Styjun

Recently Azure functions released support for TypeScript:

import AzureFunction, Context, HttpRequest from '@azure/functions';

@some_decorator - ???
const httpTrigger: AzureFunction = async function (context: Context, 
req: HttpRequest): Promise<void> 



export default httpTrigger;

I'm looking for a good approach to implement a pre-function call.
For instance, the pre-function could do authorization checks or whatever else is necessary before the function in question is executed.

I'm wondering which TypeScript decorators would be the best and cleanest option but not sure about the implementation.

That's a great question!

Decorators are basically functions that wrap the function on which they are applied on. You can read more about how they work in their official doc.

Now coming to Azure Functions, typescript functions are actually compiles to JavaScript and are then run. So, decorators will work out of the box. :)

Note that you will have to set the experimentalDecorators flag in your tsconfig.json file to true for this to work as mentioned in the docs

Here is simple sample of how you could implement your own custom decorator

import AzureFunction, Context, HttpRequest from '@azure/functions';

function checkUserId(userId: string) 
 return function(
 target: Object,
 propertyKey: string,
 descriptor: PropertyDescriptor
 ) 
 const originalValue = descriptor.value;

 descriptor.value = async function(...args: any[]) 
 const context: Context = args[0];
 const req: HttpRequest = args[1];

 if (req.headers['x-func-user-id'] !== userId) 
 context.res = 
 status: 403,
 body: 'User not authorized!'
 ;
 return;
 

 // Call the original function
 await originalValue.apply(this, args);
 return;
 ;
 ;


class HttpTrigger 
 @checkUserId('azure-user')
 static async function(context: Context, req: HttpRequest): Promise<void> (req.body && req.body.name);

 if (name) 
 context.res = ;
 else 
 context.res = 
 status: 400,
 body: 'Please pass a name on the query string or in the request body'
 ;
 
 


export default HttpTrigger.function;

And you can test this function like this

curl --request GET 
 --url 'http://localhost:7071/api/HttpTrigger?name=Azure' 
 --header 'x-func-user-id: azure-user'