The for...in loop is used to iterate through the keys of an object. Syntax of JavaScript for…in Loop

for (key in object) {
    // body of for...in
};

Here object – The object whose keys we want to iterate over and key – A variable that stores a single key belonging to object.

const employee = {
    name: "John",
    class: 8
};

// loop through the keys of student object
for (let key in employee) {

    // display the key-value pairs
    console.log(`${key} => ${employee[key]}`);
};

// Output:
// name => John
// class => 8

You can use for...in to iterate over an iterable such arrays and strings but you should avoid using for...in for iterables.