Built-in API

The global Deno namespace contains APIs that are not web standard, including APIs for reading from files, opening TCP sockets, serving HTTP, and executing subprocesses, etc.

For a full list of Deno Built-in APIs, see the reference. Below we highlight some of the most important.

Errors

The Deno runtime comes with 20 error classes that can be raised in response to a number of conditions.

Some examples are:

Deno.errors.NotFound;
Deno.errors.WriteZero;

They can be used as below:

try {
  const file = await Deno.open("./some/file.txt");
} catch (error) {
  if (error instanceof Deno.errors.NotFound) {
    console.error("the file was not found");
  } else {
    // otherwise re-throw
    throw error;
  }
}

File System

The Deno runtime comes with various functions for working with files and directories. You will need to use --allow-read and --allow-write permissions to gain access to the file system.

Refer to the links below for code examples of how to use the file system functions.

• Reading files in several different ways
• Reading files in streams
• Reading a text file (Deno.readTextFile)
• Writing a text file (Deno.writeTextFile)

I/O

The Deno runtime comes with built-in functions for working with resources and I/O.

Refer to the links below for code examples for common functions.

• Closing resources (Deno.close)
• Seeking a certain position within the resource (Deno.seek)

Network

The Deno runtime comes with built-in functions for dealing with connections to network ports.

Refer to the links below for code examples for common functions.

• Connect to the hostname and port (Deno.connect)
• Announcing on the local transport address (Deno.listen)

Sub Process

The Deno runtime comes with built-in functions for spinning up subprocesses.

Refer to the links below for code samples of how to create a subprocess.

• Creating a subprocess (Deno.run)