promise.md

Verifying Promises

On April 2, 2018 AWS Lambda introduced support for Node.js 8.10 and Lambda handlers that can return a promise directly - eliminating the need to invoke the callback previously passed to the handler or the earlier context.fail(), context.succeed() and context.done() methods.

Verifying Promise.resolve()

When the Lambda handler returns a Promise that resolves, verification can be performed using expectResolve(). If the handler calls anything other function or the Promise rejects, then the test will fail.

const LambdaTester = require( 'lambda-tester' );

const myHandler = require( '../index' ).handler;

describe( 'handler', function() {

	it( 'test callback( null, result )', function() {

		return LambdaTester( myHandler )
			.event( { name: 'Fred' } )
			.expectResolve();
	});
});

To verify the result value from Promise.resolve:

const LambdaTester = require( 'lambda-tester' );

// Mocha using Chai
const expect = require( 'chai' ).expect;

const myHandler = require( '../index' ).handler;

describe( 'handler', function() {

	it( 'test callback( null, result )', function() {

		return LambdaTester( myHandler )
			.event( { name: 'Fred' } )
			.expectResolve( ( result, additional ) => {

                expect( result.userId ).to.exist;
                expect( result.user ).to.equal( 'fredsmith' );
            });
	});
});

The additional parameter contains information such as AWS x-ray data and execution information. The function passed to expectResolve() can return a Promise to perform asynchronous operation. Operations that require a callback for asynchronous operations can use the done parameter:

const LambdaTester = require( 'lambda-tester' );

// Mocha using Chai
const expect = require( 'chai' ).expect;

const myHandler = require( '../index' ).handler;

const asyncOperation = require( '../my-async-op' );

describe( 'handler', function() {

	it( 'test callback( null, result )', function() {

		return LambdaTester( myHandler )
			.event( { name: 'Fred' } )
			.expectResolve( ( result, additional, done ) => {

                // this function will call done when complete
                asyncOperation( result, done );
            });
	});
});

Verifying Promise.reject()

When the Lambda handler returns a Promise that rejects, verification can be performed using expectReject(). If the handler calls anything other function or the Promise resolves, then the test will fail.

const LambdaTester = require( 'lambda-tester' );

const myHandler = require( '../index' ).handler;

describe( 'handler', function() {

	it( 'test callback( err )', function() {

		return LambdaTester( myHandler )
			.event( { name: 'Unknown' } )
			.expectReject();
	});
});

To verify the value of err from Promise.reject(), pass a validator to expectReject():

const LambdaTester = require( 'lambda-tester' );

// Mocha using Chai
const expect = require( 'chai' ).expect;

const myHandler = require( '../index' ).handler;

describe( 'handler', function() {

	it( 'test callback( err )', function() {

		return LambdaTester( myHandler )
			.event( { name: 'Unknown' } )
			.expectReject( ( err, additional ) => {

				expect( err.message ).to.equal( 'User not found' );
			});
	});
});

The additional parameter contains information such as AWS x-ray data and execution information. The function passed to expectReject() can return a Promise to perform asynchronous operation. Operations that require a callback for asynchronous operations can use the done parameter:

const LambdaTester = require( 'lambda-tester' );

// Mocha using Chai
const expect = require( 'chai' ).expect;

const myHandler = require( '../index' ).handler;

const asyncOperation = require( '../my-async-op' );

describe( 'handler', function() {

	it( 'test callback( err )', function() {

		return LambdaTester( myHandler )
			.event( { name: 'Unknown' } )
			.expectReject( ( err, additional, done ) => {

                asyncOperation( err, done );
			});
	});
});

Note: Promise verification cannot be used to verify callback(), context.succeed(), context.fail() or context.done().