Stringify ObjectID in JSON.stringify when running js in MongoDB terminal

json-stringify-objectid.js

// when running a js script directly from mongo terminal
// and want the output saved in a file
// JSON.stringify the query result will result in empty object {} for ObjectId
// use the following function as a replacer 

function stringifyObjectId(key, value) {
    if (value instanceof ObjectId) {
        return value.str; // example result "6100f43d0e2d24b8c7cd58af"
    }
    return value;
}

// example
JSON.stringify(result, stringifyObjectId, 4)