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diff --git a/js/src/tests/web-platform-test-shims.js b/js/src/tests/web-platform-test-shims.js
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+// This is used when jstests.py runs web-platform-tests in the shell.
+// Some tests require browser features that the shell doesn't have built-in.
+
+// window.self
+// <https://html.spec.whatwg.org/multipage/window-object.html#dom-self>
+if (!("self" in this)) {
+ this.self = this;
+}
+
+// window.setTimeout, clearTimeout, setInterval, clearInterval
+// <https://html.spec.whatwg.org/multipage/timers-and-user-prompts.html#dom-settimeout>
+if (!("setTimeout" in this)) {
+ // Not-very-compliant polyfill for setTimeout, based on Promise.
+
+ // When this array becomes non-empty, call scheduleWork.
+ let timeouts = [];
+
+ // Fake clock used to order timeouts.
+ let now = 0;
+
+ // Least positive integer not yet used as a timer id.
+ let next_id = 1;
+
+ // Add an active timer record to timeouts.
+ function enqueue(id, t, cb) {
+ if (typeof cb !== 'function') {
+ let code = "" + cb;
+ cb = () => eval(code);
+ }
+ t += now;
+ timeouts.push({id, t, cb});
+ timeouts.sort((a, b) => a.t - b.t);
+ if (timeouts.length === 1) {
+ scheduleWork();
+ }
+ }
+
+ // Register a microtask to make sure timers are actually processed. There
+ // should be at most one of these microtasks pending at a time.
+ function scheduleWork() {
+ // Number of times to let the microtask queue spin before running a
+ // timeout. The idea is to let all Promises resolve between consecutive
+ // timer callbacks. Of course, this is a hack; whatever number is used
+ // here, an async function can simply await that many times plus one to
+ // observe that timeouts are being handled as microtasks, not events.
+ let delay = 10;
+
+ function handleTimeouts() {
+ if (--delay > 0) {
+ Promise.resolve().then(handleTimeouts);
+ return;
+ }
+ if (timeouts.length > 0) {
+ let {t, cb} = timeouts.shift();
+ if (now < t) {
+ now = t;
+ }
+ if (timeouts.length > 0) {
+ scheduleWork(); // There's more to do after this.
+ }
+ cb();
+ }
+ }
+ Promise.resolve().then(handleTimeouts);
+ }
+
+ this.setTimeout = function (cb, t, ...args) {
+ let id = next_id++;
+ enqueue(id, t, () => cb(...args));
+ return id;
+ };
+
+ this.clearTimeout = function (id) {
+ timeouts = timeouts.filter(obj => obj.id !== id);
+ };
+
+ this.setInterval = function (cb, t, ...args) {
+ let id = next_id++;
+ let func = () => {
+ enqueue(id, t, func);
+ cb(...args);
+ };
+ enqueue(id, t, func);
+ return id;
+ };
+
+ this.clearInterval = this.clearTimeout;
+}
+
+// Some tests in web platform tests leave promise rejections unhandled.
+if ("ignoreUnhandledRejections" in this) {
+ ignoreUnhandledRejections();
+}