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For...in

Easy

In plain terms

for...in iterates over enumerable property keys of an object. Returns keys (not values). Avoid using on arrays - use for...of instead. Use hasOwnProperty to skip inherited properties.

What you need to know

  • •Iterates object keys
  • •Includes inherited (use hasOwnProperty)
  • •Returns strings for array indices

Try it yourself

Copy the code below and run it in your browser console or a code editor:

const obj = { a: 1, b: 2, c: 3 };
for (const key in obj) {
  console.log(key, obj[key]);  // a 1, b 2, c 3
}

// Arrays (keys are indices - avoid, use for...of)
const arr = [10, 20, 30];
for (const i in arr) {
  console.log(i, arr[i]);  // "0" 10, "1" 20, "2" 30
}

Learn more

Dive deeper with these trusted resources:

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