Built-in functions

Function	What it does
print(a, b, ...)	prints the values separated by spaces
println(a, b, ...)	the same and a line break
len(x)	length of a string, array, table, Java collection, map or array
str(x)	the value as a string, as println shows it
type(x)	"int", "long", "double", "boolean", "string", "table", "array", "function", "null", a class name, or the Java class name of a Java object
int(x), long(x), double(x)	convert a number or a string; a string that is not a number gives null
keys(t)	the keys of a table or a Java map, as an array
range(n), range(a, b), range(a, b, step)	an array of integers from a (0) up to b, not included
array(n, fill)	an array of n elements, all fill
table()	a new empty table
isa(v, C)	whether v is an instance of a Jumper class or a Java class C
format(fmt, ...)	String.format
error(msg)	throws an error with this message
assert(cond, msg)	throws Assertion failed: msg when cond is false
millis(), nanoTime()	the current time, as in System

println(len("abc"), len([1, 2]), len({ a: 1 }));   // 3 2 1
println(type(1), type(1.5), type("s"), type([]));  // int double string array
println(int("42") + 1, int("x"));                  // 43 null
println(range(1, 7, 2));                           // [1, 3, 5]
println(format("%.2f", 3.14159));                  // 3.14
println(isa(1, Integer), isa("s", Number));        // true false

A host can add its own functions and values to scripts, see Embedding.