Initial Commit

node_modules/web-vitals/src/onFCP.ts

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+/*
+ * Copyright 2020 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import {onBFCacheRestore} from './lib/bfcache.js';
+import {bindReporter} from './lib/bindReporter.js';
+import {doubleRAF} from './lib/doubleRAF.js';
+import {getActivationStart} from './lib/getActivationStart.js';
+import {getVisibilityWatcher} from './lib/getVisibilityWatcher.js';
+import {initMetric} from './lib/initMetric.js';
+import {observe} from './lib/observe.js';
+import {whenActivated} from './lib/whenActivated.js';
+import {FCPMetric, MetricRatingThresholds, ReportOpts} from './types.js';
+
+/** Thresholds for FCP. See https://web.dev/articles/fcp#what_is_a_good_fcp_score */
+export const FCPThresholds: MetricRatingThresholds = [1800, 3000];
+
+/**
+ * Calculates the [FCP](https://web.dev/articles/fcp) value for the current page and
+ * calls the `callback` function once the value is ready, along with the
+ * relevant `paint` performance entry used to determine the value. The reported
+ * value is a `DOMHighResTimeStamp`.
+ */
+export const onFCP = (
+  onReport: (metric: FCPMetric) => void,
+  opts?: ReportOpts,
+) => {
+  // Set defaults
+  opts = opts || {};
+
+  whenActivated(() => {
+    const visibilityWatcher = getVisibilityWatcher();
+    let metric = initMetric('FCP');
+    let report: ReturnType<typeof bindReporter>;
+
+    const handleEntries = (entries: FCPMetric['entries']) => {
+      entries.forEach((entry) => {
+        if (entry.name === 'first-contentful-paint') {
+          po!.disconnect();
+
+          // Only report if the page wasn't hidden prior to the first paint.
+          if (entry.startTime < visibilityWatcher.firstHiddenTime) {
+            // The activationStart reference is used because FCP should be
+            // relative to page activation rather than navigation start if the
+            // page was prerendered. But in cases where `activationStart` occurs
+            // after the FCP, this time should be clamped at 0.
+            metric.value = Math.max(entry.startTime - getActivationStart(), 0);
+            metric.entries.push(entry);
+            report(true);
+          }
+        }
+      });
+    };
+
+    const po = observe('paint', handleEntries);
+
+    if (po) {
+      report = bindReporter(
+        onReport,
+        metric,
+        FCPThresholds,
+        opts!.reportAllChanges,
+      );
+
+      // Only report after a bfcache restore if the `PerformanceObserver`
+      // successfully registered or the `paint` entry exists.
+      onBFCacheRestore((event) => {
+        metric = initMetric('FCP');
+        report = bindReporter(
+          onReport,
+          metric,
+          FCPThresholds,
+          opts!.reportAllChanges,
+        );
+
+        doubleRAF(() => {
+          metric.value = performance.now() - event.timeStamp;
+          report(true);
+        });
+      });
+    }
+  });
+};