JavaScript Array: Get "Range" of Items

JavaScript Array: get range of items

Use the array.slice(begin [, end]) function.

var a = ['a','b','c','d','e','f','g'];
var sliced = a.slice(0, 3); //will contain ['a', 'b', 'c']

The last index is non-inclusive; to mimic ruby's behavior you have to increment the end value. So I guess slice behaves more like a[m...n] in ruby.

return an index range of elements in an array

You can use slice

var array = ["1", "2", "3"]
let indexRange = (arr, start, end) => {  return arr.slice(start, end)}console.log(indexRange(array, 0, 3));

Does JavaScript have a method like range() to generate a range within the supplied bounds?

It works for characters and numbers, going forwards or backwards with an optional step.

var range = function(start, end, step) {
    var range = [];
    var typeofStart = typeof start;
    var typeofEnd = typeof end;

    if (step === 0) {
        throw TypeError("Step cannot be zero.");
    }

    if (typeofStart == "undefined" || typeofEnd == "undefined") {
        throw TypeError("Must pass start and end arguments.");
    } else if (typeofStart != typeofEnd) {
        throw TypeError("Start and end arguments must be of same type.");
    }

    typeof step == "undefined" && (step = 1);

    if (end < start) {
        step = -step;
    }

    if (typeofStart == "number") {

        while (step > 0 ? end >= start : end <= start) {
            range.push(start);
            start += step;
        }

    } else if (typeofStart == "string") {

        if (start.length != 1 || end.length != 1) {
            throw TypeError("Only strings with one character are supported.");
        }

        start = start.charCodeAt(0);
        end = end.charCodeAt(0);

        while (step > 0 ? end >= start : end <= start) {
            range.push(String.fromCharCode(start));
            start += step;
        }

    } else {
        throw TypeError("Only string and number types are supported");
    }

    return range;

}

jsFiddle.

If augmenting native types is your thing, then assign it to Array.range.

var range = function(start, end, step) {    var range = [];    var typeofStart = typeof start;    var typeofEnd = typeof end;
    if (step === 0) {        throw TypeError("Step cannot be zero.");    }
    if (typeofStart == "undefined" || typeofEnd == "undefined") {        throw TypeError("Must pass start and end arguments.");    } else if (typeofStart != typeofEnd) {        throw TypeError("Start and end arguments must be of same type.");    }
    typeof step == "undefined" && (step = 1);
    if (end < start) {        step = -step;    }
    if (typeofStart == "number") {
        while (step > 0 ? end >= start : end <= start) {            range.push(start);            start += step;        }
    } else if (typeofStart == "string") {
        if (start.length != 1 || end.length != 1) {            throw TypeError("Only strings with one character are supported.");        }
        start = start.charCodeAt(0);        end = end.charCodeAt(0);
        while (step > 0 ? end >= start : end <= start) {            range.push(String.fromCharCode(start));            start += step;        }
    } else {        throw TypeError("Only string and number types are supported");    }
    return range;
}
console.log(range("A", "Z", 1));console.log(range("Z", "A", 1));console.log(range("A", "Z", 3));

console.log(range(0, 25, 1));
console.log(range(0, 25, 5));console.log(range(20, 5, 5));

How to get 'range' of items in descending order?

Use .reverse:

var myArray = [{ x: 1 }, { x: 2 }, { x: 3 }, { x: 4 }];
myArray = myArray.slice(2,4).reverse();
console.log(myArray);

Get range of numbers from the sorted array

This is a perfect use-case of for...of loop.

const array = [100, 400, 700, 1000, 1300, 1600];

function foobar(array,min,max) {
    let new_array = [];
    for (let val of array) {
        if(min<=val && val<=max) {
            new_array.push(val);
        } else if(val>max) {
            new_array.push(val);
            break;
        }

    }
    return new_array;
}

console.log(foobar(array,0,15));     // outputs [100]
console.log(foobar(array,250,1010)); // outputs [400, 700, 1000, 1300]

It's simple, follows conventional programming paradigm and it traverses the array only once.

How to return a specific range of indexes from within in a JSON Array?

If you can use ES2019+:

const getPastCasesByCountry = (response, country, pastDays) => Object.fromEntries(
  Object.entries(response[country].dates)
    .sort(([date1], [date2]) => date1.localeCompare(date2))
    .slice(-pastDays)
);

const last7DaysAN = getPastCasesByCountry(response, 'AN', 7);

This uses

the handy Object.entries + Object.fromEntries combo which gives you the ability to work with an object essentialy the same way as with an array
the fact that Array.slice can take a negative start index, which is an offset from the end – as we needed here

Convert to data set

To convert to DataSet as described in your question, I'd change the function a bit:

const getPastCasesByCountry = (response, country, pastDays) => 
  Object.entries(response[country].dates)
    .sort(([date1], [date2]) => date1.localeCompare(date2))
    .slice(-pastDays)

const DataSet = (casesByDate) => casesByDate.map(([dateString, cases]) => ({
  x: new Date(dateString),
  y: cases.total.confirmed
}))

const data = DataSet(getPastCasesByCountry(response, 'AN', 7));

JavaScript Array: Get "Range" of Items