Difference between concat and push?
Why does a return of the push
method cause
Uncaught TypeError: acc.push is not a function
But a return concat
results in the correct solution?
[1, 2, 3, 4].reduce(function name(acc, curr) {
if (even(curr)) {
return acc.push(curr);
}
return acc;
}, []);
function even(number) {
if (number % 2 === 0) {
return true;
}
return false;
}
[1, 2, 3, 4].reduce(function name(acc, curr) {
if (even(curr)) {
return acc.concat(curr);
}
return acc;
}, []);
function even(number) {
if (number % 2 === 0) {
return true;
}
return false;
}
Solution 1:
The push()
adds elements to the end of an array and returns the new length of the array. Thus your return here is invalid.
The concat()
method is used to merge arrays. Concat does not change the existing arrays, but instead returns a new array.
Better to filter, if you want a NEW array like so:
var arr = [1, 2, 3, 4];
var filtered = arr.filter(function(element, index, array) {
return (index % 2 === 0);
});
Note that assumes the array arr is complete with no gaps - all even indexed values. If you need each individual, use the element
instead of index
var arr = [1, 2, 3, 4];
var filtered = arr.filter(function(element, index, array) {
return (element% 2 === 0);
});
Solution 2:
acc
should not be an array. Look at the documentation. It can be one, but..
It makes no sense at all to reduce
an array to an array. What you want is filter
. I mean, reduce
using an array as the accumulator and concat
ing each element to it technically does work, but it is just not the right approach.
var res = [1, 2, 3, 4].filter(even);
console.log(res);
function even(number) {
return (number % 2 === 0);
}