I want to create a type or an interface with typescript from a javascript object that I don't know how it's created.

for example I want to create a type Request to use it in my function so I can make sure I pass the right parameter to the function :

let req = require("somewhere"); // my javascript object

function myfunction(request : Request) {
// some code
}

myfunction(req);// ok
myfunction(20);// Error

how can I create the Request type

I want to create a type or an interface with typescript from a javascript object that I don't know how it's created.

for example I want to create a type Request to use it in my function so I can make sure I pass the right parameter to the function :

let req = require("somewhere"); // my javascript object

function myfunction(request : Request) {
// some code
}

myfunction(req);// ok
myfunction(20);// Error

how can I create the Request type

• javascript
• typescript

• You need to know in advance how the "javascript object" looks like, otherwise you can not create a type for it – Nitzan Tomer Commented Jan 14, 2017 at 23:55
• @NitzanTomer my read of the question is how to make the parameter type and the required type covary – Paarth Commented Jan 14, 2017 at 23:58
• its an object that is created using the function constructor http.IncomingMessage (nodeJs) its has a lot of properties – El houcine bougarfaoui Commented Jan 15, 2017 at 0:02

1 Answer 1

You can use the typeof keyword.

function myfunction(request : typeof req) {
// some code
}

Though be careful, if req is any you won't receive the type checking you want.

That said, if you want to access the request interface defined in express I believe you can access it as follows

import express = require('express')
function myfunction(request : express.Request) {
// some code
}