Iterables

An object is deemed iterable if it has an implementation for theSymbol.iterator property.Some built-in types likeArray,Map,Set,String,Int32Array,Uint32Array, etc. have theirSymbol.iterator property already implemented.Symbol.iterator function on an object is responsible for returning the list of values to iterate on.

Iterable interface

Iterable is a type we can use if we want to take in types listed above which are iterable. Here is an example:

ts
functiontoArray<X>(xs:Iterable<X>):X[] {
return [...xs]
}

for..of statements

for..of loops over an iterable object, invoking theSymbol.iterator property on the object.Here is a simplefor..of loop on an array:

ts
letsomeArray = [1,"string",false];
for (letentryofsomeArray) {
console.log(entry);// 1, "string", false
}

for..of vs.for..in statements

Bothfor..of andfor..in statements iterate over lists; the values iterated on are different though,for..in returns a list ofkeys on the object being iterated, whereasfor..of returns a list ofvalues of the numeric properties of the object being iterated.

Here is an example that demonstrates this distinction:

ts
letlist = [4,5,6];
for (letiinlist) {
console.log(i);// "0", "1", "2",
}
for (letioflist) {
console.log(i);// 4, 5, 6
}

Another distinction is thatfor..in operates on any object; it serves as a way to inspect properties on this object.for..of on the other hand, is mainly interested in values of iterable objects. Built-in objects likeMap andSet implementSymbol.iterator property allowing access to stored values.

ts
letpets =newSet(["Cat","Dog","Hamster"]);
pets["species"] ="mammals";
for (letpetinpets) {
console.log(pet);// "species"
}
for (letpetofpets) {
console.log(pet);// "Cat", "Dog", "Hamster"
}

Code generation

Targeting ES5

When targeting an ES5-compliant engine, iterators are only allowed on values ofArray type.It is an error to usefor..of loops on non-Array values, even if these non-Array values implement theSymbol.iterator property.

The compiler will generate a simplefor loop for afor..of loop, for instance:

ts
letnumbers = [1,2,3];
for (letnumofnumbers) {
console.log(num);
}

will be generated as:

js
varnumbers = [1,2,3];
for (var_i =0;_i <numbers.length;_i++) {
varnum =numbers[_i];
console.log(num);
}

Targeting ECMAScript 2015 and higher

When targeting an ECMAScript 2015-compliant engine, the compiler will generatefor..of loops to target the built-in iterator implementation in the engine.