User properties

User properties are a powerful feature in PostHog which enable you to better manage, analyze, and control data. You can create filters or cohorts based on user properties, which can then be used to create insights, feature flags, and more.

How to set user properties

The recommended way to set user properties is to include them when capturing an event:

posthog.capture(
    'event_name', 
    { 
        $set: { name: 'Max Hedgehog'  },
        $set_once: { initial_url: '/blog' },
    }
)

You can also set user properties when you call the identify method:

posthog.identify(
    'distinct_id',
    { email: 'max@hedgehogmail.com', name: 'Max Hedgehog' } 
    {}, 
);

User property values can be strings, booleans, numbers, objects, or arrays. For objects and arrays, you can use HogQL to access nested properties in the PostHog app.

Note that user properties are set in the order the events are ingested, and not according to event timestamps. Since we typically ingest events as soon as we receive them, you only need to take this into consideration when you're importing historical data.

What is the difference between set and set_once?

When using set, it will replace any value that may have been set on a person for a specific property. In contrast, set_once will only set the property if it has never been set before.

set is typically used for properties that may change over time – e.g., email, current plan, organization name. set_once is typically only used for information that is guaranteed to never change – e.g., the first URL a user visited, or the date a user first logged in.

For example:

posthog.capture(
    'event_name', 
    { 
        $set: { name: 'Max Hedgehog'  },
        $set_once: { initial_url: '/blog' },
    }
)
posthog.capture(
    'event_name', 
    { 
        $set: { name: 'Mr. Fox'  },
        $set_once: { initial_url: '/pricing' },
    }
)
// name: 'Mr. Fox'
// initial_url: '/blog'

Further reading

For detailed information on how to use user properties in PostHog, check out our full user properties docs.