# Bug updating global arrays

**URL:** <https://discourse.nodered.org/t/bug-updating-global-arrays/2403>\
**Category:** General\
**Created:** [19 August 2018 16:08 UTC](https://discourse.nodered.org/t/bug-updating-global-arrays/2403 "2018-08-19T16:08:54Z")\
**Posts on this page:** 6\
**Page:** 1

<div class="post-metadata">

**Author:** ![blechdom](https://sea2.discourse-cdn.com/flex026/user_avatar/discourse.nodered.org/blechdom/32/707_2.png) [@blechdom](https://discourse.nodered.org/u/blechdom)\
**Post date:** [19 August 2018 16:08 UTC](https://discourse.nodered.org/t/bug-updating-global-arrays/2403/1 "2018-08-19T16:08:54Z")

</div>

I've been hitting my head against a certain code behavior which I believe`I've captured in the following flow.  
I'm trying to do something quite simple.  
I have two global arrays (flow arrays): current\_array and last\_array  
I want to be able to update the current\_array one element at a time, and at certain times, I would like to copy the current\_array to the last\_array.

This works fine when I update the entire array like this:

flow.set("current\_array", [1,2,3,4,5,6]);

but it fails (sometimes) when I update it like this:

var current\_array = flow.get("current\_array");  
current\_array[0] = 500;  
flow.set("current\_array", current\_array);

It only fails after I have copied a current\_array to the last\_array. Almost always immediately after I have done so. It's as if, updating the current\_array also updates last\_array, or that by updating current\_array, it triggers the function containing the "set last\_array to current\_array". If I comment out the "set last\_array to current\_array", it doesn't happen, but also the code stops working. I don't understand why the "set last\_array to current\_array" triggers without input, only when I update a global variable, sometimes.

I am happy to explain the order of operations in order to make the bug happen, if it is unclear.

Any help is greatly appreciated.

```auto
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flow.get(\"last_array\");\nreturn msg;","outputs":1,"noerr":0,"x":619.0000152587891,"y":202.00000381469727,"wires":[["c35d6fc7.c0cae"]]},{"id":"c35d6fc7.c0cae","type":"debug","z":"6fb2422e.195b2c","name":"","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"false","x":849.0000152587891,"y":202.00000381469727,"wires":[]},{"id":"d6183394.aec2f","type":"inject","z":"6fb2422e.195b2c","name":"set last array to current array","topic":"","payload":"","payloadType":"date","repeat":"","crontab":"","once":false,"onceDelay":0.1,"x":360.00001525878906,"y":720.0000076293945,"wires":[["234ef22a.b130be"]]},{"id":"234ef22a.b130be","type":"function","z":"6fb2422e.195b2c","name":"set last_array to current_array","func":"var current_array = flow.get(\"current_array\");\nflow.set(\"last_array\", current_array);\nmsg.payload = \"set last array to current array\";\nreturn msg;","outputs":1,"noerr":0,"x":630.0000152587891,"y":720.0000076293945,"wires":[["9990ef93.d4e31"]]},{"id":"9990ef93.d4e31","type":"debug","z":"6fb2422e.195b2c","name":"","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"false","x":869.0000152587891,"y":720.0000114440918,"wires":[]},{"id":"b64a4983.51a9a8","type":"inject","z":"6fb2422e.195b2c","name":"load preset 1","topic":"","payload":"","payloadType":"date","repeat":"","crontab":"","once":false,"onceDelay":0.1,"x":450,"y":320,"wires":[["8bf9b031.9fcdd"]]},{"id":"ff41b1cb.1daa9","type":"inject","z":"6fb2422e.195b2c","name":"load preset 2","topic":"","payload":"","payloadType":"date","repeat":"","crontab":"","once":false,"onceDelay":0.1,"x":449.00001525878906,"y":361.0000057220459,"wires":[["87cfdf89.43fc7"]]},{"id":"7f760e6b.b2b4c","type":"inject","z":"6fb2422e.195b2c","name":"load preset 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\" + flow.get(\"current_array\");\nreturn msg;","outputs":1,"noerr":0,"x":699.0000152587891,"y":400.0000057220459,"wires":[[]]},{"id":"b1e3cb41.30f1b8","type":"inject","z":"6fb2422e.195b2c","name":"update 1 part of array","topic":"","payload":"","payloadType":"date","repeat":"","crontab":"","once":false,"onceDelay":0.1,"x":482.00001525878906,"y":483.00000953674316,"wires":[["6f554c71.7c2224"]]},{"id":"9dc50cbe.e2cc5","type":"inject","z":"6fb2422e.195b2c","name":"update 1 part of array","topic":"","payload":"","payloadType":"date","repeat":"","crontab":"","once":false,"onceDelay":0.1,"x":482.00000762939453,"y":524.0000076293945,"wires":[["2f310b17.a747b4"]]},{"id":"d99fafa4.6e51f","type":"inject","z":"6fb2422e.195b2c","name":"update 1 part of 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msg;","outputs":1,"noerr":0,"x":702.0000076293945,"y":524.0000076293945,"wires":[[]]},{"id":"e6dca23a.5e329","type":"function","z":"6fb2422e.195b2c","name":"setting current array","func":"var current_array = flow.get(\"current_array\");\ncurrent_array[5] = 1000;\nflow.set(\"current_array\", current_array);\nmsg.payload = \"setting current array to preset 1: \" + flow.get(\"current_array\");\nreturn msg;","outputs":1,"noerr":0,"x":704.0000152587891,"y":566.0000085830688,"wires":[[]]}]

```

---

<div class="post-metadata">

**Author:** ![ghayne](https://sea2.discourse-cdn.com/flex026/user_avatar/discourse.nodered.org/ghayne/32/39_2.png) [@ghayne](https://discourse.nodered.org/u/ghayne)\
**Post date:** [19 August 2018 16:39 UTC](https://discourse.nodered.org/t/bug-updating-global-arrays/2403/2 "2018-08-19T16:39:40Z")

</div>

Kristin,

if you edit your post and put 3 backticks ``` before and after your code it will get formatted correctly.

---

<div class="post-metadata">

**Author:** ![Andrei](https://sea2.discourse-cdn.com/flex026/user_avatar/discourse.nodered.org/andrei/32/10446_2.png) [@Andrei](https://discourse.nodered.org/u/Andrei)\
**Post date:** [19 August 2018 17:00 UTC](https://discourse.nodered.org/t/bug-updating-global-arrays/2403/3 "2018-08-19T17:00:20Z")

</div>

> [@blechdom](#):
>
> It only fails after I have copied a current\_array to the last\_array. Almost always immediately after I have done so. It's as if, updating the current\_array also updates last\_array,

HI, indeed, this unwanted behavior will happen when you try to copy an array to another array using the assignment operator (or using the flow.set like you did). In JavaScript assigning current\_array to last\_array will make last\_array to point to the same elements of current\_array, i.e, changing current\_array will affect last\_array and vice\_versa.

```auto
var current_array = flow.get("current_array");
flow.set("last_array", current_array);
msg.payload = "set last array to current array";
return msg;

```

Try to use this statement to copy current\_array to last\_array:

```auto
last_array = [...current_array];

```

Only then you save last\_array to the flow context

```auto
flow.set("last_array", last_array);

```

edit:

Therefore this is how your last function will looks like:

```auto
var current_array = flow.get("current_array");
last_array = [...current_array];
flow.set("last_array", last_array);
msg.payload = "set last array to current array";
return msg;

```

---

<div class="post-metadata">

**Author:** ![blechdom](https://sea2.discourse-cdn.com/flex026/user_avatar/discourse.nodered.org/blechdom/32/707_2.png) [@blechdom](https://discourse.nodered.org/u/blechdom)\
**Post date:** [19 August 2018 17:12 UTC](https://discourse.nodered.org/t/bug-updating-global-arrays/2403/4 "2018-08-19T17:12:09Z")

</div>

Alas, everything in the world makes sense again.  
thank you!

---

<div class="post-metadata">

**Author:** ![Andrei](https://sea2.discourse-cdn.com/flex026/user_avatar/discourse.nodered.org/andrei/32/10446_2.png) [@Andrei](https://discourse.nodered.org/u/Andrei)\
**Post date:** [19 August 2018 18:30 UTC](https://discourse.nodered.org/t/bug-updating-global-arrays/2403/5 "2018-08-19T18:30:17Z")

</div>

For those who want to understand better what it means passing by reference and the shalow copy solution from this post here is a good video: [https://www.youtube.com/watch?v=duyshh9Fs1U](https://www.youtube.com/watch?v=duyshh9Fs1U)

---

<div class="post-metadata">

**Author:** ![orimate](https://sea2.discourse-cdn.com/flex026/user_avatar/discourse.nodered.org/orimate/32/71872_2.png) [@orimate](https://discourse.nodered.org/u/orimate)\
**Post date:** [3 December 2022 13:56 UTC](https://discourse.nodered.org/t/bug-updating-global-arrays/2403/6 "2022-12-03T13:56:02Z")

</div>

You saved my day. Thank you for this link.