Invoking a Lambda Function from within another Lambda function is quite simple using the aws-sdk which is available in every Lambda.

I suggest starting with something simple first.
This is the "Hello World" of intra-lambda invocation:

Lambda_A invokes Lambda_B with a Payload containing a single parameter name:'Alex'.
Lambda_B responds with Payload: "Hello Alex".

First create Lambda_B which expects a name property on the event parameter
and responds to request with "Hello "+event.name:

Lambda_B

exports.handler = function(event, context) {
console.log('Lambda B Received event:', JSON.stringify(event, null, 2));
context.succeed('Hello ' + event.name);
};

Ensure that you give Lambda_B and Lambda_A the same role.
E.g: create a role called lambdaexecute which has AWSLambdaRole, AWSLambdaExecute and AWSLambdaBasicExecutionRole (All are required):

Lambda_A

var AWS = require('aws-sdk');
AWS.config.region = 'eu-west-1';
var lambda = new AWS.Lambda();


exports.handler = function(event, context) {
var params = {
FunctionName: 'Lambda_B', // the lambda function we are going to invoke
InvocationType: 'RequestResponse',
LogType: 'Tail',
Payload: '{ "name" : "Alex" }'
};


lambda.invoke(params, function(err, data) {
if (err) {
context.fail(err);
} else {
context.succeed('Lambda_B said '+ data.Payload);
}
})
};

Once you have saved both these Lambda functions, Test run Lambda_A:

Once you have the basic intra-lambdda invocation working you can easily extend it to invoke more elaborate Lambda functions.

The main thing you have to remember is to set the appropriate ARN Role for all functions.

As of Dec 3, 2016, you can simply use an AWS Step function to put Lambda function Lambda_B as the sequential step of Lambda_A.

With AWS Step Functions, you define your application as a state machine, a series of steps that together capture the behavior of the app. States in the state machine may be tasks, sequential steps, parallel steps, branching paths (choice), and/or timers (wait). Tasks are units of work, and this work may be performed by AWS Lambda functions, Amazon EC2 instances of any type, containers, or on premises servers—anything that can communicate with the Step Functions API may be assigned a task.

So the following state machine should meet your need.

Here is the code corresponding to the state machine.

{
"Comment": "A simple example of the Amazon States Language using an AWS Lambda Function",
"StartAt": "Lambda_A",


"States": {
"Lambda_A": {
"Type": "Task",
"Resource": "arn:aws:lambda:REGION:ACCOUNT_ID:function:FUNCTION_NAME",
"Next": "Lambda_B"
},
"Lambda_B":{
"Type": "Task",
"Resource": "arn:aws:lambda:REGION:ACCOUNT_ID:function:FUNCTION_NAME",
"End": true
}


}
}

Moreover, you can add much more sophisticated logics in a state machine, such as parallel steps and catch failures. It even logs the details of every single execution which makes debugging a much better experience, especially for lambda functions.

Everything mentioned by @nelsonic is correct, except for the roles.

I tried choosing the roles that he mentioned above:

• AWSLambdaExecute
• AWSLambdaBasicExecutionRole

But it did not allow me to invoke my other lambda function, so I changed the role to the below:

• AWSLambdaRole
• AWSLambdaBasicExecutionRole

The reason behind is AWSLambdaExecute only provides Put, Get access to S3 and full access to CloudWatch Logs. but AWSLambdaRole provides Default policy for AWS Lambda service role. if you observe its permission policy it will talk about the invokeFunction

{
"Version": "2012-10-17",
"Statement": [
{
"Effect": "Allow",
"Action": [
"lambda:InvokeFunction"
],
"Resource": [
"*"
]
}
]
}

Note: it is OK to proceed without AWSLambdaBasicExecutionRole policy as it only enables the logging in the cloud watch nothing much. But AWSLambdaRole is absolutely necessary.

It's easier to invoke a lambda using the AWS.Lambda promises interface in aws-sdk than using callbacks.

This example function lets you make a synchronous invocation of a lambda from another lambda (it uses 'RequestResponse' as InvocationType, so you'll can get the value returned by the invoked lambda).

If you use 'Event' (for asynchronous invocation), you can't get the value returned by the called lambda, only be able to detect whether the lambda could be invoked with success or not. It is intended for cases when you don't need to obtain a returned value from the invoked lambda.

//
// Full example of a lambda that calls another lambda
//
// (create a lambda in AWS with this code)
//
'use strict';


//
// Put here the name of the function you want to call
//
const g_LambdaFunctionName = 'PUT_HERE_THE_INVOKED_LAMBDA_NAME'; // <======= PUT THE DESIRED VALUE


const AWS    = require('aws-sdk');
const lambda = new AWS.Lambda;


//
// Expected use:
//
//   // (payload can be an object or a JSON string, for example)
//   let var = await invokeLambda(lambdaFunctionName, payload);
//
const invokeLambda = async (lambdaFunctionName, payload) => {


console.log('>>> Entering invokeLambda');


// If the payload isn't a JSON string, we convert it to JSON
let payloadStr;
if (typeof payload === 'string')
{
console.log('invokeLambda:  payload parameter is already a string: ', payload);
payloadStr = payload;
}
else
{
payloadStr = JSON.stringify(payload, null, 2);
console.log('invokeLambda: converting payload parameter to a string: ', payloadStr);
}


let params = {
FunctionName   : lambdaFunctionName,               /* string type, required */
// ClientContext  : '',                               /* 'STRING_VALUE' */
InvocationType : 'RequestResponse',                /* string type: 'Event' (async)| 'RequestResponse' (sync) | 'DryRun' (validate parameters y permissions) */
// InvocationType : 'Event',


LogType        : 'None',                           /* string type: 'None' | 'Tail' */
// LogType        : 'Tail',
Payload        : payloadStr,                       /* Buffer.from('...') || 'JSON_STRING' */ /* Strings will be Base-64 encoded on your behalf */
//  Qualifier      : '',                             /* STRING_VALUE' */
};


//
// TODO/FIXME: add try/catch to protect this code from failures (non-existent lambda, execution errors in lambda)
//
const lambdaResult = await lambda.invoke(params).promise();


console.log('Results from invoking lambda ' + lambdaFunctionName + ': ' , JSON.stringify(lambdaResult, null, 2) );


// If you use LogType = 'Tail', you'll obtain the logs in lambdaResult.LogResult.
// If you use 'None', there will not exist that field in the response.
if (lambdaResult.LogResult)
{
console.log('Logs of lambda execution: ',  Buffer.from(lambdaResult.LogResult, 'base64').toString());
}


console.log('invokeLambdaSync::lambdaResult: ', lambdaResult);


console.log('<<< Returning from invokeLambda, with lambdaResult: ', JSON.stringify(lambdaResult, null, 2));


// The actual value returned by the lambda it is lambdaResult.Payload
// There are other fields (some of them are optional)
return lambdaResult;
};


//
// We'll assign this as the calling lambda handler.
//
const callingFunc = async (event) => {


//
// in this example We obtain the lambda name from a global variable
//
const lambdaFunctionName = g_LambdaFunctionName;
   

// const payload            = '{"param1" : "value1"}';
const payload            = event;


//
// invokeLambda has to be called from a async function
// (to be able to use await)
//
const result = await invokeLambda(lambdaFunctionName, payload);


console.log('result: ', result);
};


// Assing handler function
exports.handler = callingFunc;

Notice that you should use await before invokeLambda:

...
//
// Called from another async function
//
const result = await invokeLambda(lambdaFunctionName, payload);
...

Some relevant links with additional information:

• AWS Reference about invoke call: https://docs.aws.amazon.com/AWSJavaScriptSDK/latest/AWS/Lambda.html#invoke-property
• AWS documentation about invoking a lambda: https://docs.aws.amazon.com/lambda/latest/dg/API_Invoke.html
• Promises interface in AWS: https://aws.amazon.com/es/blogs/compute/node-js-8-10-runtime-now-available-in-aws-lambda/
• Examples (explanations in Spanish): https://www.it-swarm.dev/es/node.js/invocar-aws-lambda-desde-otra-lambda-de-forma-asincronica/826852446/
• Handling errors, avoiding coupling between lambdas: https://www.rehanvdm.com/serverless/13-aws-lambda-design-considerations-you-need-to-know-about-part-2/index.html

Invoke Lambda AWS SDK Typescript

I wrote my own class to do this, parse the response and check errors. I've posted it here to save anyone else who wants the effort :) This requires aws-sdk and ts-log.

import { AWSError, Lambda } from 'aws-sdk'
import { Logger } from 'tslog';


export class LambdaClient {
awsLambda: Lambda;
logger: Logger;
constructor(region: string) {
this.awsLambda = new Lambda({ region })
this.logger = new Logger({ name: "LambdaClient" })
}


trigger({ functionName, payload }): Promise<any> {
return new Promise(
(resolve, reject) => {
const params = {
FunctionName: functionName,
InvocationType: 'RequestResponse',
LogType: 'Tail',
Payload: JSON.stringify(payload)
};
this.awsLambda.invoke(params, (err: AWSError, data: Lambda.InvocationResponse) => {
if (err) {
this.logger.error({ message: "error while triggering lambda", errorMessage: err.message })
return reject(err)
}
if (data.StatusCode !== 200 && data.StatusCode !== 201) {
this.logger.error({ message: "expected status code 200 or 201", statusCode: data.StatusCode, logs: base64ToString(data.LogResult) })
return reject(data)
}
const responsePayload = data.Payload
return resolve(JSON.parse(responsePayload.toString()))
})


}
)
}
}




function base64ToString(logs: string) {
try {
return Buffer.from(logs, 'base64').toString('ascii');
} catch {
return "Could not convert."
}
}