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Question

I have two arrays:

firstArray = [1, 2, 3, 4];
secondArray = [5, 6, 7, 8];

I must use their elements to compute another one inside a forEach.

_.forEach(firstArray, (data, i) => {
myValue: firstArray[i] + secondArray[i]
});

This works fine. But I want to ignore the last element of the second array all the time. So in this case, for i = 3 the result should be 4 , not 4+8 , ignoring the last value of the second array.

I know that using an if statement wouldn't work but is there a way to do it?

I mean inside the forEach, removing the last element before the function doesn't work in this case.

UPDATE : I received some good answers but all of them were using something different than forEach . I would like to know if there is a way to do it with this function or not.

Answer 1

You can just check if i is the last element of the second array using array.length .

I don't really understand your forEach code, but you could use ternary operators in it:

_.forEach(firstArray, (data, i) => {
    myValue: firstArray[i] + (i === secondArray.length - 1 ? 0 : secondArray[i])
});

Answer 2

Use map

firstArray = [1, 2, 3, 4];
secondArray = [5, 6, 7, 8];
var output = firstArray.map( ( s, i, ar ) => s + (i == ar.length - 1 ?  0 : secondArray[i] ) );

UPDATE: I received some good answers but all of them were using something different than forEach. I would like to know if there is a way to do it with this function or not.

using forEach , you would need another array to capture the sum

firstArray = [1, 2, 3, 4];
secondArray = [5, 6, 7, 8];
var myValue = [];
firstArray.map( ( s, i, ar ) => myValue.push( s + (i == ar.length - 1 ?  0 : secondArray[i] ) ) );

now myValue is

[6, 8, 10, 4]

Hence map seems to be a less-verbose approach.

Answer 3

You could reduce and map the arrays.

 var firstArray = [1, 2, 3, 4], secondArray = [5, 6, 7, 8], result = [firstArray, secondArray].reduce((a, b) => a.map((v, i) => v + b[i])); console.log(result); 

Answer 4

I would do this:

firstArray = [1, 2, 3, 4];
secondArray = [5, 6, 7, 8];

for (let i = 0; i < firstArray.length; i++){
    if(i === firstArray.length - 1) {
         myValue = firstArray[i];
    } else {
        myValue = firstArray[i] + secondArray[i]
    }
}