Pure beginner javascript - array and map

Question

Im brand new to javascript. I simply have an exercise where i need the array numbers to add each index to the next one.

The result should be:

var result = [4,8,15,21,11];

I tried something like this. It works on the first 4 indexes, however gives NaN on last since it doesnt have a next number to add to.

var numbers = [1, 3, 5, 10, 11];

var result = numbers.map((x, index) => x + numbers[index + 1])

console.log(result)

Is there any smarter way to do this and how do i get index 4 to stay as 11? And please explain as im trying to learn.

Ilmari Karonen · Accepted Answer · 2020-09-29 15:51:51Z

That's a perfectly valid way to do it. You can handle the last index by taking advantage of the fact that numbers[index] will be undefined (not an error) if index is beyond the end of the array, and undefined is a falsy value, so you can use || 0 to use 0 instead of undefined if it's there:

var result = numbers.map((x, index) =>  x + (numbers[index+1] || 0));

Live Example:

var numbers = [1, 3, 5, 10, 11];

var result = numbers.map((x, index) => x + (numbers[index + 1] || 0));

console.log(result);

|| is the logical OR operator, but it's a bit special in JavaScript: It evaluates its left-hand operand and, if that value is truthy¹, takes that value as its result; if the left-hand is falsy, the operator evalutes the right-hand operand and takes that value as its result.

In ES2020+ you can use the new nullish coalescing operator (??) instead of ||:

var result = numbers.map((x, index) =>  x + (numbers[index+1] ?? 0));

That will only use the 0 if numbers[index] is null or undefined, rather than if it's any falsy value.

¹ "truthy" and "falsy" - A truthy value is a value that evaluates to true when used as a condition (for instance, if (x) or x || y); a falsy value is a value that evalutes to false. The falsy values are 0, "", NaN, null, undefined, and of course, false. All other values are truthy. (Except there's a special case on browsers: document.all is falsy for legacy reasons.)

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