RED.util.prepareJSONataExpression and RED.util.evaluateJSONataExpression

[skavan]

Hi,

I have written a custom node and in my test harness, with jsonata installed, it works fine, a simplified example is here:

expression = jsonata(`{"topic": $jaTopic, "retain": true, "delay": 0,

               "payload" : $map($lookup(valuemaps,$jaCommand),

                function($v, $i, $a){{"displayValue": $v.displayValue, "command" : $v.standardValue[0] }})}`);

expression.assign('jaTopic', var1);

expression.assign('jaCommand', var2);

return expression.evaluate(jsonData);

Now, as I head over to my node-red (rpi) test-bed, I belatedly notice that the approach used by other nodes is to use:

RED.util.prepareJSONataExpression

and

RED.util.evaluateJSONataExpression

So, I guess I have two questions:

1. Should I avoid npm'ing a jsonata install for my own nodes.

2. How do I map the code above into the RED.util functions? I can't find any docs!

Thanks.

[Nick O'Leary]

Hi,

you can see the definition of these two functions here: https://github.com/node-red/node-red/blob/master/red/runtime/util.js#L335

The prepareJSONataExpression does a couple extra things:

 - it defines the $flowContext and $globalContext functions that allow an expression to lookup values from context

 - it provides a custom clone function - needed as JSONata's native clone function cannot clone some node.js types (as its meant for pure JSON). This is needed in particular when used in a flow that starts with HTTP In as the msg.req and msg.res objects are not cloneable.

 - it also assumes you are going to be evaluating 

RED.util.prepareJSONataExpression is pretty much a drop-in replacement for the jsonata function. It does take a second argument, the node evaluating the expression if you want to use the flowContext/globalContext functions.

    expression = RED.util.prepareJSONataExpression(`{"topic": $jaTopic, "retain": true, "delay": 0,

                "payload" : $map($lookup(valuemaps,$jaCommand), 

                function($v, $i, $a){{"displayValue": $v.displayValue, "command" : $v.standardValue[0] }})}`);

    expression.assign('jaTopic', var1);

    expression.assign('jaCommand', var2);          

    return RED.util.evaluateJSONataExpression(expression,jsonData);

Nick

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[skavan]

Thanks. Super helpful.

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