Add isStream

snippets/isStream.md

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+### isStream
+
+Checks if the given argument is a stream.
+
+Check if the value is different from `null`, use `typeof` to check if a value is of type `object` and the `pipe` property is of type `function`.
+
+```js
+const isStream = val => val !== null && typeof val === 'object' && typeof val.pipe === 'function';
+```
+
+```js
+const fs = require('fs');
+isStream(fs.createReadStream('test.txt')); // true
+```