console: assert() static method

assertion

Any boolean expression. If the assertion is false, a generic message indicating assertion failure is written to the console.

val1 … valN

A list of JavaScript values to output. A representation of each of these values is output to the console after a generic assertion failure message (which may be different from the message output when these values are not present) in the order given with some type of separation between the message and between each of them. There is a special case if val1 is a string, which is described subsequently.

msg

A JavaScript string containing zero or more substitution strings, which are replaced with subst1 through substN in consecutive order up to the number of substitution strings. A colon, a space, and then the substituted string are appended to the generic assertion message to form a detailed assertion message, and the result is output to the console. See Using string substitutions for a description of how substitutions work.

subst1 … substN

JavaScript values with which to replace substitution strings within msg. If there are more substitution values than there are substitution strings, the extra values are themselves written to the console after the detailed assertion message in the same manner as when there's no format string.

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

None (undefined).

The following code example demonstrates the use of a JavaScript object following the assertion:

const errorMsg = "the # is not even";
for (let number = 2; number <= 5; number++) {
  console.log(`the # is ${number}`);
  console.assert(number % 2 === 0, "%o", { number, errorMsg });
}
// output:
// the # is 2
// the # is 3
// Assertion failed: {number: 3, errorMsg: "the # is not even"}
// the # is 4
// the # is 5
// Assertion failed: {number: 5, errorMsg: "the # is not even"}

• Microsoft Edge's documentation for console.assert()
• Node.js documentation for console.assert()
• Google Chrome's documentation for console.dir()