Checking with console.assert

Learn to log an error when a condition fails with console.assert.

Checking with console.assert

console.assert(condition, message) prints message as an error when condition is false; it does nothing when true. This is useful for checking assumptions in your code.

For example, assume a function expects a positive number. With console.assert(value > 0, "value must be positive") you get a warning when this expectation breaks. Such a check is called an assertion.

Where are assertions used?

Assertions check conditions that must hold for the rest of the code to work correctly. You can verify that a loop index stays in bounds or that a function argument has the expected type.

But assertions are not for controlling normal flow; they exist to surface bugs temporarily. Cleaning up unnecessary assertions before going to production is a good habit.

Exercise Check a variable with console.assert

Define a function called divide(a, b). It should check that b is not 0 with console.assert(b !== 0, "b cannot be zero"). Then it should return a / b. Finally call divide(10, 2).

Show example solution

function divide(a, b) {
  console.assert(b !== 0, "b cannot be zero");
  return a / b;
}

divide(10, 2);

Test yourself

When does console.assert(cond, msg) print the message?
- When the condition is true
- Always
- Never
- When the condition is false
Show answer

When the condition is false — Correct. Assert warns when the condition fails.
Match the usage

console.assert(true, msg)

Prints nothing
console.assert(false, msg)

Prints the message as an error
console.assert(x > 0, ...)

Warns when x is not positive
console.log(msg)

Always prints the message

Complete the console.assert call

function checkAge(age) {
  console.assert(____, "age cannot be negative");
}

Show answer

function checkAge(age) {
  console.assert(age >= 0, "age cannot be negative");
}

Next lesson: Common bug patterns

We Are Enhancing Your Experience

Cookie Preferences

Strictly Necessary Cookies

Analytical Cookies