feat(core utils): Add threshold_list helper for intersection observers.

Author: thet

src/core/utils.js

@@ -689,6 +689,25 @@ const date_diff = (date_1, date_2) => {
     return Math.floor((utc_1 - utc_2) / _MS_PER_DAY);
 };
 
+/**
+ * Build intersection observer threshold list.
+ *
+ * See: https://developer.mozilla.org/en-US/docs/Web/API/Intersection_Observer_API#building_the_array_of_threshold_ratios
+ *
+ * @param {Number} num_steps - The number of steps to use.
+ *
+ * @returns {Array} - Returns the threshold list.
+ */
+const threshold_list = (num_steps = 0) => {
+    let thresholds = [];
+
+    for (let i = 1.0; i <= num_steps; i++) {
+        thresholds.push(i / num_steps);
+    }
+    thresholds.push(0);
+    return thresholds.sort();
+};
+
 var utils = {
     jqueryPlugin: jqueryPlugin,
     escapeRegExp: escapeRegExp,
@@ -720,6 +739,7 @@ var utils = {
     is_iso_date_time: is_iso_date_time,
     is_iso_date: is_iso_date,
     date_diff: date_diff,
+    threshold_list: threshold_list,
     getCSSValue: dom.get_css_value, // BBB: moved to dom. TODO: Remove in upcoming version.
 };
 

src/core/utils.test.js

@@ -780,3 +780,34 @@ describe("date_diff ...", function () {
         expect(utils.date_diff(date_1, date_2)).toBe(1);
     });
 });
+
+describe("threshold_list ...", function () {
+    it("returns a list with 0 for num_steps 0", () => {
+        expect(utils.threshold_list(0)).toEqual([0]);
+    });
+
+    it("returns a list of thresholds for num_steps 2", () => {
+        expect(utils.threshold_list(2)).toEqual([0, 0.5, 1]);
+    });
+
+    it("returns a list of thresholds for num_steps 4", () => {
+        expect(utils.threshold_list(4)).toEqual([0, 0.25, 0.5, 0.75, 1]);
+    });
+
+    it("returns a list of thresholds for num_steps 5", () => {
+        expect(utils.threshold_list(5)).toEqual([0, 0.2, 0.4, 0.6, 0.8, 1]);
+    });
+
+    it("returns a list of thresholds for num_steps 10", () => {
+        expect(utils.threshold_list(10)).toEqual([
+            0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1,
+        ]);
+    });
+
+    it("returns a list of thresholds for num_steps 20", () => {
+        expect(utils.threshold_list(20)).toEqual([
+            0, 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 0.55, 0.6, 0.65,
+            0.7, 0.75, 0.8, 0.85, 0.9, 0.95, 1,
+        ]);
+    });
+});