Typescript : how to create a type from a javascript object

Question

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

You need to know in advance how the "javascript object" looks like, otherwise you can not create a type for it — Nitzan Tomer
– 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
– 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
– El houcine bougarfaoui, Commented Jan 15, 2017 at 0:02

Paarth · Accepted Answer · 2017-01-16 02:59:48Z

22

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
}

edited Jan 16, 2017 at 2:59

answered Jan 14, 2017 at 23:57

Paarth

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4 Comments

El houcine bougarfaoui Over a year ago

thank you for your answer, but I want something visible without using typeof

Paarth Over a year ago

In that case I feel your question is unclear. Are you asking what type to use for the interface that you imported? If that's what you'd want you'll need to tell us exactly what you're importing and exactly how you're using it.

El houcine bougarfaoui Over a year ago

yes that's exactly what I want, what I import is the request object defined in the expressJS library, as I said it has a lot of properties

Paarth Over a year ago

@BougarfaouiElhoucine I updated the answer, does that work for you?

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