How to flatten array with the given callback in Javascript?

Question

The function must project each element of the specified array to a sequence and flatten the resulting sequences into one array.

I have the function that must return the flattened array based on the given selector function (childrenSelector) but facing issues when applying slice() function.

When applying slice as a selector function, it says

TypeError: x.slice is not a function

function flattenArray(arr, childrenSelector) {
 return arr.reduce((accumArr, currVal) => {
 console.log(currVal);
 return Array.isArray(currVal) 
 ? accumArr.concat(currVal.map(childrenSelector)) 
 : accumArr.concat(childrenSelector(currVal))
 }, []
 );
}
flattenArray([[11, 12, 13, 14, 15], [21, 22, ,23, 24, 25], [31, 32, 34, 35]], x => x.slice(0, 2))

Is there any solution for this issue?

• CertainPerformance: What's the desired output?
• Jack Bashford: So, it should do the slices for each child array first, then it should combine those all into one flat array?

Answer 1

The problem is that, when iterating over the outer array, the conditional

Array.isArray(currVal)

is fulfilled, so

accumArr.concat(currVal.map(childrenSelector))

runs when currVal is an array of numbers. But numbers don't have a .slice method.

Instead, call childrenSelector on the currVal, without .map (so that the array is sliced):

function flattenArray(arr, childrenSelector) {
 return arr.reduce((accumArr, currVal) => {
 return accumArr.concat(childrenSelector(currVal));
 }, []);
}
console.log(
 flattenArray([
 [11, 12, 13, 14, 15],
 [21, 22, , 23, 24, 25],
 [31, 32, 34, 35]
 ], x => x.slice(0, 2))
);

You can also use flatMap:

const flattenArray = (arr, childrenSelector) => arr.flatMap(childrenSelector);
console.log(
 flattenArray([
 [11, 12, 13, 14, 15],
 [21, 22, , 23, 24, 25],
 [31, 32, 34, 35]
 ], x => x.slice(0, 2))
);