substring (ECMAScript 5.1 Specification Section 126.96.36.199) is the first logical choice to retrieve a part of the string. This
substring function can accept two numbers, the start and the end (exclusive) character position, respectively. In case the end value is smaller than the start, substring is smart enough to swap the values before doing the string extraction. An example of substring is illustrated in this code snippet:
var a = 'The Three Musketeers'; a.substring(4, 9); 'Three' a.substring(9, 4); 'Three'
substr (Section B.2.3). However, the parameters for
substr are the start character position and the numbers of characters to be extracted, respectively. This is shown in the following fragment:
var b = 'The Three Musketeers'; b.substr(4, 9); 'Three Mus' b.substr(9, 4); ' Mus'
This pair of functions, when they are both available, can be really confusing. It is so easy to mistake one for another and thereby leading to an unexpected outcome. It also does not help that the names,
substr, are too similar. Without looking at the documentation or the specification, there is a chance of picking a wrong one.
To add more confusion to this mixture, a String object also supports
slice (Section 188.8.131.52), just like in Array’s slice. For all intents and purposes,
slice has a behavior very close to
substring (accepting start and end position). However, there is a minor difference. If the end value is smaller than the start,
slice will not internally swap the values. In other words, it follows what is expected for Array’s
slice in the same situation and thus it returns an empty string instead.
var c = 'The Three Musketeers'; c.slice(4, 9); 'Three' c.slice(9, 4); ''
Prevention is the best cure. Be advised of your function name!