fetch TL;DR

Azle canisters use a custom fetch implementation to perform cross-canister calls and to perform HTTPS outcalls.

Here's an example of performing a cross-canister call:

import { serialize } from 'azle/experimental';
import express from 'express';

const app = express();

app.use(express.json());

app.post('/cross-canister-call', async (req, res) => {
    const to: string = req.body.to;
    const amount: number = req.body.amount;

    const response = await fetch(`icp://dfdal-2uaaa-aaaaa-qaama-cai/transfer`, {
        body: serialize({
            candidPath: '/token.did',
            args: [to, amount]
        })
    });
    const responseJson = await response.json();

    res.json(responseJson);
});

app.listen();

Keep these important points in mind when performing a cross-canister call:

Use the icp:// protocol in the URL
The canister id of the canister that you are calling immediately follows icp:// in the URL
The canister method that you are calling immediately follows the canister id in the URL
The candidPath property of the body is the path to the Candid file defining the method signatures of the canister that you are calling. You must obtain this file and copy it into your canister. See the Assets chapter for info on copying files into your canister
The args property of the body is an array of the arguments that will be passed to the canister method that you are calling

Here's an example of performing an HTTPS outcall:

import express from 'express';

const app = express();

app.use(express.json());

app.post('/https-outcall', async (_req, res) => {
    const response = await fetch(`https://httpbin.org/headers`, {
        headers: {
            'X-Azle-Request-Key-0': 'X-Azle-Request-Value-0',
            'X-Azle-Request-Key-1': 'X-Azle-Request-Value-1',
            'X-Azle-Request-Key-2': 'X-Azle-Request-Value-2'
        }
    });
    const responseJson = await response.json();

    res.json(responseJson);
});

app.listen();

fetch

Azle has custom fetch implementations for clients and canisters.

The client fetch is used for authentication, and you can learn more about it in the Authentication chapter.

Canister fetch is used to perform cross-canister calls and HTTPS outcalls. There are three main types of calls made with canister fetch:

Cross-canister calls to a candid canister
Cross-canister calls to an HTTP canister
HTTPS outcalls

Cross-canister calls to a candid canister

Examples:

Cross-canister calls to an HTTP canister

We are working on better abstractions for these types of calls. For now you would just make a cross-canister call using icp:// to the http_request and http_request_update methods of the canister that you are calling.

HTTPS outcalls