Spread and Rest

Spread

... spreads an array into an array literal or into a call’s arguments:

let morning = ["standup", "review"];
let evening = ["deploy"];
print([...morning, "lunch", ...evening]); // ["standup", "review", "lunch", "deploy"]

let scores = [72, 95, 88];
print(math.max(...scores)); // 95

Both also take a set, in its order, a map as [key, value] pairs and bytes as integers: [...tags], f(...counts), [...b"hi"] is [104, 105].

An object or a struct spreads into an object literal. Keys are applied in order and later ones win, so put the defaults first to override them, or last to fill in only what is missing:

let defaults = { timeout: 30, retries: 3 };

print({ ...defaults, timeout: 5 }); // {timeout: 5, retries: 3}
print({ timeout: 5, ...defaults }); // {timeout: 30, retries: 3}

struct Point { x, y = 0 }
let p = Point { x: 1, y: 2 };
print({ ...p, z: 3 });              // {x: 1, y: 2, z: 3}

A struct literal takes a struct of its own type, or an object of some of its fields, which makes an updated copy (see Structs):

struct Point { x, y = 0 }
let p = Point { x: 1, y: 2 };
print(Point { ...p, y: 5 }); // Point {x: 1, y: 5}

Spreading anything else is a TypeError:

print([..."abc"]); // TypeError: Cannot spread string into an array.

Rest Parameters

A last parameter ...name (or ...mut name) takes the arguments after the others as an array, possibly empty.

fun log(level, ...parts) {
    print("[{level}] " + parts.join(" "));
}

log("info", "started", "in", 3, "ms"); // [info] started in 3 ms

let count = (...args) => args.len();
print(count(1, 2, 3)); // 3
print(count());        // 0

Rest and spread go together to forward arguments:

fun sum(...nums) { return nums.reduce((a, b) => a + b, 0); }

let xs = [1, 2, 3];
print(sum(...xs, 10)); // 16

Fewer arguments than the parameters before the rest is an ArgumentError:

fun log(level, ...parts) {}
log(); // ArgumentError: Expected at least 1 argument but got 0.