Question

Say I have a function:

function linesReverser(lines) {
  var localLines = lines.slice(); 
  localLines[1].reverse(); 
  return _.flatten(localLines);
}

And using it like so:

var input = [["Hello"],["Hello", "World"]["Attention", "Please"]];
var output1 = linesReverser(input); //["Hello", "World", "Hello", "Attention", "Please"]
var output2 = linesReverser(input); //["Hello", "Hello", "World", "Attention", "Please"]

Notice how the object reference is being shared. I am new to JS, but I thought copying the values would alleviate this issue (line.slice()), but it doesn’t seem to work. Is this because of the nested arrays?

How can I non-destructively/immutably perform a reverse?

Answer 1

You’re making a shallow copy of the lines array. To copy the nested arrays, you need to slice each one.

var localLines = lines.map(function(arr) {
    return arr.slice();
});

The .map method will return a new Array of the return values, which are a slice of each nested Array.

---

FWIW, here’s a shorter version that will work in modern browsers, though I’d probably stick with the first one.

var localLines = lines.map(Array.apply.bind(Array, null));