge of this exercise is to figure out how to iterate through the properties of obj. We showed two ways in this chapter: for/in with hasOwnProperty() and Object.keys(). The former involves a bit more work, so we use Object.keys() combined with map() to extract and uppercase the keys into an array.

We can also use forEach, though it requires a bit more effort:

let upperKeys = [];
let objKeys = Object.keys(obj);
objKeys.forEach(function(key) {
  upperKeys.push(key.toUpperCase());
});
console.log(upperKeys); // => [ 'B', 'A', 'C' ]

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