diff --git a/biome.json b/biome.json
index dabb2cce..fe1b09d4 100644
--- a/biome.json
+++ b/biome.json
@@ -15,6 +15,9 @@
       "style": {
         "useBlockStatements": "off",
         "noShoutyConstants": "warn"
+      },
+      "performance": {
+        "noDelete": "off"
       }
     }
   },
diff --git a/umap/static/umap/js/umap.features.js b/umap/static/umap/js/umap.features.js
index 061030f6..c2764884 100644
--- a/umap/static/umap/js/umap.features.js
+++ b/umap/static/umap/js/umap.features.js
@@ -312,7 +312,7 @@ U.FeatureMixin = {
     // Retrocompat
     if (this.properties._umap_options.clickable === false) {
       this.properties._umap_options.interactive = false
-      this.properties._umap_options.clickable = undefined
+      delete this.properties._umap_options.clickable
     }
   },
 
@@ -377,7 +377,7 @@ U.FeatureMixin = {
     const properties = L.extend({}, this.properties)
     properties._umap_options = L.extend({}, properties._umap_options)
     if (Object.keys && Object.keys(properties._umap_options).length === 0) {
-      properties._umap_options = undefined // It can make a difference on big data sets
+      delete properties._umap_options // It can make a difference on big data sets
     }
     return properties
   },
@@ -396,7 +396,6 @@ U.FeatureMixin = {
     const geojson = this.parentClass.prototype.toGeoJSON.call(this)
     geojson.properties = this.cloneProperties()
     geojson.id = this.id
-    // biome-ignore lint/performance/noDelete: we do not want the key at all.
     delete geojson.properties._storage_options
     return geojson
   },
@@ -579,8 +578,8 @@ U.FeatureMixin = {
 
   clone: function () {
     const geoJSON = this.toGeoJSON()
-    geoJSON.id = undefined
-    geoJSON.properties.id = undefined
+    delete geoJSON.id
+    delete geoJSON.properties.id
     const layer = this.datalayer.geojsonToFeatures(geoJSON)
     layer.isDirty = true
     layer.edit()
@@ -1100,7 +1099,7 @@ U.Polyline = L.Polyline.extend({
       U.Utils.flattenCoordinates(geojson.geometry.coordinates),
     ]
 
-    geojson.id = undefined // delete the copied id, a new one will be generated.
+    delete geojson.id // delete the copied id, a new one will be generated.
 
     const polygon = this.datalayer.geojsonToFeatures(geojson)
     polygon.edit()
@@ -1235,8 +1234,8 @@ U.Polygon = L.Polygon.extend({
 
   toPolyline: function () {
     const geojson = this.toGeoJSON()
-    geojson.id = undefined
-    geojson.properties.id = undefined
+    delete geojson.id
+    delete geojson.properties.id
     geojson.geometry.type = 'LineString'
     geojson.geometry.coordinates = U.Utils.flattenCoordinates(
       geojson.geometry.coordinates
diff --git a/umap/static/umap/js/umap.forms.js b/umap/static/umap/js/umap.forms.js
index 1243b466..0d698624 100644
--- a/umap/static/umap/js/umap.forms.js
+++ b/umap/static/umap/js/umap.forms.js
@@ -1133,7 +1133,7 @@ U.FormBuilder = L.FormBuilder.extend({
         }
       }
       // FormBuilder use this key for the input type itself
-      schema.type = undefined
+      delete schema.type
       this.defaultOptions[key] = schema
     }
   },
diff --git a/umap/static/umap/js/umap.js b/umap/static/umap/js/umap.js
index b3ca3b5c..79348124 100644
--- a/umap/static/umap/js/umap.js
+++ b/umap/static/umap/js/umap.js
@@ -104,7 +104,7 @@ U.Map = L.Map.extend({
     }
     if (this.options.advancedFilterKey) {
       this.options.facetKey = this.options.advancedFilterKey
-      this.options.advancedFilterKey = undefined
+      delete this.options.advancedFilterKey
     }
 
     // Global storage for retrieving datalayers and features
@@ -130,14 +130,14 @@ U.Map = L.Map.extend({
       if (!this.options.onLoadPanel) {
         this.options.onLoadPanel = 'caption'
       }
-      this.options.displayCaptionOnLoad = undefined
+      delete this.options.displayCaptionOnLoad
     }
     if (this.options.displayDataBrowserOnLoad) {
       // Retrocompat
       if (!this.options.onLoadPanel) {
         this.options.onLoadPanel = 'databrowser'
       }
-      this.options.displayDataBrowserOnLoad = undefined
+      delete this.options.displayDataBrowserOnLoad
     }
     if (this.options.datalayersControl === 'expanded') {
       this.options.onLoadPanel = 'datalayers'
@@ -901,7 +901,7 @@ U.Map = L.Map.extend({
     importedData.layers.forEach((geojson) => {
       if (!geojson._umap_options && geojson._storage) {
         geojson._umap_options = geojson._storage
-        geojson._storage = undefined
+        delete geojson._storage
       }
       delete geojson._umap_options?.id // Never trust an id at this stage
       const dataLayer = this.createDataLayer(geojson._umap_options)
diff --git a/umap/static/umap/js/umap.layer.js b/umap/static/umap/js/umap.layer.js
index e1e9432f..2d4d0133 100644
--- a/umap/static/umap/js/umap.layer.js
+++ b/umap/static/umap/js/umap.layer.js
@@ -568,11 +568,11 @@ U.DataLayer = L.Evented.extend({
     // Retrocompat
     if (this.options.remoteData?.from) {
       this.options.fromZoom = this.options.remoteData.from
-      this.options.remoteData.from = undefined
+      delete this.options.remoteData.from
     }
     if (this.options.remoteData?.to) {
       this.options.toZoom = this.options.remoteData.to
-      this.options.remoteData.to = undefined
+      delete this.options.remoteData.to
     }
     this.backupOptions()
     this.connectToMap()
@@ -834,7 +834,7 @@ U.DataLayer = L.Evented.extend({
   },
 
   setOptions: function (options) {
-    options.geojson = undefined
+    delete options.geojson
     this.options = U.Utils.CopyJSON(U.DataLayer.prototype.options) // Start from fresh.
     this.updateOptions(options)
   },
@@ -1202,7 +1202,7 @@ U.DataLayer = L.Evented.extend({
   clone: function () {
     const options = U.Utils.CopyJSON(this.options)
     options.name = L._('Clone of {name}', { name: this.options.name })
-    options.id = undefined
+    delete options.id
     const geojson = U.Utils.CopyJSON(this._geojson)
     const datalayer = this.map.createDataLayer(options)
     datalayer.fromGeoJSON(geojson)
@@ -1222,8 +1222,8 @@ U.DataLayer = L.Evented.extend({
     this.map.off('zoomend', this.onZoomEnd, this)
     this.off()
     this.clear()
-    this._loaded = undefined
-    this._dataloaded = undefined
+    delete this._loaded
+    delete this._dataloaded
   },
 
   reset: function () {
@@ -1701,7 +1701,7 @@ U.DataLayer = L.Evented.extend({
       if (data.geojson) {
         this.clear()
         this.fromGeoJSON(data.geojson)
-        data.geojson = undefined
+        delete data.geojson
       }
       this._reference_version = response.headers.get('X-Datalayer-Version')
       this.sync.update('_reference_version', this._reference_version)
diff --git a/umap/static/umap/js/umap.share.js b/umap/static/umap/js/umap.share.js
index 567b8fd0..a2ab6106 100644
--- a/umap/static/umap/js/umap.share.js
+++ b/umap/static/umap/js/umap.share.js
@@ -21,7 +21,6 @@ U.Share = L.Class.extend({
         map.eachFeature((feature) => {
           const row = feature.toGeoJSON().properties
           const center = feature.getCenter()
-          // biome-ignore lint/performance/noDelete: we do not want the key at all.
           delete row._umap_options
           row.Latitude = center.lat
           row.Longitude = center.lng
@@ -225,7 +224,7 @@ U.IframeExporter = L.Evented.extend({
     if (this.options.viewCurrentFeature && this.map.currentFeature) {
       this.queryString.feature = this.map.currentFeature.getSlug()
     } else {
-      this.queryString.feature = undefined
+      delete this.queryString.feature
     }
     if (this.options.keepCurrentDatalayers) {
       this.map.eachDataLayer((datalayer) => {
@@ -235,7 +234,7 @@ U.IframeExporter = L.Evented.extend({
       })
       this.queryString.datalayers = datalayers.join(',')
     } else {
-      this.queryString.datalayers = undefined
+      delete this.queryString.datalayers
     }
     const currentView = this.options.currentView ? window.location.hash : ''
     const queryString = L.extend({}, this.queryString, options)
diff --git a/umap/static/umap/test/DataLayer.js b/umap/static/umap/test/DataLayer.js
index 26ba7d61..201f8a7f 100644
--- a/umap/static/umap/test/DataLayer.js
+++ b/umap/static/umap/test/DataLayer.js
@@ -316,7 +316,7 @@ describe('U.DataLayer', () => {
       let icon = qs('div.umap-div-icon .icon_container')
       poly.properties.mycolor = 'DarkGoldenRod'
       marker.properties.mycolor = 'DarkRed'
-      marker.properties._umap_options.color = undefined
+      delete marker.properties._umap_options.color
       assert.notOk(qs('path[fill="DarkGoldenRod"]'))
       assert.equal(icon.style.backgroundColor, 'olivedrab')
       datalayer.options.color = '{mycolor}'