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Access web APIs as JavaScript objects. Jo (pronounced "Yo!") is a modest and thin library to write simpler client code.

Chuck Norris, Jo

Jo supports both verb-oriented APIs (e.g., /getJoke?{id:1}) and resource-oriented APIs (e.g., /jokes/1). You can use both interchangeably, which is particularly useful when you have to interact with microservices that adopt different API styles.

Jo can be used with any web server. It uses JSON as data format (we plan on adding more formats in the future). It includes native support for Jolie API Gateways (AKA Jolie redirections).

You can find a brief tutorial for building a microservice-based web application with Jo here:

Usage: Verb-oriented APIs

Verb-oriented APIs can be accessed through the global Jo object.

Invoking an operation from the server

Syntax: Jo.operation( [data, params] ) where

Invoking this method returns a promise.

This method uses POST as default HTTP method. To change it, use the optional parameters. For example, to use GET: Jo.operation( data, { method: 'GET' } ).


Suppose the originating web server offers an operation greet that returns a string given a tree with subnode name. (Nanoservices are great for examples, not so much for production: do this at home only!)

You can invoke it as follows.

Jo.greet( { name: "Homer" } )
	.then( response => console.log( response.greeting ) ) // Jo uses promises
	.catch( error => {		// an error occurred
		if ( error.isFault ) {	// It's an application error
			console.log( JSON.stringify( error.fault ) );
		} else {		// It's a middleware error
			console.log( error.message );
	} );

Here is how this operation would be implemented in a Jolie service.

greet( request )( response ) {
	response.greeting = "Hello " +

Catching errors, the alternative way

Distinguishing application and middleware errors might be boring. Use JoHelp.parseError (JoHelp is pronounced "Yo! Help!") to get that part done for you automatically. You will get a string containing the error message, if it is a middleware error, or a JSON.stringify of the carried data, if it is an application error.

Jo.greet( { name: "Homer" } )
	.then( response => console.log( response.greeting ) )
	.catch( JoHelp.parseError ).catch( console.log );

Jolie Redirections

Jo supports redirection (not to be confused with HTTP redirections), the Jolie primitive to build API gateways with named subservices. (Unnamed subservices in the gateway, obtained by aggregation, are available as normal operations, so they can be called with the previous syntax.)

Suppose that the originating Jolie service has a redirection table as follows.

Redirects: Greeter => GreeterService

If GreeterService has our operation greet, we can invoke it as follows.

Jo("Greeter").greet( { name: "Homer" } )
	.then( response => console.log( response.greeting ) )
	.catch( JoHelp.parseError ).catch( console.log );

If your Jolie API gateway points to another API gateway, you can nest them!

Jo("SubGateway1/SubGateway2/Greeter").greet( { name: "Homer" } )
	.then( response => console.log( response.greeting ) )
	.catch( JoHelp.parseError ).catch( console.log );

Usage: Resource-oriented APIs

Resource-oriented APIs can be accessed through the global Jor object.

Syntax: Jor.resource.http_method( data [, params] ) where

  • resource is the name of the resource you want to access;
  • http_method is an HTTP method (use lowercase): get, post, delete, put, head, patch, or options;
  • the data to be sent is a JSON object;
  • the optional params are parameters for the underlying fetch operation (

If resource has a name that cannot be written in JavaScript, you can use the alternative syntax Jor["resource"].http_method( data [, params] ).


Suppose the web server offers a resource /jokes, and you want to get all of them.

	.then( response => /* handle all the jokes */ )
	.catch( JoHelp.parseError ).catch( console.log );


Redirections are supported by Jor just as for verb-based APIs. Suppose the server offers the /jokes resource through the subservice ChuckNorris. Then we can access it as follows.

	.then( response => /* handle all the jokes */ )
	.catch( JoHelp.parseError ).catch( console.log );


<script type="text/javascript" src=""></script>

Or, download Jo ( and use it locally.

Pull requests with better ways to distribute Jo are welcome.


There are no dependencies on other libraries. However, Jo uses some recent features offered by web browsers.


How do I handle basic values in root nodes sent by Jolie? (AKA response.$)

In JSON, an element can either be a basic value (e.g., strings, numbers), an object, or an array. In Jolie, there are no restrictions: an element is always a tree, and each node can contain both a basic value and subnodes (similarly to markup languages). For example, this is valid Jolie: "Homer" { .children[0] = "Bart", .children[1] = "Lisa" }. It gives a tree containing the string Homer in its root node, which has an array subnode children with two elements. If you receive this tree using Jo in variable response, you can access the value contained in the root node ("Homer" in our example) by response.$.