console.assert(boolean_expression)

[BobStein]

Works in Chrome 51 and Firefox 47. How universal is it? What will it break?

MDN says don't use:

This feature is non-standard and is not on a standards track. Do not use it on production sites facing the Web: it will not work for every user. There may also be large incompatibilities between implementations and the behavior may change in the future.

There are smart ways to work around not having console, nor not having console.assert.

[garrik]

+1
Now I think there is a specification here in the Console
Living Standard

[Flimm]

+1

[GPHemsley]

https://developer.mozilla.org/docs/Web/API/console/assert

Looks like this is now covered by MDN data:
https://caniuse.com/mdn-api_console_assert

(via mdn/browser-compat-data#1145)