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.


Writes an error message to the console if the assertion is false. If the assertion is true, nothing will happen.

Note: This feature is available in Web Workers.


console.assert(assertion, obj1 [, obj2, ..., objN]);
console.assert(assertion, msg [, subst1, ..., substN]);


Any boolean expression. If the assertion is false, the message will get written to the console.
obj1 ... objN
A list of JavaScript objects to output. The string representations of each of these objects are appended together in the order listed and output.
A JavaScript string containing zero or more substitution strings.
subst1 ... substN
JavaScript objects with which to replace substitution strings within msg. This gives you additional control over the format of the output.

See Outputting text to the console in the documentation of console for details.


Console Object API

Browser compatibility

Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari
Basic support (Yes) 28.0 (28.0) (Yes) (Yes) (Yes)
Available in workers (Yes) 38.0 (38.0) (Yes) (Yes) (Yes)
Feature Android Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
Basic support ? 28.0 (28.0) ? ? ?
Available in workers ? 38.0 (38.0) ? ? ?

See also

Document Tags and Contributors

Contributors to this page: chrisdavidmills, Sebastianz, cvrebert, teoli, denschub
Last updated by: chrisdavidmills,