Control Flow & Loops

To iterate over the values of an array
To iterate over the enumerable properties of an object
The for...in loop in JavaScript is designed to iterate over the enumerable properties of an object. For example, for(let key in obj) { console.log(key, obj[key]); } will iterate over each enumerable property key in the object obj. While for...in can be used with arrays (since arrays are objects in JavaScript), it's generally not recommended because it also iterates over any other enumerable properties of the array, not just the numeric indices. For arrays, it's better to use a standard for loop, Array.forEach(), or the newer for...of loop.
To iterate over a specified number of times
To iterate over the characters in a string