This is basically a JavaScript expression evaluator, so any JavaScript expression can be used, from simple arithmetic like 4 + 5 to function calls like max(3, 2, 1) to variable declarations like a=64,b=32 and use of them like a * b + 4 and even function declarations like foo=function(){ return a+b; }.
The functions abs, acos, acosh, asin, asinh, atan, atan2, atanh, cbrt, ceil, cos, cosh, exp, floor, log, max, min, pow, round, sin, sinh, sqrt, tan, tanh and trunc as well as the constants E, LN2, LN10, LOG2E, LOG10E and PI from the JS Math object are available directly without using the Math object.
The function random has the forms random(), random(max) or random(min, max). The functions deg and rad convert to degrees and radians respectively. The functions hex and dec convert numbers to hexadecimals and hexadecimals to numbers respectively. The functions human and hsize return human readable numbers and byte sizes respectively. The function clear clears the output and the function now returns the current date and time. The function help shows this text.
Functions that take no arguments can be executed without adding empty parentheses, e.g. typing just clear is the same as typing clear();.
Putting = or / in front of the expression (e.g. /1920*1080*4) is a shortcut for calling the human or hsize functions respectively.