willWarn() takes either a String or a RegExp as its expected warning, 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 warning logged through [can-log/dev/dev.warn]. In addition, if fn is provided, it is fired on every warning with the content of the warning message and whether it matched expected.

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

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

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

canDev.warn("something evil");

finishWarningCheck(); // -> 1