sheetjs-clone/docbits/21_readstream.md

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### Streaming Read
<details>
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<summary><b>Why is there no Streaming Read API?</b> (click to show)</summary>
The most common and interesting formats (XLS, XLSX/M, XLSB, ODS) are ultimately
ZIP or CFB containers of files. Neither format puts the directory structure at
the beginning of the file: ZIP files place the Central Directory records at the
end of the logical file, while CFB files can place the storage info anywhere in
the file! As a result, to properly handle these formats, a streaming function
would have to buffer the entire file before commencing. That belies the
expectations of streaming, so we do not provide any streaming read API.
</details>
When dealing with Readable Streams, the easiest approach is to buffer the stream
and process the whole thing at the end. This can be done with a temporary file
or by explicitly concatenating the stream:
<details>
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<summary><b>Explicitly concatenating streams</b> (click to show)</summary>
```js
var fs = require('fs');
var XLSX = require('xlsx');
function process_RS(stream/*:ReadStream*/, cb/*:(wb:Workbook)=>void*/)/*:void*/{
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var buffers = [];
stream.on('data', function(data) { buffers.push(data); });
stream.on('end', function() {
var buffer = Buffer.concat(buffers);
var workbook = XLSX.read(buffer, {type:"buffer"});
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/* DO SOMETHING WITH workbook IN THE CALLBACK */
cb(workbook);
});
}
```
More robust solutions are available using modules like `concat-stream`.
</details>
<details>
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<summary><b>Writing to filesystem first</b> (click to show)</summary>
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This example uses [`tempfile`](https://npm.im/tempfile) to generate file names:
```js
var fs = require('fs'), tempfile = require('tempfile');
var XLSX = require('xlsx');
function process_RS(stream/*:ReadStream*/, cb/*:(wb:Workbook)=>void*/)/*:void*/{
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var fname = tempfile('.sheetjs');
console.log(fname);
var ostream = fs.createWriteStream(fname);
stream.pipe(ostream);
ostream.on('finish', function() {
var workbook = XLSX.readFile(fname);
fs.unlinkSync(fname);
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/* DO SOMETHING WITH workbook IN THE CALLBACK */
cb(workbook);
});
}
```
</details>