Node.js - assert.doesNotReject() Function

The Node.js assert.doesNotReject() function is an inbuilt function of the assert module of Node.js. It is used to check if the given promise is not rejected.

In the assert.doesNotReject() function if the provided parameter is a promise, it awaits the promise. If the provided parameter is a function, then it is called immediately and awaits the return promise to complete. This function will check if the provided promise is rejected or not. This function behaves identically to assert.doesNotThrow().

The assert.doesNotReject() is not useful to use because there is only a little benefit in catching an error and rejecting the error again. Instead, it would be better if we add appropriate comments to a particular code path that should not be rejected and keeping error message as expressive as possible.

Syntax

Following is the syntax of Node.js assert.doesNotReject() function −

assert.doesNotReject(asyncFn[, error][, message]);

Parameters

This function accepts three parameters. The same are described below.

• asyncFn − (Required) This parameter holds an async function that will throw errors synchronously.

• error − (Optional) This parameter can hold Class, Regular Expression, Validation function, or an object where each property will be tested.

• message − (Optional) This parameter can hold Class, Regular Expression, Validation function, or an object where each property will be tested.

Return Value

The function assert.doesNotReject() will return the rejected promise to the output.

Example

Following is the usage of the Node.js assert.doesNotReject() function.

const assert = require('assert');
(async () => {
   await assert.doesNotReject(
      async () => {
         throw new TypeError('Wrong value');
      },
         TypeError
   );
}
)();

Output

Following is the output of the above code −

(node:40094) UnhandledPromiseRejectionWarning: AssertionError [ERR_ASSERTION]: Got unwanted rejection.
Actual message: "Wrong value"
   at process._tickCallback (internal/process/next_tick.js:68:7)
   at Function.Module.runMain (internal/modules/cjs/loader.js:746:11)
   at startup (internal/bootstrap/node.js:238:19)
   at bootstrapNodeJSCore (internal/bootstrap/node.js:572:3)
(node:40094) UnhandledPromiseRejectionWarning: Unhandled promise rejection.
   This error originated either by throwing inside of an async function without
   a catch block, or by rejecting a promise which was not handled with .catch(). (rejection id: 2)
(node:40094) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated.
   In the future, promise rejections that are not handled will terminate the Node.js
   process with a non-zero exit code.

Example

Following is the usage of the Node.js assert.doesNotReject() function in another scenario.

const assert = require('assert');
(async () => {
   await assert.doesNotReject(
      async () => {
         throw new TypeError('Error occured!!!');
      },
         SyntaxError
   );
}
)();

Output

Following is the output of the above code −

(node:43403) UnhandledPromiseRejectionWarning: TypeError: Error occured!!!
   at assert.doesNotReject (/home/cg/root/639c2bf348ea8/main.js:6:23)
   at waitForActual (assert.js:518:21)
   at Function.doesNotReject (assert.js:620:39)at /home/cg/root/639c2bf348ea8/main.js:4:22
   at Object.<anonymous> (/home/cg/root/639c2bf348ea8/main.js:11:2)
   at Module._compile (internal/modules/cjs/loader.js:702:30)
   at Object.Module._extensions..js (internal/modules/cjs/loader.js:713:10)
   at Module.load (internal/modules/cjs/loader.js:612:32)
   at tryModuleLoad (internal/modules/cjs/loader.js:551:12)
   at Function.Module._load (internal/modules/cjs/loader.js:543:3)
(node:43403) UnhandledPromiseRejectionWarning: Unhandled promise rejection.
   This error originated either by throwing inside of an async function without a catch block,
   or by rejecting a promise which was not handled with .catch(). (rejection id: 2)
(node:43403) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated.
   In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.