Working with objects in JavaScript

Keys, values, entries, spread, and the two kinds of freeze.

const tour = { slug: "basics", steps: 30, sets: { count: 6 } };

console.log(Object.keys(tour), Object.values(tour).slice(0, 2));
console.log(Object.entries(tour).map(([k]) => k));

const renamed = Object.fromEntries(
  Object.entries(tour).map(([key, value]) => [key.toUpperCase(), value]),
);
console.log(Object.keys(renamed));

const copy = { ...tour, steps: 31 };
copy.sets.count = 99; // shared: spread is shallow
console.log(tour.sets.count, copy.steps);

const frozen = Object.freeze({ a: 1 });
console.log(Object.isFrozen(frozen), "b" in frozen, tour?.sets?.count);

How it works

1. Object.entries pairs with fromEntries to transform a whole object.
2. Spread copies one level: nested objects are still shared.
3. Object.freeze is shallow, and silent unless in strict mode.

Keywords and builtins used here

• Object
• const

The run, in numbers

Lines

16

Characters to type

571

Tokens

193

Three-star pace

95 tpm

At the three-star pace of 95 tokens a minute, this run takes about 122 seconds.

Type this snippet

Step 2 of 8 in Objects & collections, step 28 of 43 in Language basics.

← Previous Next →