AssertionError

Summary

Thrown when an assertion fails. Mirrors the shape of Node.js's built-in assert.AssertionError so that test reporters can treat them uniformly.

Signature

class AssertionError {
  constructor(options: {
    actual?: unknown
    expected?: unknown
    generatedMessage?: boolean
    message?: unknown
    operator: string
  }): AssertionError

  // Properties
  actual: unknown
  cause?: unknown
  code: string
  expected: unknown
  generatedMessage: boolean
  message: string
  name: string
  operator: string
  stack?: string
  stackTraceLimit: number

  // Methods
  captureStackTrace(targetObject: object, constructorOpt: Function): void
  prepareStackTrace(err: Error, stackTraces: CallSite[]): any
}

Constructor

Creates a new AssertionError with the given message, actual/expected values, and operator.

Properties

`actual`

`cause`

`code`

`expected`

`generatedMessage`

`message`

`name`

`operator`

`stack`

`stackTraceLimit`

The Error.stackTraceLimit property specifies the number of stack frames collected by a stack trace (whether generated by new Error().stack or Error.captureStackTrace(obj)).

The default value is 10 but may be set to any valid JavaScript number. Changes will affect any stack trace captured after the value has been changed.

If set to a non-number value, or set to a negative number, stack traces will not capture any frames.

Methods

`captureStackTrace(targetObject: object, constructorOpt: Function): void`

Creates a .stack property on targetObject, which when accessed returns a string representing the location in the code at which Error.captureStackTrace() was called.

const myObject = {};
Error.captureStackTrace(myObject);
myObject.stack;  // Similar to `new Error().stack`

The first line of the trace will be prefixed with ${myObject.name}: ${myObject.message}.

The optional constructorOpt argument accepts a function. If given, all frames above constructorOpt, including constructorOpt, will be omitted from the generated stack trace.

The constructorOpt argument is useful for hiding implementation details of error generation from the user. For instance:

function a() {
  b();
}

function b() {
  c();
}

function c() {
  // Create an error without stack trace to avoid calculating the stack trace twice.
  const { stackTraceLimit } = Error;
  Error.stackTraceLimit = 0;
  const error = new Error();
  Error.stackTraceLimit = stackTraceLimit;

  // Capture the stack trace above function b
  Error.captureStackTrace(error, b); // Neither function c, nor b is included in the stack trace
  throw error;
}

a();