I want to create a new Array method without modifying the original array.

my_array.push('foo') //modifies my_array
my_array.slice(2) //returns a new array without modifying my_array

I'd like to make a new Array.prototype method that returns an array without modifying the array it was called upon. So this would be possible:

//[define new method here] Array.prototype.add_foo = function() {...
var my_array = ['poo'];

my_array.add_foo(); //would return ['poo', 'foo'];
my_array; //would return ['poo'];

I want to create a new Array method without modifying the original array.

my_array.push('foo') //modifies my_array
my_array.slice(2) //returns a new array without modifying my_array

I'd like to make a new Array.prototype method that returns an array without modifying the array it was called upon. So this would be possible:

//[define new method here] Array.prototype.add_foo = function() {...
var my_array = ['poo'];

my_array.add_foo(); //would return ['poo', 'foo'];
my_array; //would return ['poo'];

• javascript
• arrays

• Just clone the array first. – elclanrs Commented Aug 29, 2014 at 2:44
• Okay, create a new array and return it instead of modifying this – Paul Commented Aug 29, 2014 at 2:45

2 Answers 2

my_array.concat('foo');

concat doesn't alter the original array.

Array.prototype.add_foo = function(){
    var ret = this.slice(0); //make a clone
    ret.push("foo"); //add the foo
    return ret; //return the modified clone
}