What is the difference between String.slice and String.substring?

slice() works like substring() with a few different behaviors.

Syntax: string.slice(start, stop);
Syntax: string.substring(start, stop);

What they have in common:

1. If start equals stop: returns an empty string
2. If stop is omitted: extracts characters to the end of the string
3. If either argument is greater than the string’s length, the string’s length will be used instead.

Distinctions of substring():

1. If start > stop, then substring will swap those 2 arguments.
2. If either argument is negative or is NaN, it is treated as if it were 0.

Distinctions of slice():

1. If start > stop, slice() will return the empty string. ("")
2. If start is negative: sets char from the end of string, exactly like substr() in Firefox. This behavior is observed in both Firefox and IE.
3. If stop is negative: sets stop to: string.length – Math.abs(stop) (original value), except bounded at 0 (thus, Math.max(0, string.length + stop)) as covered in the ECMA specification.

Source: Rudimentary Art of Programming & Development: Javascript: substr() v.s. substring()