There is no built-in assert() function in JavaScript. However if rather simple to write such a function yourself

It's important to note that in NodeJS there is a standard library assert that includes such functions and even more.

What is the purpose of the assert function and how is it used?

The purpose of an assert() function is to check if a given condition is true. If the condition is true the function does nothing. However, if the condition is false, the assert() function throws an error, which can be caught and used to identify the issue. This method is used mainly in automated tests and when debugging.

Example of assert() function in JavaScript

function assert(condition) {
  if (!condition) {
    throw new Error("Assertion failed");

Example test case where assert function comes in handy.

// Let's say we have a function which whenever called returns 1 each time.
function returnOnlyOne() {
  return 1;

// If we want to test this functions just to make sure it works as expected
// We can do the following

console.log('Start tests');
assert(returnOnlyOne() == 1);
console.log('Test 1 passed!');


$ Start tests
$ Test 1 passed!

So we've validated that our function behaves as expected!


If we swap the returnOnlyOne function to give in the wrong behavior with

function returnOnlyOne() {
  return 2;

Then the output will be

$ Start tests
$ Error: Assertion failed

The assert statement will catch the error and throw an exception.