# Indexing into flow/global scope array element using variable

**URL:** https://discourse.nodered.org/t/indexing-into-flow-global-scope-array-element-using-variable/28518
**Category:** General
**Created:** [18 June 2020 05:34 UTC](https://discourse.nodered.org/t/indexing-into-flow-global-scope-array-element-using-variable/28518 "2020-06-18T05:34:20Z")
**Posts on this page:** 3
**Page:** 1

<div class="post-metadata">

### Author: ![Eric](https://avatars.discourse-cdn.com/v4/letter/e/898d66/32.png) [@Eric](https://discourse.nodered.org/u/Eric)
#### Post date: [18 June 2020 05:34 UTC](https://discourse.nodered.org/t/indexing-into-flow-global-scope-array-element-using-variable/28518/1 "2020-06-18T05:34:21Z")

</div>

I have a flow scoped array that I initialized like this:

`flow.set('status',[true,true,true,true,true,true,true,true,true,true]);`

Later on, I'm doing some processing to figure out which element of that array I want to make a change to. So I can do this if I want to change element [5] of that array.

`flow.set('status[5]', msg.payload);`

That works fine. But I actually want to use a variable instead of hard coding in the index value, so more like this:

`flow.set('status[index_match]', msg.payload);`

But node-red throws an error when I use a variable "index\_match".

> function : (error)"Error: Invalid property expression: unexpected i at position 7"

I can can kinda see why, a variable inside the single quotes is weird. Is there some other syntax for addressing an element of a flow scoped array? I'd rather not have to resort to this ugliness:

```auto
if (index_match == 0)
  flow.set('status[0]', msg.payload);
else if (index_match == 1)
 flow.set('status[1]', msg.payload);
else if (index_match == 2)
 flow.set('status[2]', msg.payload);

```

Edit: I also tried this syntax with the index outside the quotes.

`flow.set('status'[index_match], msg.payload);`

This runs, but doesn't actually change the array element, so doesn't do what I'm intending.

---

<div class="post-metadata">

### Author: ![kuema](https://sea2.discourse-cdn.com/flex026/user_avatar/discourse.nodered.org/kuema/32/6542_2.png) [@kuema](https://discourse.nodered.org/u/kuema)
#### Post date: [18 June 2020 06:40 UTC](https://discourse.nodered.org/t/indexing-into-flow-global-scope-array-element-using-variable/28518/2 "2020-06-18T06:40:07Z")

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The key of the context functions is actually an expression evaluated by the runtime, it's behaves not like a map or dictionary. That is the reason why something like `flow.set('status[5]', msg.payload);` works in the first place.

Now if you do `flow.set('status[index_match]', msg.payload);`, the runtime evaluates `status[index_match]`, but your local `index_match` variable is unknown to it. So it throws an error.

I see two possible solutions:

Get the status array from context and use it in your script like a normal array

```auto
let status = flow.get('status');
status[index_match] = msg.payload;

```

That will work because your array (or any object) is accessed by reference.

OR

use a template string or string concatenation and build the string beforehand.

```javascript

flow.set(`status[${index_match}]`, msg.payload);

```

---

<div class="post-metadata">

### Author: ![system](https://us1.discourse-cdn.com/flex026/uploads/nodered/original/1X/d073cd938eafa2e558d7c2cd59003b3ef4963033.png) [@system](https://discourse.nodered.org/u/system)
#### Post date: [17 August 2020 06:40 UTC](https://discourse.nodered.org/t/indexing-into-flow-global-scope-array-element-using-variable/28518/3 "2020-08-17T06:40:21Z")

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