Javascript Array processing

Question

If I have an array:

var names = ['John', 'Jim', 'Joe'];

and I want to create a new array from names which afterwards will look like:

newNames = ['John John', 'Jim Jim', 'Joe Joe']

What I came up with is the following:

var newNames = [];
var arr = null;
var loop = 0;
$.each(names, function (i, item) {
 arr = [];
 loop = 2;
 while (loop--) {
 arr.push(item);
 }
 newNames.push(arr.join(' '));
});

Seems like there should be a shorter, easier way to do this. As you can see we can repeat the names with a space n times. I'm experimenting with different ideas/concepts, having fun.

• konijn: Since then, JavaScript has added String.repeat().

Answer 1

Create a function that operates on values in the manner you want...

function dupe(n) { return n + " " + n }

---

then use Array.prototype.map...

var newNames = names.map(dupe)

---

or jQuery's not quite compliant version, $.map...

var newNames = $.map(names, dupe)

---

You can also create a function factory that will make a dupe function that will add the operate on the value a given number of times.

function dupeFactory(i) {
 return function(n) {
 var j = i-1
 , m = n
 while (j--)
 m += " " + n
 return m
 }
}

Then use it like this...

var newNames = names.map(dupeFactory(3))

---

Or make reuse of the functions created from the factory, by storing them in variables...

var dupe3 = dupeFactory(3),
 dupe6 = dupeFactory(6)
names.map(dupe6)

• David Whitten: Dang, this seems to be pretty straightforward and very clean. Thanks much.
• user10711: @DavidWhitten: You're welcome. I also added a solution that lets you choose how many times you want to append the name.
• Bergi: Your dupeFactory does not work, it creates 2^n repetitions...
• user10711: jsfiddle.net/6THSR

Answer 2

You could use the native Array.map method in modern browsers (see MDN)

const newNames = ['John', 'Jim', 'Joe'].map( name => `${name} ${name}`);
console.log(newNames);

Answer 3

I guess you want to have a variable loop value:

var names = [...];
var newNames = names.slice(0); // copy the names for the first part
for (var i=0; i<names.length; i++)
 for (var j=1; j<2; j++) // sic
 newNames[i] += " "+names[i];

Else, for a constantly two times a simple string concatenation will be shorter. Together with .map():

var newNames = names.map(function(name) {
 return name+" "+name;
});