Javascript Read Excel File (Into Array & JSON)

Welcome to a tutorial on how to read an Excel file into an array or JSON in Javascript. Yes, that is right. We can actually read Excel files in client-side Javascript, without having to do all those “upload to a temp folder on the server then read the Excel file”. Read on for the example!

 

 

TABLE OF CONTENTS

 

 

PART 1) DUMMY EXCEL FILE

NAME	EMAIL
Jo Doe	jo@doe.com
Job Doe	job@doe.com
Joe Doe	joe@doe.com
Jog Doe	jog@doe.com
Joh Doe	joh@doe.com
Joi Doe	joi@doe.com
Jol Doe	jol@doe.com
Jon Doe	jon@doe.com
Jou Doe	jou@doe.com
Joy Doe	joy@doe.com

First, let us start with the Excel file… There is nothing “special” here, just a list of dummy users.

 

 

PART 2) THE HTML

<!-- (A) JAVASCRIPT -->
<!-- https://sheetjs.com/ -->
<script src="https://cdnjs.cloudflare.com/ajax/libs/xlsx/0.18.5/xlsx.full.min.js"></script>
<script defer src="3a-read-array.js"></script>
<!--
<script defer src="3b-read-json.js"></script>
-->
 
<!-- (B) FILE PICKER -->
<input type="file" id="demo" accept=".xls,.xlsx">

Yep, that’s all for the HTML.

1. Javascript does not have native functions to read Excel files. So we will be using a library called SheetJS – Check out their Github page if you want to follow up, links are below in the extras section.
2. Just a regular file input field to pick an Excel file.

 

 

PART 3A) READ INTO AN ARRAY

document.getElementById("demo").onchange = evt => {
  // (A) NEW FILE READER
  var reader = new FileReader();
 
  // (B) ON FINISH LOADING
  reader.addEventListener("loadend", evt => {
    // (B1) GET THE FIRST WORKSHEET
    var workbook = XLSX.read(evt.target.result, {type: "binary"}),
        worksheet = workbook.Sheets[workbook.SheetNames[0]],
        range = XLSX.utils.decode_range(worksheet["!ref"]);
 
    // (B2) READ CELLS IN ARRAY
    var data = [];
    for (let row=range.s.r; row<=range.e.r; row++) {
      let i = data.length;
      data.push([]);
      for (let col=range.s.c; col<=range.e.c; col++) {
        let cell = worksheet[XLSX.utils.encode_cell({r:row, c:col})];
        data[i].push(cell.v);
      }
    }
    console.log(data);
  });
 
  // (C) START - READ SELECTED EXCEL FILE
  reader.readAsArrayBuffer(evt.target.files[0]);
};

Not going to explain this line-by-line (very boring), so here’s a quick walkthrough.

1. Create a var reader = new FileReader() to read the selected Excel file.
2. What to do when the Excel file is loaded. I.E. Start reading the cells of the worksheet.
   • (B1) Use the SheetJS library to get the first worksheet.
   • (B2) Then loop through the rows/columns, and keep them in the data array.
3. reader.readAsArrayBuffer(evt.target.files[0]) In English, “read the selected Excel file as an array buffer”. This will effectively start (B) and extract the cell values into an array.

 

PART 3B) READ INTO JSON STRING

// (B2) READ HEADER ROW
var data = {}, keys = [];
for (let col=range.s.c; col<=range.e.c; col++) {
  let cell = worksheet[XLSX.utils.encode_cell({r:0, c:col})];
  data[cell.v] = [];
  keys.push(cell.v);
} 
 
// (B3) READ DATA ROWS
for (let row=range.s.r + 1; row<=range.e.r; row++) {
  for (let col=range.s.c; col<=range.e.c; col++) {
    let cell = worksheet[XLSX.utils.encode_cell({r:row, c:col})];
    data[keys[col]].push(cell.v);
  }
}
 
// (B4) JSON ENCODE
data = JSON.stringify(data);
console.log(data);

For you guys who need a different “data arrangement” or JSON encoded string – The process is the same, we are only organizing the data in a different structure.

• (B2) First, we deal with the “header row” and “data key”.
  • Take note, var data is now an object, not an array.
  • Read the header row to extract the key – data = { NAME:[], EMAIL:[] }.
  • Also, keep the keys in an array – keys = ["NAME", "EMAIL"].
• (B3) As usual, loop through the rows and columns to “fill up” data.
• (B4) Lastly, JSON encode data into a string.

 

 

 

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EXAMPLE CODE DOWNLOAD

Click here for the source code on GitHub gist, just click on “download zip” or do a git clone. I have released it under the MIT license, so feel free to build on top of it or use it in your own project.

 

 

COMPATIBILITY CHECKS

• Arrow Functions – CanIUse
• File Reader – CanIUse

This example will work on all modern “Grade A” browsers.

 

TABLE FORMAT & ERROR HANDLING

Captain Obvious at your service – We are assuming that the data is always in the first worksheet, and the first row is always the header. If that is not the case, you will have to change the entire section B2 in the Javascript accordingly. Also, you may want to add some sort of error handling when the worksheet cannot be read properly.

 

PERFORMANCE WARNING

Yes, we are reading an entire Excel file into an array, this works fine for small/medium Excel files. But you will face performance issues and memory limitations for massive files. So you may also want to implement some sort of “limiter” on your own; Maybe stop reading at row 1000, or process it “batch-by-batch”.

 

LINKS & REFERENCES

• SheetJS – GitHub
• How To Read Files In Javascript – Code Boxx