Layer Set Mask Feather Value

In it's simplest form:

app.activeDocument.activeLayer.layerMaskFeather = 25;

However, in practice, you would probably wish to set up a test to see if the active layer actually had a layer mask applied beforehand:

if (hasLayerMask() == true) { // Only if it has a layer mask
    app.activeDocument.activeLayer.layerMaskFeather = 25;
}

//github.com/ES-Collection/Photoshop-Scripts/blob/master/Link%20All%20Masks.jsx

///////////////////////////////////////////////////////////////////////////////
// Function: hasLayerMask
// Usage: see if there is a raster layer mask
// Input: <none> Must have an open document
// Return: true if there is a vector mask
///////////////////////////////////////////////////////////////////////////////
function hasLayerMask() {
    var hasLayerMask = false;
    try {
        var ref = new ActionReference();
        var keyUserMaskEnabled = app.charIDToTypeID('UsrM');
        ref.putProperty(app.charIDToTypeID('Prpr'), keyUserMaskEnabled);
        ref.putEnumerated(app.charIDToTypeID('Lyr '), app.charIDToTypeID('Ordn'), app.charIDToTypeID('Trgt'));
        var desc = executeActionGet(ref);
        if (desc.hasKey(keyUserMaskEnabled)) {
            hasLayerMask = true;
        }
    } catch (e) {
        hasLayerMask = false;
    }
    return hasLayerMask;
}