How to duplicate a value of array and push it same array in javascript?

Question

I am try to learn more about the JavaScript and I have one of the assignment where I have array and I need to duplicate that and return the result as below :

var smValue = [1,2,3];

smValue.duplicate() will give the output like :

[1,2,3,1,2,3]

I got stuck on that. I have tried to create function like that :

var smValue = [1,2,3]; 
fuction duplicate (){
    for(i=0;i<=smValue.length;i++){
        array[smValue.length + 1] = smValue.push(smValue[i]);  
    }
return array;
}

smValue.duplicate();

But failed. Please help me to resolve it. It may the vary basic but I have never seen this before.

Theoretical: You loop thru the array and push each key back into the array Practical: for(var i = 0, len = smValue.length;i < len;i++) { smValue.push(smValue[i]); } — Henrik Peinar
– Henrik Peinar, Commented Feb 13, 2015 at 11:41
Yes same I thought but how can I assign the key to new item of the array? — Toretto
– Toretto, Commented Feb 13, 2015 at 11:43
You are pushing all values over array[4] key. In addition array wasn't declarated. — mgamon
– mgamon, Commented Feb 13, 2015 at 11:44
fails because your function isn't part of Array.prototype so you can't chain it to array itself — charlietfl
– charlietfl, Commented Feb 13, 2015 at 11:46

Chris Charles · Accepted Answer · 2015-02-13 11:44:57Z

4

A simple way would be to concat the array with itself:

function duplicate (arr){
    return arr.concat(arr)
}

Demo

Or if you want to call it on the array itself.

Array.prototype.duplicate = function(){
    return this.concat(this)
}

smValue.duplicate() // returns [1,2,3,1,2,3]

answered Feb 13, 2015 at 11:44

Chris Charles

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5 Comments

Cerbrus Over a year ago

This is the only answer that actually implements duplicate on the prototype, and doesn't use a loop... +1

Toretto Over a year ago

If i can do that normally with concat function then why we would use Array.protoType , Is there any specific reason?

Andy Over a year ago

Because if you want to do var newarr = arr.duplicate(); you need to attach the method to the array prototype like Chris and I have shown in our answers. Otherwise you'll need to do var newarr = duplicate(arr);. Depends on the functionality you want, but in your question you wanted the first options.

Toretto Over a year ago

okay that's fine, But can we do that without using concat function?

Chris Charles Over a year ago

@Toretto, why dont you want to use concat? (if you dont then there are plenty of looping answers here already)

Andy · Accepted Answer · 2015-02-13 11:49:11Z

2

The alternative (easier?) way is to use concat:

function duplicate(arr) {
  return arr.concat(arr);
}

To add this as a method on the array prototype:

if (!('duplicate' in Array.prototype)) {
  Array.prototype.duplicate = function () {
    return this.concat(this);
  };
}

[4, 5, 6].duplicate(); // [4, 5, 6, 4, 5, 6]

DEMO

edited Feb 13, 2015 at 11:49

answered Feb 13, 2015 at 11:43

Andy

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1 Comment

Cerbrus Over a year ago

He wants (/is required) to be able to call duplicate on the array.

W van Rij · Accepted Answer · 2015-02-13 11:44:14Z

1

You can do the following:

var duplicate = smValue.concat(smValue);

Concat 'merges' array's together, or a better word is joining them together.

answered Feb 13, 2015 at 11:44

W van Rij

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Comments

Adnan Ahmed · Accepted Answer · 2015-02-13 11:46:19Z

0

Here's a simple solution:

var smValue = [1,2,3];
smValue = smValue.concat(smValue);

answered Feb 13, 2015 at 11:46

Adnan Ahmed

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Comments

Henrik Peinar · Accepted Answer · 2015-02-13 11:49:56Z

Theoretical: You loop over the array and push each key back into the array using Array.prototype.push

Practical:

var smValue = [1,2,3];
for(var i = 0, len = smValue.length;i < len;i++) { 
  smValue.push(smValue[i]); 
}

If you want to define new function that does it on every array, you want to do this:

var myArray = [1,2,3];


Object.defineProperty(Array.prototype, 'duplicate', {
    enumerable: false,
    value: function duplicate() {
        for(var i = 0, len = this.length;i < len;i++) {
            this.push(this[i]);    
        }

        return this;
    }
});

console.log(myArray.duplicate());

http://jsfiddle.net/4a8pw4u5/

I'm using Object.defineProperty to create non-enumerable property as Array.prototype.newDuplicate would add enumerable property.

And yes, you can use concat, but considering you tried to do it with a loop, I used for loop to show how to do it with that.

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