JSON.parse() 导致错误：`SyntaxError: Unexpected token in JSON at position 0`

Question

I'm writing my first application in Node.js. I am trying to read some data from a file where the data is stored in the JSON format.

I get this error:

SyntaxError: Unexpected token  in JSON at position 0

at Object.parse (native)

Here is this part of the code:

//read saved addresses of all users from a JSON file
fs.readFile('addresses.json', function (err, data) {
    if (data) {
        console.log("Read JSON file: " + data);
        storage = JSON.parse(data);

Here is the console.log output (and I checked the .json file itself, it's the same):

Read JSON file: {

    "addresses": []

}

That seems to me like a correct JSON. Why does JSON.parse() fail then?

Answer 1

You have a strange char at the beginning of the file.

data.charCodeAt(0) === 65279

I would recommend:

fs.readFile('addresses.json', function (err, data) {
if (data) {
    console.log("Read JSON file: " + data);
    data = data.trim(); 
    //or data = JSON.parse(JSON.stringify(data.trim()));
    storage = JSON.parse(data);
 }});

Answer 2

JSON.parse() does not allow trailing commas. So, you need to get rid of it:

JSON.parse(JSON.stringify(data));

You can find more about it here .

Answer 3

It might be the BOM[ 1 ]. I have done a test by saving a file with content {"name":"test"} with UTF-8 + BOM, and it generated the same error.

> JSON.parse(fs.readFileSync("a.json"))
SyntaxError: Unexpected token  in JSON at position 0

And based on a suggestion here [ 2 ], you can replace it or drop it before you call JSON.parse() .

For example:

var storage = {};

fs.readFile('a.json', 'utf8', function (err, data) {
    if (data) {
        console.log("Read JSON file: " + data);
        console.log(typeof(data))
        storage = JSON.parse(data.trim());
    }
});

or

var storage = {};
fs.readFile('a.json', function (err, data) {
    if (data) {
        console.log("Read JSON file: " + data);
        console.log(typeof(data))
        storage = JSON.parse(data.toString().trim());
    }
})

You can also remove the first 3 bytes (for UTF-8) by using Buffer.slice() .

Answer 4

try it like this

fs.readFile('addresses.json','utf-8', function (err, data) {
    if (data) {
        console.log("Read JSON file: " + data);
        storage = JSON.parse(data);

its because of the BOM that needs an encoding to be set before reading the file. its been issued in nodejs respository in github

https://github.com/nodejs/node-v0.x-archive/issues/186

Answer 5

To further explain @Luillyfe's answer:

Ah-ha! fs.readFileSync("data.json") returns a Javascript object!

Edit: Below is incorrect...But summarizes what one might think at first!

I had through that was a string. So if the file was saved as UTF-8/ascii, it would probably not have an issue? The javascript object returned from readFileSync would convert to a string JSON.parse can parse? No need to call JSON.stringify?

I am using powershell to save the file. Which seems to save the file as UTF-16 (too busy right now to check). So I get "SyntaxError: Unexpected token in JSON at position 0."

However, JSON.stringify(fs.readFileSync("data.json")) correctly parses that returned file object into a string JSON can parse.

Clue for me is my json file contents looking like the below (after logging it to the console):

�{ " R o o m I D _ L o o k u p " :   [
         {
                 " I D " :     1 0 ,
                 " L o c a t i o n " :     " f r o n t " ,
                 " H o u s e " :     " f r o n t r o o m "
         }
}

That doesn't seem like something a file would load into a string...

Incorrect being (this does not crash...but instead converts the json file to jibberish!):

const jsonFileContents = JSON.parse(JSON.stringify(fs.readFileSync("data.json")));

I can't seem to find this anywhere. But makes sense!

Edit: Um... That object is just a buffer. Apologies for the above!

Solution:

const fs = require("fs");

function GetFileEncodingHeader(filePath) {
    const readStream = fs.openSync(filePath, 'r');
    const bufferSize = 2;
    const buffer = new Buffer(bufferSize);
    let readBytes = 0;

    if (readBytes = fs.readSync(readStream, buffer, 0, bufferSize, 0)) {
        return buffer.slice(0, readBytes).toString("hex");
    }   

    return "";
}

function ReadFileSyncUtf8(filePath) {
    const fileEncoding = GetFileEncodingHeader(filePath);
    let content = null;

    if (fileEncoding === "fffe" || fileEncoding === "utf16le") {
        content = fs.readFileSync(filePath, "ucs2"); // utf-16 Little Endian
    } else if (fileEncoding === "feff" || fileEncoding === "utf16be") {
        content = fs.readFileSync(filePath, "uts2").swap16(); // utf-16 Big Endian
    } else {
        content = fs.readFileSync(filePath, "utf8");
    }

    // trim removes the header...but there may be a better way!
    return content.toString("utf8").trimStart();
}

function GetJson(filePath) {
    const jsonContents = ReadFileSyncUtf8(filePath);
    console.log(GetFileEncodingHeader(filePath));

    return JSON.parse(jsonContents);
}

Usage:

GetJson("data.json");

Note: I don't currently have a need for this to be async yet. Add another answer if you can make this async!

Answer 6

As mentioned by TamusJRoyce I ended up using the util.TextDecoder class to come up with a robust way to read both UTF-8 (without BOM) and UTF-8 (with BOM). Here's the snippit, assuming that file input.json is UTF-8 (with or without BOM) and contains valid JSON.

const fs = require('fs');
const util = require('util');

const rawdata = fs.readFileSync('input.json');
const textDecoder = new util.TextDecoder('utf-8');
const stringData = textDecoder.decode(rawdata);
const objects = JSON.parse(stringData);

Answer 7

const fs = require('fs');
const myConsole = new console.Console(fs.createWriteStream('./output.json'));
myConsole.log(object);

This will create an output file with all the output which can been triggered through console.log(object) .

This is the easiest way to convert the console.log() output into a file.`