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Fetch Data

Concepts​

  • Like browsers, Deno implements web standard APIs - such as fetch.
  • Deno is secure by default, meaning explicit permission must be granted to access the network.
  • See also: Deno's permissions model.

Overview​

When building a web application developers will usually need to retrieve data from somewhere else on the web. This works no differently in Deno than in any other JavaScript application, using the fetch() method. For more information read the MDN documentation on fetch.

The difference with Deno occurs when running a script which makes a call over the web. Deno is secure by design which means access to IO (Input / Output) is prohibited by default. To make a call over the web Deno must be explicitly told it is ok to do so. This is achieved by adding the --allow-net flag to the deno run command.

Example​

Command: deno run --allow-net fetch.ts

/**
 * Output: JSON Data
 */
const jsonResponse = await fetch("https://api.github.com/users/denoland");
const jsonData = await jsonResponse.json();

console.log(jsonData);

/**
 * Output: HTML Data
 */
const textResponse = await fetch("https://deno.land/");
const textData = await textResponse.text();

console.log(textData);

/**
 * Output: Error Message
 */
try {
  await fetch("https://does.not.exist/");
} catch (error) {
  console.log(error);
}

Files and Streams​

Like in browsers, sending and receiving large files is possible thanks to the Streams API. Deno.FsFile API provides two properties: readable and writable, which can be used to convert a Deno file into a writable or readable stream.

Command: deno run --allow-read --allow-write --allow-net fetch_file.ts

/**
 * Receiving a file
 */
const fileResponse = await fetch("https://deno.land/logo.svg");

if (fileResponse.body) {
  const file = await Deno.open("./logo.svg", { write: true, create: true });
  
  await fileResponse.body.pipeTo(file.writable);
}

/**
 * Sending a file
 */
const file = await Deno.open("./logo.svg", { read: true });

await fetch("https://example.com/", {
  method: "POST",
  body: file.readable,
});