Read FDF as JSON, promisified

README.md

@@ -154,6 +154,20 @@ pdftk
     });
 ```
 
+#### Get names and values of form fields ####
+
+```javascript
+pdftk
+    .input(fs.readFileSync('./file.pdf'))
+    .readFormFieldValuesAsJSON()
+    .then(json => for (const [name, value] of Object.entries(json)) {
+        // print values or fill the form with modifications
+    })
+    .catch(err => {
+        // handle errors
+    });
+```
+
 #### Useful chaining ####
 
 ```javascript

index.js

@@ -177,6 +177,41 @@ class PdfTk {
         return new Buffer(str, encoding);
     }
 
+    /**
+     * Creates JSON data from fdf buffer output.
+     * @static
+     * @public
+     * @param {Buffer} fdfBuffer - fdf buffer to transform to JSON.
+     * @returns {object} Fdf field data as a JSON.
+     */
+    static fdfToJSON(fdfBuffer) {
+        const fieldsRegExp = /\/Fields\s*\[\s*<<([\s\S]*?)>>\s*\]\s*>>/;
+        const singleFieldRegExp = /\/([VT])\s*[(]((?:\\[)]|[^)])*)[)]/g;
+
+        const fieldsMatch = fdfBuffer.toString().match(fieldsRegExp);
+        if (!fieldsMatch) {
+            throw TypeError('Function must be called on generated FDF output');
+        }
+        const fdfJson = fieldsMatch[1].split(/>>\s*<</)
+            .reduce((json, field) => {
+                let fieldName = null;
+                let fieldValue = null;
+                // eslint-disable-next-line comma-dangle
+                for (const [, letter, contents] of field.matchAll(singleFieldRegExp)) {
+                    switch (letter) {
+                        case 'V': fieldValue = contents; break;
+                        case 'T': fieldName = contents; break;
+                        default: break;
+                    }
+                }
+                if (fieldValue !== null && fieldName !== null) {
+                    json[fieldName] = fieldValue;
+                }
+                return json;
+            }, {});
+        return fdfJson;
+    }
+
     /**
      * Sanitizes fdf input
      * @statuc
@@ -496,6 +531,16 @@ class PdfTk {
         return this;
     }
 
+    /**
+     * Read PDF form fields into JSON data. Implies generateFdf().
+     * @public
+     * @returns {Promise} Promise that resolves the form fill data as JSON, field name as key and field value as value.
+     * @see {@link https://www.pdflabs.com/docs/pdftk-man-page/#dest-op-generate-fdf}
+     */
+    readFormFieldValuesAsJSON() {
+        return this.generateFdf().output().then(buffer => PdfTk.fdfToJSON(buffer));
+    }
+
     /**
      * Fill a PDF form from JSON data.
      * @public

test/files/emptyform.json

@@ -0,0 +1,4 @@
+{
+  "email": "",
+  "name": ""
+}

test/files/filledform.json

@@ -0,0 +1,4 @@
+{
+  "email": "test@email.com",
+  "name": "John Doe"
+}

test/files/filledformwithnumber.json

@@ -0,0 +1,4 @@
+{
+  "email": "test@email.com",
+  "name": "123"
+}

test/readFormFieldValuesAsJSON.spec.js

@@ -0,0 +1,61 @@
+/* globals describe, it */
+const chai = require('chai');
+
+const { expect, } = chai;
+
+const pdftk = require('../');
+const fs = require('fs');
+const path = require('path');
+
+describe('readFormFieldValuesAsJSON', function () {
+
+    it('should generate a json file from the fdf of an unfilled pdf', function () {
+
+        const testFile = JSON.parse(fs.readFileSync(path.join(__dirname, './files/emptyform.json')));
+        const input = path.join(__dirname, './files/form.pdf');
+
+        return pdftk
+            .input(input)
+            .readFormFieldValuesAsJSON()
+            .then(json => expect(json).to.eql(testFile))
+            .catch(err => expect(err).to.be.null);
+    });
+
+    it('should generate an json file from the fdf of a filled pdf', function () {
+
+        const testFile = JSON.parse(fs.readFileSync(path.join(__dirname, './files/filledform.json')));
+        const input = fs.readFileSync(path.join(__dirname, './files/filledform.temp.pdf'));
+
+        return pdftk
+            .input(input)
+            .readFormFieldValuesAsJSON()
+            .then(json => expect(json).to.deep.equal(testFile))
+            .catch(err => expect(err).to.be.null);
+    });
+
+    it('should generate an json file from the fdf of a number filled pdf', function () {
+
+        const testFile = JSON.parse(fs.readFileSync(path.join(__dirname, './files/filledformwithnumber.json')));
+        const input = fs.readFileSync(path.join(__dirname, './files/filledformwithnumber.temp.pdf'));
+
+        return pdftk
+            .input(input)
+            .readFormFieldValuesAsJSON()
+            .then(json => expect(json).to.eql(testFile))
+            .catch(err => expect(err).to.be.null);
+    });
+
+    it('should throw and TypeError if the buffer is not an fdf', function () {
+
+        const input = path.join(__dirname, './files/form.pdf');
+
+        return pdftk
+            .input(input)
+            .readFormFieldValuesAsJSON()
+            .then(json => json)
+            .catch(err => {
+                expect(err.name).to.equal('TypeError');
+                expect(err.message).to.equal('Function must be called on generated FDF output');
+            });
+    });
+});