How does list mapping in dart actually work? - list

Question

I am coming from a javascript background, and in javascript if i had an array with elements [1,2,3] I can map through this array and perform a function on each iteration as so:
[1,2,3].map((i) => {
console.log(i);
return;
})
and i get
1
2
3
Trying same in dart
void main() {
[1, 2, 3].map((i) {
print(i);
return;
});
}
No output is printed, what am I missing?

Answer 1

JavaScript is different from Dart.
According to the documentation, the map method on an Iterable is meant to
return a new lazy Iterable with elements that are created by calling f on each element of this Iterable in iteration order.
In other words, the function in the map method isn't called until it is needed because it is lazily invoked. It will not return anything until something asks for what it is supposed to return. As long as the returned Iterable is not iterated over, the supplied function f will not be invoked.
Try in dartpad.dev
void main() {
var arr = [1, 2, 3];
var arr2 = arr.map((item)=> {
item * 2
});
print ('$arr2');
}
In the example above, the function in the map method of arr is called/invoked because it is requested for by $arr2 in the print function. Because of this, it runs the code inside it and the parameter in the print method gets printed out
Try
print([1, 2, 3].map((i) {
print(i);
return;
}));
You'll see that you get your desired result, because the function in the map method was invoked by the print method.
Since the method is lazy, it will not be invoked until it is requested. Adding a map method to an array doesn't invoke it. You've got to require what it returns for the function in the map method to be invoked.