Formulas

A Formula property computes its value from the page’s other properties. The formula is a small JavaScript function in the collection’s code. You don’t need the rest of the plugin API to write one.

Add a formula

1. In Collection Settings…, add a property of type Formula. Call it Total.
2. Add the inputs it needs, say Number properties Hours and Rate.
3. Choose Edit formula in Code Editor on the Formula property.
4. Register the calculation in onLoad():

class Plugin extends CollectionPlugin {
    onLoad() {
        this.properties.formula("Total", ({record}) => {
            const hours = record.number("Hours");
            const rate = record.number("Rate");
            if (hours == null || rate == null) return null;
            return hours * rate;
        });
    }
}

Preview, check a page with values, one without, and one with zero, then save. If the collection already has code, add the formula call to its existing onLoad() instead of replacing the class.

Writing them

Return null when there is no sensible answer, so an empty input stays empty instead of becoming zero. Formulas run every time a row is drawn, so keep them quick: read the properties you need and return a value, without network requests or writes.

Readers: record.number(), record.text(), record.choice(), record.users(), record.linkedRecord() and record.linkedRecords(). A choice comes back as its id; PluginProperty.choiceLabel() gives the label.

Two more examples

A progress percentage from two numbers:

this.properties.formula("Progress", ({record}) => {
    const done = record.number("Completed");
    const planned = record.number("Planned");
    if (done == null || planned == null || planned <= 0) return null;
    return Math.round(100 * done / planned);
});

The number of linked projects, from a multi-value Page Link, which is how you build a rollup:

this.properties.formula("Project count", ({record}) => {
    return record.linkedRecords("Projects").length;
});

For custom rendering, sorting and views, continue with Collection plugins.