if statements in JavaScript are structured like this:

if (condition) {


    // execute if true


}

To execute some other code if the condition is false, use this structure:

if (condition) {


    // execute if true


} else {


    // execute if false


}

Here’s how I applied that to your script:

let repoT = base.getTable('Repo');


let sourceT = base.getTable('Source');





let record = await input.recordAsync('Pick a record', sourceT);


// let name = record.getCellValue('Part 1') + '-' + record.getCellValue('Part 2');





if (!record.getCellValue('Repo')) {


    let name = record.getCellValue('Data');


    // create the record in the repo table


    let newRepoRecord = await repoT.createRecordAsync({


            'Name': name


    })


        // set variable for today's date


    let now = new Date().toLocaleDateString("en-GB");


    //note that newRepoRecord is the record ID of the created record


    // see this by logging the result:





    console.log(newRepoRecord);





    // now take the newly created record ID


    // and set the linked field from this


    let updatedRecord = await sourceT.updateRecordAsync(record, {


        'Repo': :{id: newRepoRecord}],


        'Part Created': "Part Entered " + now


    })


} else {


    console.log("Repo is already linked for this record");


}

record.getCellValue('Repo') will return null if the field is empty, and null is equivalent to false for the purposes of these conditional tests, while any data returned would be equivalent to true. Because you want to run the remaining code if it is empty, the exclamation point is added in front of the condition to invert the result, making false become true and vice versa. In short, the if structure as written above effectively says, “if the field is empty, run this stuff; otherwise display a message”.

Absolutely brilliant @Justin_Barrett , thanks so much!!!

This works like a charm


Best wishes


Tom