Dashes not underscores

snippets/truncate-a-string.md

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+### Truncate a String
+
+Determine if the string's `length` is greater than `num`. 
+Return the string truncated to the desired length, with `...` appended to the end or the original string.
+
+```
+const truncate = (str, num) =>
+  str.length > num ? str.slice(0, num > 3 ? num-3 : num) + '...' : str;
+// truncate('boomerang', 7) -> 'boom...'
+```