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.

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.
Living Standard Initial definition

Browser compatibility

We're converting our compatibility data into a machine-readable JSON format. This compatibility table still uses the old format, because we haven't yet converted the data it contains. Find out how you can help!

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

See also

Document Tags and Contributors

 Last updated by: FloraFauna,