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108 lines
3 KiB
Markdown
108 lines
3 KiB
Markdown
# Read and write files
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## Concepts
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- Deno's runtime API provides the
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[Deno.readTextFile](https://doc.deno.land/builtin/stable#Deno.readTextFile)
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and
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[Deno.writeTextFile](https://doc.deno.land/builtin/stable#Deno.writeTextFile)
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asynchronous functions for reading and writing entire text files.
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- Like many of Deno's APIs, synchronous alternatives are also available. See
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[Deno.readTextFileSync](https://doc.deno.land/builtin/stable#Deno.readTextFileSync)
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and
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[Deno.writeTextFileSync](https://doc.deno.land/builtin/stable#Deno.writeTextFileSync).
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- Use `--allow-read` and `--allow-write` permissions to gain access to the file
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system.
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## Overview
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Interacting with the filesystem to read and write files is a common requirement.
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Deno provides a number of ways to do this via the
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[standard library](https://deno.land/std) and the
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[Deno runtime API](https://doc.deno.land/builtin/stable).
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As highlighted in the [Fetch Data example](./fetch_data) Deno restricts access
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to Input / Output by default for security reasons. Therefore when interacting
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with the filesystem the `--allow-read` and `--allow-write` flags must be used
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with the `deno run` command.
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## Reading a text file
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The Deno runtime API makes it possible to read text files via the
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`Deno.readTextFile()` method, it just requires a path string or URL object. The
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method returns a promise which provides access to the file's text data.
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**Command:** `deno run --allow-read read.ts`
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```typescript
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/**
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* read.ts
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*/
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const text = Deno.readTextFile("./people.json");
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text.then((response) => console.log(response));
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/**
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* Output:
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*
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* [
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* {"id": 1, "name": "John", "age": 23},
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* {"id": 2, "name": "Sandra", "age": 51},
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* {"id": 5, "name": "Devika", "age": 11}
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* ]
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*/
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```
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## Writing a text file
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The Deno runtime API allows developers to write text to files via the
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`Deno.writeTextFile()` method. It just requires a file path and text string. The
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method returns a promise which resolves when the file was successfully written.
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To run the command the `--allow-write` flag must be supplied to the `deno run`
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command.
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**Command:** `deno run --allow-write write.ts`
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```typescript
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/**
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* write.ts
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*/
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const write = Deno.writeTextFile("./hello.txt", "Hello World!");
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write.then(() => console.log("File written to ./hello.txt"));
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/**
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* Output: File written to ./hello.txt
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*/
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```
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By combining `Deno.writeTextFile` and `JSON.stringify` you can easially write
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serialized JSON objects to a file. This example uses synchronous
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`Deno.writeTextFileSync`, but this can also be done asynchronously using
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`await Deno.writeTextFile`.
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To execute the code the `deno run` command needs the write flag.
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**Command:** `deno run --allow-write write.ts`
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```typescript
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/**
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* write.ts
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*/
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function writeJson(path: string, data: object): string {
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try {
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Deno.writeTextFileSync(path, JSON.stringify(data));
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return "Written to " + path;
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} catch (e) {
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return e.message;
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}
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}
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console.log(writeJson("./data.json", { hello: "World" }));
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/**
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* Output: Written to ./data.json
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*/
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```
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