duplicating arrays javascript splicing

Question

I have come across a strange bug in my code and I cannot understand why it happens.

I have an array array1. I duplicate array1 by making array2 equal to array1. I then modify array2 using splice to add a number. Array1 should not be touched? But both output the same change.

var array1 = [0,1,2,3,4,5];
var array2 = array1;
array2.splice(1,0,1) //add  
console.log(array1);
console.log(array2);

I am assuming I am confusing array assignment? What is the proper way to duplicate arrays without this happening?

Cheers

Turns out, if I simply do "var array2 = array1.splice();" This makes it independent. Cannot believe I never knew this... — myol
– myol, Commented Aug 18, 2011 at 17:28

Lekensteyn · Accepted Answer · 2011-08-18 17:28:11Z

4

Use array1.concat() to duplicate the array instead of passing a reference to array1:

var array1 = [0,1,2,3,4,5];
var array2 = array1.concat();
array2.splice(1,0,1) //add  
console.log(array1);
console.log(array2);

array.concat() can concatenate multiple arrays, but if you pass an empty argument, you're effectively concatenating an array with nothing: cloning the array.

Note that any array and object elements are still references:

var a = [ [1], 2];
var b = a.concat();
b[0][0] = 0;
console.log(b); // gives 0,2
console.log(c); // gives 0,2 too!

edited Aug 18, 2011 at 17:28

answered Aug 18, 2011 at 17:20

Lekensteyn

67k25 gold badges169 silver badges205 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

shawndumas Over a year ago

good job; this is ther right way to do this. see --> w3schools.com/jsref/jsref_concat_array.asp

Norman Joyner · Accepted Answer · 2011-08-18 17:31:25Z

0

If you are using jQuery you can do:

var array1 = [0,1,2,3,4,5];
var array2 = array1.slice();
array2.splice(1,0,1) //add  
console.log(array1);
console.log(array2);

Check out this example.

edited Aug 18, 2011 at 17:31

answered Aug 18, 2011 at 17:22

Norman Joyner

9656 silver badges12 bronze badges

2 Comments

Maverick Over a year ago

clone() is not a native javascript function.

Lekensteyn Over a year ago

array.clone is not a Javascript built-in, it's provided by Mootools in your example.

SeanCannon · Accepted Answer · 2011-08-18 17:20:51Z

-1

Arrays and objects are copied by reference. Try this:

Object.prototype.clone = function() {
  var newObj = (this instanceof Array) ? [] : {};
  for (i in this) {
    if (i == 'clone') continue;
    if (this[i] && typeof this[i] == "object") {
      newObj[i] = this[i].clone();
    } else newObj[i] = this[i]
  } return newObj;
}

var array1 = [0,1,2,3,4,5];
var array2 = array1.clone();

answered Aug 18, 2011 at 17:20

SeanCannon

78.1k17 gold badges126 silver badges145 bronze badges

1 Comment

shawndumas Over a year ago

don't extend Object.prototype --> erik.eae.net/archives/2005/06/06/22.13.54

Collectives™ on Stack Overflow

duplicating arrays javascript splicing

3 Answers 3

1 Comment

2 Comments

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

1 Comment

2 Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related