Add decode method

src/index.js

@@ -39,4 +39,34 @@ b64.encode = (input, opts) => new Promise(resolve => {
 	});
 });
 
+b64.decode = (input, opts) => new Promise(resolve => {
+	opts = applyDefaultOpts(opts);
+
+	if (typeof input !== 'string') {
+		throw new TypeError('input must be a base64 string');
+	}
+
+	const chunkMultiple = 4;
+	opts.chunkSize = Math.ceil(opts.chunkSize / chunkMultiple) * chunkMultiple;
+
+	if (opts.chunkSize === 0) {
+		throw new Error('chunkSize must be larger than 0');
+	}
+
+	const stringLength = input.length;
+	let currentIndex = 0;
+	let output = Buffer.alloc(0);
+
+	setImmediate(function encodeChunk() {
+		const chunk = input.slice(currentIndex, currentIndex + opts.chunkSize);
+		output = Buffer.concat([output, Buffer.from(chunk, 'base64')]);
+		currentIndex += opts.chunkSize;
+		if (currentIndex < stringLength) {
+			setImmediate(encodeChunk);
+		} else {
+			resolve(output);
+		}
+	});
+});
+
 module.exports = b64;