JavaScript Arrays - Select elements with even values

Question

Given an array in JavaScript, I would like to select only the elements that are even. For example

array = [1,2,3,4,5,6,7,8,9,10]

It would return

[2,4,6,8,10]

Here is my code

array = [1,2,3,4,5,6,7,8,9,10]
for (i = 0; i < array.length; i++) {
  newArray = [];
  if (i[0] % 2 == 0 {
    newArray.push(i)
   return newArray;}}

=> Illegal return statement

I have no idea what I'm doing wrong, but any help would be appreciated. Thank you.

You cannot have a return not in a function body, and you are adding the indexes, not the elements to newArray — kennebec
– kennebec, Commented Jul 17, 2014 at 14:16
To tmarwen - That wasn't a method. But that was all the code I had. In JavaScript, I think you don't have to close loops(if for, etc.). I thought that logically that code would work but I guess not. — George Lucas
– George Lucas, Commented Jul 17, 2014 at 14:18
Matching up parentheses () and braces {} is very important in programming. Right now your return statement is inside the () of your if. And the only line inside your for is newArray = [];. — Russell Zahniser
– Russell Zahniser, Commented Jul 17, 2014 at 14:18
@GeorgeLucas since there is no method, as kennebec said you can not add a return statement except in a method. — tmarwen
– tmarwen, Commented Jul 17, 2014 at 14:20

Mritunjay · Accepted Answer · 2014-07-17 14:14:22Z

3

Better you use native filter method. It will filter those elements which will match a criteria given in a callback.

var evens = array.filter(function(element){
               return element%2==0
            });

answered Jul 17, 2014 at 14:14

Mritunjay

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4 Comments

George Lucas Over a year ago

Nice, it worked perfectly and is a lot easier than my method. I am trying to learn JavaScript from Ruby so it's a bit of a challenge. But your help is greatly appreciated. Thank you

Xotic750 Over a year ago

Array.prototype.filter is much more concise in my opinion, and would be my preference. But is must be noted that this is only available in ECMA5, though shims are available. Also, the native for loop has better performance accross all environments as it has to do less work, see spec ecma-international.org/ecma-262/5.1/#sec-15.4.4.20

Mritunjay Over a year ago

But I try native functions because when there are things done for me, why should I not use them.

Xotic750 Over a year ago

Yes, by all means use it, I was just pointing out some info for the OP. :)

Xotic750 · Accepted Answer · 2014-07-17 14:19:23Z

1

Your code corrected, you were getting close. Hopefully you can see where you were going wrong.

Javascript

var array = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10],
    newArray = [],
    i;

for (i = 0; i < array.length; i++) {
    if (array[i] % 2 === 0) {
        newArray.push(array[i]);
    }
}

console.log(newArray);

On jsFiddle

answered Jul 17, 2014 at 14:19

Xotic750

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6 Comments

George Lucas Over a year ago

I see. So, I needed to declare newArray OUTSIDE of the for loop instead of inside it. And also, instead of using the == operator I need to use the === operator to compare equality of both value and type. Out of curiosity though, why is that needed? Also is there a need to declare an i variable twice? Since we're already declaring it inside the for loop, setting i equal to 0.

Xotic750 Over a year ago

In this case you could use == or ===, but it is good practice to use ===. There is good information here on SO about the different comparison methods. One such question stackoverflow.com/questions/359494/…

Xotic750 Over a year ago

Other things to notice where the var keyword. and yes declaring the newArray outside is important otherwise each iteration would create the variable anew.

George Lucas Over a year ago

I see, I also explored the var keyword. I have tried creating variables both with it and without it and it works both ways. Most likely, the convention is to use the var keyword. Is this true?

Xotic750 Over a year ago

Without var the variable becomes a global variable, not good practice. this question may help you there stackoverflow.com/questions/1470488/…

|

sergeyz · Accepted Answer · 2014-07-17 15:00:40Z

1

Not as concise as previous answer but also can be used

var array = [1,2,3,4,5,6,7,8,9,10];

var evens = arr.reduce(function(prev, cur) {
             return (cur%2 === 0) ? prev.concat(cur) : prev; 
            }, []);

answered Jul 17, 2014 at 15:00

sergeyz

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