willError() takes either a String or a RegExp as its expected error, and does a full, case-sensitive String match in the case of a String, or a regex test in the case of a RegExp, for every error logged through [can-log/dev/dev.error]. In addition, if fn is provided, it is fired on every error logged by dev.error with the content of the error message and whether it matched expected.

willError() returns a teardown function, which must be called at least once to disable the tracking of the matched error. when called, the teardown function returns the number of times expected was matched by a dev error.

var dev = require('can-log/dev/dev');
var devHelpers = require('can-test-helpers/lib/dev');

var finishErrorCheck = devHelpers.willError("something evil", function(message, match) {
     message; // -> "something evil"
     match; // true
});

canDev.error("something evil");

finishErrorCheck(); // -> 1