slice works for strings, too.

 const
     data = ['7177576', '4672769', '2445142', '9293878', '5764392'],
     result = data.map(string => string.slice(0, 3));

console.log(result);

JavaScript has many methods for manipulating arrays. The map method creates a new array populated with the results of calling a provided function on every element in the calling array. As Nina Scholz mentions above you can use slice on strings as well.

The issue you are having is that you are calling slice on the array and not on the items of the array.

You need to do something like Nina Suggest which is to map over the main array and call your slice function for each item.