Looping over objects

Visit every key of an object

• An object has no index 0, 1, 2, so a plain for loop cannot walk through it.
• for (const key in obj) gives you each key in turn.
• Read the value with square brackets: obj[key]. The dot form, obj.key, would look for a key literally called "key".

const stock = { apples: 4, pears: 0, plums: 7 };
for (const fruit in stock) {
  console.log(fruit, stock[fruit]);
}
// → apples 4
// → pears 0
// → plums 7

Keys, values and entries as arrays

• Object.keys(obj) gives an array of the keys.
• Object.values(obj) gives an array of the values.
• Object.entries(obj) gives an array of [key, value] pairs. You can loop over it with for…of.

const stock = { apples: 4, pears: 0, plums: 7 };
console.log(Object.keys(stock));            // → ["apples","pears","plums"]
console.log(Object.values(stock).length);   // → 3
for (const [fruit, count] of Object.entries(stock)) {
  if (count === 0) {
    console.log(fruit + " sold out");
  }
}
// → pears sold out

Worked example: total and best

Question: an object holds each student's score. Find the total, and the name with the highest score.

const scores = { Ada: 82, Ben: 95, Cy: 77 };
let total = 0;
let best = "";
for (const name in scores) {
  total += scores[name];
  if (best === "" || scores[name] > scores[best]) {
    best = name;
  }
}
console.log(total);   // → 254
console.log(best);    // → Ben

The loop keeps the name of the best student, not just the score, because the score alone would not tell you who it belongs to.

Watch out

• for…in is for objects. On an array, use for…of, which gives the values.
• Object.keys(obj).length counts the keys. An object has no .length of its own.

Common mistakes

• Writing stock.fruit inside the loop. It looks for a key named fruit and gives undefined. Use stock[fruit].
• Using for…of directly on an object. It throws "stock is not iterable". Loop over Object.entries(stock) instead.
• Starting best at 0 when it should hold a name. Start from "" or from the first key.

Now you try

• Loop over an object's keys, then add up its values.
• Press Check answer to test it.