Read only the necessary bytes when parsing the header

src/header.js

@@ -8,24 +8,42 @@ export default class Header {
     }
 
     parse(callback) {
-        fs.readFile(this.filename, (err, buffer) => {
+        fs.open(this.filename, 'r', (err, fd) => {
             if (err) throw err;
 
-            this.type = buffer.slice(0, 1).toString(this.encoding);
-            this.dateUpdated = this.parseDate(buffer.slice(1, 4));
-            this.numberOfRecords = this.convertBinaryToInteger(buffer.slice(4, 8));
-            this.start = this.convertBinaryToInteger(buffer.slice(8, 10));
-            this.recordLength = this.convertBinaryToInteger(buffer.slice(10, 12));
+            const startBuffer = Buffer.alloc(2);
 
-            const result = [];
-            for (let i = 32, end = this.start - 32; i <= end; i += 32) {
-                result.push(buffer.slice(i, i+32));
-            }
+            fs.read(fd, startBuffer, 0, 2, 8, (err, bytesRead, buffer) => {
+                if (err) throw err;
 
-            this.fields = result.map(this.parseFieldSubRecord.bind(this));
+                const start = this.convertBinaryToInteger(buffer);
 
-            callback(this);
-        });
+                const headerBuffer = Buffer.alloc(start);
+
+                fs.read(fd, headerBuffer, 0, start, 0, (err, bytesRead, buffer) => {
+                    if (err) throw err;
+
+                    this.type = buffer.slice(0, 1).toString(this.encoding);
+                    this.dateUpdated = this.parseDate(buffer.slice(1, 4));
+                    this.numberOfRecords = this.convertBinaryToInteger(buffer.slice(4, 8));
+                    this.start = this.convertBinaryToInteger(buffer.slice(8, 10));
+                    this.recordLength = this.convertBinaryToInteger(buffer.slice(10, 12));
+
+                    const result = [];
+                    for (let i = 32, end = this.start - 32; i <= end; i += 32) {
+                        result.push(buffer.slice(i, i+32));
+                    }
+
+                    this.fields = result.map(this.parseFieldSubRecord.bind(this));
+
+                    callback(this);
+
+                    fs.close(fd, err => {
+                        if (err) throw err;
+                    })
+                })
+            })
+        })
     }
 
     parseDate(buffer) {