Worked with AWS Support to find a reasonable solution. They also were not sure how to do an atomic counter using the ddb document client (as opposed to the low level client which has many documented examples) but suggested the ADD command, which has the side effect of an atomic update on a numeric field.

So, with the example below, we construct our dynamic update from the object to be stored, then append the ADD statement in the update expression (without a comma!), and add what is in effect a numeric incrementor to the ExpressionAttributeValues for nImports. Like this, which should be a complete working lambda example. There's a few console.log statements to show what's happening:

const AWS = require('aws-sdk');
const docClient = new AWS.DynamoDB.DocumentClient();
async function update (tableName, item, idAttributeNames) {
    var params = {
        TableName: tableName,
        Key: {},
        ExpressionAttributeValues: {},
        ExpressionAttributeNames: {},
        UpdateExpression: "",
        ReturnValues: "UPDATED_NEW"
    };

    for (const attname of idAttributeNames) {
        params["Key"][attname] = item[attname];
    }

    let prefix = "set ";
    let attributes = Object.keys(item);
    for (let i=0; i<attributes.length; i++) {
        let attribute = attributes[i];
        if (!idAttributeNames.includes(attribute)) {
            params["UpdateExpression"] += prefix + "#" + attribute + " = :" + attribute;
            params["ExpressionAttributeValues"][":" + attribute] = item[attribute];
            params["ExpressionAttributeNames"]["#" + attribute] = attribute;
            prefix = ", ";
        }
    }
    console.log('params before adding atomic counter is:', params)
    // Add the counter using the ADD syntax
    params["UpdateExpression"] +=  " ADD #nImports :nImports"
    params["ExpressionAttributeValues"][":nImports"] = 1;
    params["ExpressionAttributeNames"]["#nImports"] = 'nImports'

    console.log('params after adding atomic counter is:', params)
    
    try {
        const result = await docClient.update(params).promise();
        console.log('after await, result is ', result);
        return result;
    } catch (err) {
        console.log('err is ', err)
    }
};



exports.handler = async (event) => {
    
    const item = {title: 'sometitle', site_url: "www.amazon.com", key: "G"};
    const body = await update('test_table', item, ['title', 'site_url']);
    const response = {
        statusCode: 200,
        body: JSON.stringify(body),
    };
    return response;
}

The kind folks at AWS did a little more digging, and also pointed out an error in the initial code that, when corrected, should increment as desired using the SET operator.

Basically the original code didn't properly target the variable for increment. So a corrected version where we add the incremented variable should be:

    console.log('params before adding atomic counter is:', params)
    // Add the counter
    params["UpdateExpression"] += ", #nImports = #nImports + :incr";
    params["ExpressionAttributeValues"][":incr"] = 1;
    //params["ExpressionAttributeValues"][":nImports"] = 0;
    params["ExpressionAttributeNames"]["#nImports"] = 'nImports'

    console.log('params after adding atomic counter is:', params)```

I'm sticking with the original ADD answer because I like the differentiation it gives over the properties inserted by the SET, but both seem valid and I wanted to include the correction as well