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

Note: This feature is available in Web Workers.

Note: The console.assert() method is implemented differently in Node.js than the console.assert() method available in browsers.

Specifically, in browsers, calling console.assert() with a falsy assertion will cause the message to be printed to the console without interrupting execution of subsequent code. In Node.js, however, a falsy assertion will cause an AssertionError to be thrown.


console.assert(assertion, obj1 [, obj2, ..., objN]);
console.assert(assertion, msg [, subst1, ..., substN]); // c-like message formatting


Any boolean expression. If the assertion is false, the message is 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 parameter gives you additional control over the format of the output.

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


Specification Status Comment
Console API
The definition of 'console.assert()' in that specification.
Editor's Draft Initial definition

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)
c-like message formatting No support 28.0 (28.0) ? ? ?
Feature Android Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
Basic support ? 28.0 (28.0) ? ? ?
Available in workers ? 38.0 (38.0) ? ? ?
c-like message formatting ? 28.0 (28.0) ? ? ?

See also


© 2016 Mozilla Contributors
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.

API console Console debugging Debugging DOM Method NeedsBrowserCompatibility web console Web Development