Javascript node-red 将 json 字符串解析为 msg.payload
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node-red parsing a json string to msg.payload
提问by Michael
Noob question
菜鸟问题
I am using the snmp function to collect data via oid lists. I have extracted the data and used the json function to parse the data into a json string seen below.
我正在使用 snmp 函数通过 oid 列表收集数据。我已经提取了数据并使用 json 函数将数据解析为如下所示的 json 字符串。
msg.payload : string[81]
"[{"oid":"1.3.6.1.4.1.38783.3.3.1.1.1.0","type":2,"value":53800,"tstr":"Integer"}]"
msg.payload : 字符串[81]
"[{"oid":"1.3.6.1.4.1.38783.3.3.1.1.1.0","type":2,"value":53800,"tstr":"Integer"}]"
I am trying to write a function to strip out "value":53800from this string and output it in msg.payload.
我正在尝试编写一个函数来"value":53800从这个字符串中剥离出来并在msg.payload.
I have tried below but it returns
我在下面尝试过，但它返回
"TypeError: Cannot assign to read only property
'_msgid'of"value":53700,"tstr":"Integer"}]"
“类型错误：无法分配给只读属性
'_msgid'的"value":53700,"tstr":"Integer"}]"
var msg = msg.payload;
var value = msg.substr(49,62);
return value;
回答by hardillb
Don't try and split the string up like that, it's too prone to errors if the value lengths ever change.
不要尝试像那样拆分字符串，如果值长度发生变化，则很容易出错。
Instead run the message through the JSON node before the function node.
而是在函数节点之前通过 JSON 节点运行消息。
This will parse the string and generate a proper JSON object.
这将解析字符串并生成正确的 JSON 对象。
You can then access value field as such:
然后，您可以像这样访问值字段：
var value = msg.payload[0].value;
msg.payload = value;
return msg;
The Cannot assign to read only propertyerror is because you returned a string from the function. You need to return a message JSON object not a string. My example sets the msg.payloadto the required value.
该Cannot assign to read only property错误是因为你返回从函数的字符串。您需要返回消息 JSON 对象而不是字符串。我的示例将 设置msg.payload为所需的值。
回答by spiralbrain
For your JSON the following code will work in the function:
对于您的 JSON，以下代码将在函数中工作：
var value = msg.payload.value;
msg.payload = value;
return msg;
