You are passing the string "field" as the value field, Try pass a real field name, for example:
const jsonFile = require('jsonFile');
const object = JSON.parse(jsonFile);
fieldName = 'email';
userSchema.statics.updated(_id, object[fieldName], false));
// You can also:
const keys = Object.keys(object);
userSchema.statics.updated(_id, object[keys[0]], false));
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