Start from second item in forEach loop

Question

I need to start from second item in array. In order to save correct context I need to use forEach instead of simple for loop.

I've done it in next way :

private convertToRanges(arr: []): any[] {
        const inputArr = arr.slice(),

        if (inputArr.length > 1) {
            let lastIndex = 0;
            inputArr.shift();
            inputArr.forEach(item => {
                ...
            });
        }
        ...
    }

I make copy and remove first item in copy.

Is there another way to start from second item and be context sure?

Answer 1

You can't tell forEach where to start, no, but you can ignore the calls you don't want:

inputArr.forEach((value, index) => {
    if (index < 1) return;
    // Code from here onward will only run for entries that aren't
    // the first entry
});

Or if you're not worried about copying most of the array, you can always use slice:

inputArr.slice(1).forEach(value => {
    // ...
});

You could also define your own forEach-style function accepting a starting index, if you liked, making it non-enumerable and choosing the name carefully to avoid conflicts.

It's also perhaps worth noting that since you're using ES2015, some of the reasons for using forEach go away a bit thanks to block scope. for is still a bit more verbose than forEach with an arrow function, but lets you start and end and increment by whatever you like:

for (let i = 1; i < inputArr.length; ++i) {
    // ...`i` and also any `let` or `const` here are scoped to
    // this specific loop iteration...
}

The part about i above is primarily a good thing, but also has a slight performance impact (at least for now) since a new i has to be created for each iteration. Not that that performance impact usually matters at all, though, and it won't be as big as calling a function a'la forEach.

Gratuitous example of the above:

const inputArr = ['a', 'b', 'c', 'd'];
for (let i = 1; i < inputArr.length; ++i) {
  // A closure to emphasize that each `i` is distinct
  setTimeout(() => {
    console.log("inputArr[" + i + "] = " + inputArr[i]);
  }, 0);
}

(Normally I'd use a template literal there, but wanted to avoid giving the impression the i behavior related to that.)