Read a json file in Deno
Deno is a secure runtine for JavaScript, so to read a file the program needs explicit permissions. We can enable file reading with the --allow-read flag in the command line:
deno run --allow-read read-json.ts
Once the permissions are straight, we need to read the file. There are a few methods on the Deno module to use, so let’s look at two options: readFile, and readTextFile.
Let’s take a look at them separately.
readFile
Deno’s readFile method returns a Promise resolves with a Uint8Array. You can call the method like this:
console.log(await Deno.readFile("data.json"));
So if we run the following code in a directory with a data.json file, you would see a log like this:
Uint8Array(29) [
123, 10, 32, 32, 32, 32, 34, 115,
111, 117, 114, 99, 101, 34, 58, 32,
34, 100, 97, 116, 97, 46, 106, 115,
111, 110, 34, 10, 125
]
In order to read this, we need a new text decoder decoder with and pass the file’s encoding:
const decoder = new TextDecoder("utf-8");
const data = await Deno.readFile("data.json");
console.log(decoder.decode(data));
Now when we run the code, we see the contents of data.json logged in the console:
{
"source": "data.json"
}
This looks good, but it is just a formatted string. To consume this data, we need to use the global JSON object’s parse method to get a usuable json object.
const decoder = new TextDecoder('utf-8')
const data = await Deno.readFile('data.json')
console.log(JSON.parse(decoder.decode(data)))
Run it again, and you will see whatever data you had stored in your json file logged to the console. You have successfully read a json file in Deno with readFile!
If you know that your data is utf8 encoded, you can skip a step with the next Deno method.
readTextFile
Deno’s readTextFile method returns a Promise that resolves to a utf8 decoded string. You can call the method like this:
console.log(await Deno.readTextFile("data.json"));
Once again, if we run this code in a directory with a data.json file, you would see the file’s contents logged in the console:
{
"source": "data.json"
}
This looks good, but just like before it is only a formatted string. To consume this data, we need to use the global JSON object’s parse method again to get a usuable json object.
const text = await Deno.readTextFile("data.json");
console.log(JSON.parse(text));
Run it again, and you will see whatever data you had stored in your json file logged to the console. You have successfully read a json file in Deno with readTextFile!
Conclusion
If you know that your file is utf8 encoded, using Deno.readTextFile is an easy way to grab the contents of a file. Otherwise, Deno.readFile and a decoder can get you on your way.
Whichever method you choose, just remember to parse the json data before trying to consume it.
Happy coding!