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assert_fail_actual_expected_message_operator_stackstartfunction.md

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  • actual {any}
  • expected {any}
  • message {any}
  • operator {string} Default: '!='
  • stackStartFunction {function} Default: assert.fail

Throws an AssertionError. If message is falsy, the error message is set as the values of actual and expected separated by the provided operator. If just the two actual and expected arguments are provided, operator will default to '!='. If message is provided only it will be used as the error message, the other arguments will be stored as properties on the thrown object. If stackStartFunction is provided, all stack frames above that function will be removed from stacktrace (see [Error.captureStackTrace]).

const assert = require('assert');

assert.fail(1, 2, undefined, '>');
// AssertionError [ERR_ASSERTION]: 1 > 2

assert.fail(1, 2, 'fail');
// AssertionError [ERR_ASSERTION]: fail

assert.fail(1, 2, 'whoops', '>');
// AssertionError [ERR_ASSERTION]: whoops

Note: Is the last two cases actual, expected, and operator have no influence on the error message.

assert.fail();
// AssertionError [ERR_ASSERTION]: Failed

assert.fail('boom');
// AssertionError [ERR_ASSERTION]: boom

assert.fail('a', 'b');
// AssertionError [ERR_ASSERTION]: 'a' != 'b'

Example use of stackStartFunction for truncating the exception's stacktrace:

function suppressFrame() {
  assert.fail('a', 'b', undefined, '!==', suppressFrame);
}
suppressFrame();
// AssertionError [ERR_ASSERTION]: 'a' !== 'b'
//     at repl:1:1
//     at ContextifyScript.Script.runInThisContext (vm.js:44:33)
//     ...