callback.md

Verifying Callbacks

The initial implementation of AWS Lambda used context.succeed(), context.fail() and context.done() to control termination of the function. On April 8, 2016 AWS Lambda introduced support for Lambda callbacks that replace the need to call context.fail(), context.succeed() and context.done().

Verifying callback( null, result )

When the Lambda handler calls callback( null, result ), verification can be performed using expectResult(). If the handler calls anything other than callback( null, result ), 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' } )
			.expectResult();
	});
});

To verify the result value from callback():

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' } )
			.expectResult( ( 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 expectResult() can return a Promise to perform asynchronous operation. Operation 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' } )
			.expectResult( ( result, additional, done ) => {

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

Verifying callback( err )

When the Lambda handler calls callback( err ), verification can be performed using expectError(). If the handler calls anything other than callback( err ), 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' } )
			.expectError();
	});
});

To verify the value of err from callback(), pass a validator to expectError():

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' } )
			.expectError( ( 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 expectError() can return a Promise to perform asynchronous operation. Operation 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' } )
			.expectError( ( err, additional, done ) => {

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

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