Control flow

Conditions always go in parentheses: if (x > 0), never if x > 0.

if and else

const hp: int = 12;

if (hp <= 0) {
    console.log("defeated");
} else if (hp < 20) {
    console.log("hurt");
} else {
    console.log("healthy");
}

const label: string = hp > 50 ? "strong" : "weak";
console.log(label);

Looping over a collection: for...of

const party: Array<string> = ["Ada", "Brin", "Cass"];

for (const name of party) {
    console.log(name);
}

for...of is the loop to reach for. It works on arrays, maps, sets and strings.

One way to loop

The language also accepts a foreach keyword as an alias. Don't use it. The style guide allows only for...of, and every sample on this site follows it.

Counting loops

let total: int = 0;
for (let i: int = 1; i <= 10; i++) {
    total += i;
}
console.log(total);    // 55

Each iteration gets a fresh i, so closures made inside the loop each see their own value.

Ranges

a..b includes both ends. a..<b leaves out the end. Loop over a range with for...in or for...of; both count the same values:

for (const i in 0..3) {
    console.log(i);    // 0, 1, 2, 3
}

for (const i in 0..<3) {
    console.log(i);    // 0, 1, 2
}

A range is also a value: an array of the same numbers, so you can store it, pass it or spread it.

const steps: int[] = 0..<3;            // [0, 1, 2]
const padded: int[] = [9, ...(1..2)];  // [9, 1, 2]

Object keys

for...in over an object gives you its keys:

const stats: Object = { str: 5, dex: 7 };
for (const key in stats) {
    console.log(key);
}

while and do...while

let n: int = 3;
while (n > 0) {
    console.log(n);
    n--;
}

let tries: int = 0;
do {
    tries++;
} while (tries < 3);
console.log(tries);

switch

Each branch needs case and usually a break. Without break, a case falls through to the next one.

function weekday(day: int): string {
    switch (day) {
        case 0:
            return "Sunday";
        case 6:
            return "Saturday";
        default:
            return "a weekday";
    }
}

console.log(weekday(6));

For enums, switch strict checks that every member is covered, and switch bitflag matches flags. See Enums.

break, continue and labels

for (let i: int = 0; i < 10; i++) {
    if (i == 5) {
        break;
    }
    if (i % 2 == 0) {
        continue;
    }
    console.log(i);    // 1, 3
}

outer: for (let row: int = 0; row < 3; row++) {
    for (let col: int = 0; col < 3; col++) {
        if (row + col > 2) {
            break outer;
        }
    }
}

Labels can only go on loops.

Running code on the way out: defer

defer runs a statement when the current scope ends, however it ends. See Errors.