Lazy call to custom stack trace formatting using Error.prepareStackTrace.

test/mjsunit/stack-traces-custom-lazy.js

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-// Test formatting method with specified date, invalid input.
-
-var dtf = new Intl.DateTimeFormat('en-US', {timeZone: 'UTC'});
-
-var someDate = dtf.format(144313200000);
-assertEquals('7/29/1974', someDate);
-
-var invalidValues = [NaN, Infinity, -Infinity];
-invalidValues.forEach(function(value) {
-  var error;
+function testPrepareStackTrace(closure) {
+  var error = undefined;
   try {
-    dtf.format(value);
+    closure();
+    assertUnreachable();
   } catch (e) {
     error = e;
   }
 
-  assertTrue(error !== undefined);
-  assertEquals('RangeError', error.name);
-});
+  // We expect custom formatting to be lazy. Setting the custom
+  // function right before calling error.stack should be fine.
+  Error.prepareStackTrace = function(e, frames) {
+    return "bar";
+  }
+
+  assertEquals("bar", error.stack);
+  Error.prepareStackTrace = undefined;
+}
+
+testPrepareStackTrace(function() { throw new Error("foo"); });
+testPrepareStackTrace(function f() { f(); });
+