JavaScript

The for...in loop is used to loop through the properties of an object. It gives you the keys (property names), not the values directly.

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✅ Syntax

for (let key in object) {
  // code to execute
}

• key is a variable that stores each property name (key) during the loop.
• You can access the corresponding value using object[key].

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📦 Example: Looping Through an Object

let person = {
  name: "John",
  age: 25,
  city: "New York"
};

for (let key in person) {
  console.log(key + ": " + person[key]);
}

Output:

name: John
age: 25
city: New York

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🔍 Looping Through an Array (Not Recommended)

You can use for...in with arrays, but it’s not ideal because it returns indexes as strings, and not in order guaranteed.

let colors = ["red", "green", "blue"];

for (let index in colors) {
  console.log(index + ": " + colors[index]);
}

Output:

0: red
1: green
2: blue

🔸 Use for...of or for loops for arrays instead (more reliable and readable).

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🧠 When to Use for...in

Use for...in when:

• You want to loop over all keys in an object.
• You need both property names and values.
• You don’t care about order (objects are unordered).

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⚠️ Tip: Use hasOwnProperty

If you’re looping through an object that inherits properties from a prototype, use .hasOwnProperty() to avoid unexpected keys.

for (let key in object) {
  if (object.hasOwnProperty(key)) {
    // Safe to use key
  }
}

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🧾 Summary

Feature	for...in Loop
Used for	Looping through object keys
Output	Property names (keys)
Use on arrays?	Not recommended
Value access	object[key]

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