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Read and Write Files

Concepts

Overview

Interacting with the filesystem to read and write files is a common requirement. Deno provides a number of ways to do this via the standard library and the Deno runtime API.

As highlighted in the Fetch Data example Deno restricts access to Input / Output by default for security reasons. Therefore when interacting with the filesystem the --allow-read and --allow-write flags must be used with the deno run command.

Reading a text file

The Deno runtime API makes it possible to read text files via the Deno.readTextFile() method, it just requires a path string or URL object. The method returns a promise which provides access to the file's text data.

Command: deno run --allow-read read.ts

/**
* read.ts
*/
const text = await Deno.readTextFile("./people.json");
console.log(text);

/**
* Output:
*
* [
* {"id": 1, "name": "John", "age": 23},
* {"id": 2, "name": "Sandra", "age": 51},
* {"id": 5, "name": "Devika", "age": 11}
* ]
*/

Writing a text file

The Deno runtime API allows developers to write text to files via the Deno.writeTextFile() method. It just requires a file path and text string. The method returns a promise which resolves when the file was successfully written.

To run the command the --allow-write flag must be supplied to the deno run command.

Command: deno run --allow-write write.ts

/**
* write.ts
*/
await Deno.writeTextFile("./hello.txt", "Hello World!");
console.log("File written to ./hello.txt");

/**
* Output: File written to ./hello.txt
*/

You can append text to a file like this:

await Deno.writeTextFile("./hello.txt", "This text will be appended.", {
append: true,
});

By combining Deno.writeTextFile and JSON.stringify you can easily write serialized JSON objects to a file. This example uses synchronous Deno.writeTextFileSync, but this can also be done asynchronously using await Deno.writeTextFile.

To execute the code the deno run command needs the write flag.

Command: deno run --allow-write write.ts

/**
* write.ts
*/
function writeJson(path: string, data: object): string {
try {
Deno.writeTextFileSync(path, JSON.stringify(data));

return "Written to " + path;
} catch (e) {
return e.message;
}
}

console.log(writeJson("./data.json", { hello: "World" }));

/**
* Output: Written to ./data.json
*/