There are a number of coverage criteria, the main ones being:
- Function coverage Has each function (or subroutine) in the program been called?
- Statement coverage Has each statement in the program been executed?
- Branch coverage Has each branch (also called DD-path) of each control structure (such as in if and case statements) been executed? For example, given an if statement, have both the true and false branches been executed? Another way of saying this is, has every edge in the program been executed?
- Line coverage has each executable line in the source file been executed?
For each case, the percentage represents executed code vs not-executed code, which equals each fraction in percent format (e.g: 50% branches, 1/2).
In the file report:
'E'stands for ‘else path not taken’, which means that for the marked if/else statement, the ‘if’ path has been tested but not the ‘else’.'I'stands for ‘if path not taken’, which is the opposite case: the ‘if’ hasn’t been tested.- The
xNin left column is the amount of times that line has been executed. - Not executed lines, or pieces of code, will be highlighted in red.
This has been verified for Istanbul v0.4.0, I’m not sure if this still applies for subsequent versions, but being that library is based on solid theoretic principles, behavior shouldn’t change too much for newer versions.
It also provides some color codes –
Pink: statements not covered.
Orange: functions not covered.
Yellow: branches not covered.
Full Istanbul docs here:
https://istanbul.js.org
For more in-depth theory on code coverage:
https://en.wikipedia.org/wiki/Code_coverage
Hope it helps!