# Passing arrays to functions

**URL:** <https://forum.code.org/t/passing-arrays-to-functions/33820>\
**Category:** Coding and Debugging Help\
**Tags:** csp-unit-5\
**Created:** [December 2, 2020, 6:16pm UTC](https://forum.code.org/t/passing-arrays-to-functions/33820 "2020-12-02T18:16:36Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![linda.henneberg](https://avatars.discourse-cdn.com/v4/letter/l/0ea827/32.png) [@linda.henneberg](https://forum.code.org/u/linda.henneberg)\
**Post date:** [December 2, 2020, 6:16pm UTC](https://forum.code.org/t/passing-arrays-to-functions/33820/1 "2020-12-02T18:16:36Z")

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**Link to the project or level:** [App Lab - Code.org](https://studio.code.org/projects/applab/qiNKTuhQ90uTqWz0PIwzHdxT1qOYAqBe_g8VT_09eBo)

**What I expect to happen:** My student is trying to create an app which displays a bunch of info for a bunch of countries. The user will pick the region, then pick the country of interest from that region. So in this piece of code, he is trying to filter a list of every country, so that all the countries in South Asia are added to a list. He wanted to do this as a function with parameters so that the function could be reused for every region.

**What actually happens:** He gets this error:

> WARNING: Line: 14: appendItem() list parameter value (southAsiaList) is not a array.ERROR: Line: 14: TypeError: Cannot set property ‘undefined’ of undefined

**What I’ve tried:** He originally tried to do this without a function by tediously filtering the country list for every region (he also wants to do GDP, flag, etc., so it was long, tedious, repetitive code).

We haven’t officially covered parameters yet, so I’m a little rusty on it. I think I can see why that error is happening, but I don’t know how to accomplish what he wants.

Here’s the code:

```
//Creating inital lists from data table
var countryList=getColumn("Countries and Territories","Country Name");
var regionList=getColumn("Countries and Territories","Region");

//Filtered Regions Lists
var southAsiaList=["South Asia"];

filterRegions("southAsia","South Asia");

function filterRegions (variable,place){
    for(var i=0;i<regionList.length;i++){
      if (regionList[i]==place){
        appendItem((variable+"List"),countryList[i]);
  
      }
    }
}

```

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<div class="post-metadata">

**Author:** ![jdonwells](https://sea2.discourse-cdn.com/flex016/user_avatar/forum.code.org/jdonwells/32/7900_2.png) [@jdonwells](https://forum.code.org/u/jdonwells)\
**Post date:** [December 3, 2020, 3:34am UTC](https://forum.code.org/t/passing-arrays-to-functions/33820/2 "2020-12-03T03:34:24Z")

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What is happening is you are passing a string to `filterRegions` for the value of `variable`. You then concatenate that with `"List"` resulting in a string. `appendItem` wants an Array, not a string.

There are three ways to do that two of which are not recommended as good code.

The first is to pass the array, not the name into the function. This is the right way to do it.

The second is to use `eval` like this `appendItem(eval(variable+"List"),countryList[i]);` This is a bad idea but it will work.

The third is to reference the global context like this `appendItem(window[variable+"List"],countryList[i]);` This is also a bad idea that works.

Why are the latter two a bad idea? Because you can pass in the name of a variable that isn’t defined. It will crash without anyone having a good idea of why. It is also very hard to figure out what it actually does because you don’t know which variables nor the types of value they hold unless you run it and stop it in that function.

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<div class="post-metadata">

**Author:** ![linda.henneberg](https://avatars.discourse-cdn.com/v4/letter/l/0ea827/32.png) [@linda.henneberg](https://forum.code.org/u/linda.henneberg)\
**Post date:** [December 3, 2020, 2:09pm UTC](https://forum.code.org/t/passing-arrays-to-functions/33820/3 "2020-12-03T14:09:40Z")

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Thank you! That is so helpful! Can you give a little more detail on what you mean by “pass the array, not the name into the function”?

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<div class="post-metadata">

**Author:** ![jdonwells](https://sea2.discourse-cdn.com/flex016/user_avatar/forum.code.org/jdonwells/32/7900_2.png) [@jdonwells](https://forum.code.org/u/jdonwells)\
**Post date:** [December 3, 2020, 2:41pm UTC](https://forum.code.org/t/passing-arrays-to-functions/33820/4 "2020-12-03T14:41:53Z")

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```javascript
//Creating inital lists from data table
var countryList=getColumn("Countries and Territories","Country Name");
var regionList=getColumn("Countries and Territories","Region");

//Filtered Regions Lists
var southAsiaList=["South Asia"];

filterRegions(southAsiaList,"South Asia");

function filterRegions (variable,place){
    for(var i=0;i<regionList.length;i++){
      if (regionList[i]==place){
        appendItem(variable,countryList[i]);
      }
    }
}

```

When you reference the variable that holds the array in the function call the array itself is passed into the function. So in the function, `variable` holds the array.

You could also change the name `variable` to something like `regions`.