从字典中提取值（在 Node-RED 中）

Question

I'm making a simple HTTP GET request

and the response, assuming the function just contains return msg; , is a dictionary, such as:

{
    "data": [{
        "time": "2020-10-01",
        "value": 998.37
    }],
    "id": "27",
    "tags": []
}

Now I want to extract just the value itself. How can I do that? If I try return msg['data']['value']; I get a TypeError TypeError: Cannot read property 'value' of undefined .

Edit: I tried Pogrindis approach, but am struggling with the Node-RED flowfiles.

The JSON of my Node-RED interface is pretty simple, the function is a one-liner

return (msg.payload)['data'][0]['value']; :

[{
    "id": "cda42b56.ad2258",
    "type": "tab",
    "label": "Test",
    "disabled": false,
    "info": ""
}, {
    "id": "95fa7937.aa8e48",
    "type": "debug",
    "z": "cda42b56.ad2258",
    "name": "Logger",
    "active": true,
    "tosidebar": true,
    "console": true,
    "tostatus": false,
    "complete": "payload",
    "targetType": "msg",
    "statusVal": "",
    "statusType": "auto",
    "x": 480,
    "y": 80,
    "wires": []
}, {
    "id": "200acc6f.e2ff54",
    "type": "function",
    "z": "cda42b56.ad2258",
    "name": "",
    "func": "return (msg.payload)['data'][0]['value'];\n",
    "outputs": 1,
    "noerr": 0,
    "initialize": "",
    "finalize": "",
    "x": 300,
    "y": 80,
    "wires": [
        ["95fa7937.aa8e48"]
    ]
}, {
    "id": "831bfe43.3d956",
    "type": "inject",
    "z": "cda42b56.ad2258",
    "name": "",
    "props": [{
        "p": "payload"
    }],
    "repeat": "",
    "crontab": "",
    "once": false,
    "onceDelay": 0.1,
    "topic": "",
    "payload": "{\"data\":[{\"time\":\"2020\",\"value\":997.62}],\"id\":\"xy123\"}",
    "payloadType": "str",
    "x": 120,
    "y": 80,
    "wires": [
        ["200acc6f.e2ff54"]
    ]
}]

Answer 1

data is an array, and value is not the index of said array.

You could use an index for data I guess

const testData = {
    "data": [{
        "time": "2020-10-01",
        "value": 998.37
    }],
    "id": "27",
    "tags": []
};

console.log(testData['data'][0]['value']);

JSFiddle

I say I guess, because with N number of data, you might be better off using some kind of iterator/filter etc but I don't know your use case.

Answer 2

The important thing to remember is that a Function node must always return a msg object, not a single value.

The error is coming from the debug node (if you mouse over the error in the sidebar it will show which node it came from) because you are returning a raw value and not a msg object.

Your function is returning just a number, this will not work.

The function should read:

msg.payload = msg.payload.data.[0].value;
return msg;

The docs about writing functions nodes can be found here

Answer 3

The flow you have given shows that you are sending a text in the inject node instead of a JSON object.

Image of injecting node

This should be changed to the JSON option. Alternatively, you could use the JSON.parse() method in your function node.

Node red nodes communicate through JSON objects, therefore the output of your function also needs to be a JSON object.

return {"value" : msg.payload.data[0].value}

Alternatively if your data list is dynamic in length you can use

return {"values" : msg.payload.data.map(object => object.value)}