Created Topic substrings

Author: kingroryg

strings/substrings.md

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+#Substrings
+
+substring is used to take a part of a string. 
+Syntax:  substring(first_index,last_index). 
+
+```js
+var a = 'Hello world!';
+document.write(a.substring(1,6));
+```
+The preceding code snippet gives ```'ello '``` . Note that the 'w' (index 6) is not part of this substring.
+
+We could also do,
+```js
+var a = 'Hello world!';
+document.write(a.substring(2));
+```
+This gives the whole string from the character with index 2. ``` 'llo world!'```
+
+##substr
+
+There is also a method substr() that works slightly differently. Instead of the second number being an index number, 
+it gives the number of characters. 
+```js
+var a = 'Hello world!';
+document.write(a.substr(1,6));
+```
+
+starts at the character with index 1 ('e') and then gives 6 characters, so the output is ```ello w```