Executing a forEach loop beginning at a specific index in JavaScript

Question

Executing a forEach loop beginning at a specific index in JavaScript

I am dealing with a large array of items and need to create a paginated list out of them. I am trying to figure out how to initiate a loop, either using forEach or for, at a specific index in the array. For instance, if each page will contain a certain number of items from the list, how can I avoid iterating over the entire array in every loop?

arr.forEach(function (item) {
  someFn(item);
})


for (var i = 0, len = arr.length; i < len; i++) {
  someFn(arr[i]);
}

javascript arrays

Answer 1

Answer №1

If you want to make a copy of an array, you can use the Array#slice method.

The slice() method creates a new array by copying a section from an existing array based on the specified start and end indexes (end index not included). The original array remains unchanged.

array.slice(10, 20).forEach(someFn); // only works with functions that follow the forEach API*

* Check out the callback function parameters

You can also iterate over a specific range in an array by starting at a given index and ending at another.

for (var i = 10, len = Math.min(20, arr.length); i < len; i++) {
    someFn(arr[i]);
}

Using

Math.min(20, arr.length)

will return a value that is either the array length or 20, depending on which is smaller. For instance, if the array only goes up to index 14, it will return 15.

Answer 2

If you want to make a copy of an array, you can use the Array#slice method.

The slice() method creates a new array by copying a section from an existing array based on the specified start and end indexes (end index not included). The original array remains unchanged.

array.slice(10, 20).forEach(someFn); // only works with functions that follow the forEach API*

* Check out the callback function parameters

You can also iterate over a specific range in an array by starting at a given index and ending at another.

for (var i = 10, len = Math.min(20, arr.length); i < len; i++) {
    someFn(arr[i]);
}

Using

Math.min(20, arr.length)

will return a value that is either the array length or 20, depending on which is smaller. For instance, if the array only goes up to index 14, it will return 15.

Answer 3

Answer №2

Regrettably, when using Array#forEach, the function iterates through every element in the array. However, you have the option to add a simple condition that determines which elements (based on specified index) will receive the function.

i > 3 ? someFn(item) : null;
^ if the index is greater than 3 - execute the function

var arr = [1,2,3,4,5,6,7];

function someFn(elem){
  console.log(elem);
}

arr.forEach(function(item, i) {
  return i > 3 ? someFn(item) : null;
})

Answer 4

Regrettably, when using Array#forEach, the function iterates through every element in the array. However, you have the option to add a simple condition that determines which elements (based on specified index) will receive the function.

i > 3 ? someFn(item) : null;
^ if the index is greater than 3 - execute the function

var arr = [1,2,3,4,5,6,7];

function someFn(elem){
  console.log(elem);
}

arr.forEach(function(item, i) {
  return i > 3 ? someFn(item) : null;
})

Answer 5

Answer №3

forEach does not have that capability available. You have a few options to consider:

Using a simple for loop
Choosing to ignore indexes that you do not want to handle (refer to Kind user's answer)
Utilizing slice method (found in Nina's answer)
Creating your own function

Below is an example of #4 as an extension to the Array.prototype (non-enumerable to prevent breaking existing code). In case extending the Array.prototype is not suitable, there is also a standalone version provided:

// Defining the function
Object.defineProperty(Array.prototype, "myEach", {
  value: function(from, to, callback, thisArg) {
    if (typeof from === "function") {
      thisArg = callback;
      callback = to;
      to = from;
      from = 0;
    }
    if (typeof to === "function") {
      thisArg = callback;
      callback = to;
      to = this.length;
    }
    for (var n = from; n < to; ++n) {
      callback.call(thisArg, this[n], n, this);
    }
  }
});

// Implementation:
var arr = ["zero", "one", "two", "three", "four", "five", "six", "seven"];
console.log("*** From 3:");
arr.myEach(3, function(e) { console.log(e); });
console.log("*** From 3 (inclusive) to 5 (exclusive):");
arr.myEach(3, 5, function(e) { console.log(e); });
console.log("*** All:");
arr.myEach(function(e) { console.log(e); });
console.log("*** Check thisArg handling on 0-2:");
var o = {answer: 42};
arr.myEach(0, 2, function(e) {
  console.log(e + " (this.answer = " + this.answer + ")");
}, o);

.as-console-wrapper {
  max-height: 100% !important;
}

Please note that this is a non-enumerable property, which is crucial when extending Array.prototype to avoid breaking existing code.

If you are creating a library to be used by others, it would be better to offer a standalone function instead:

// Defining the function
function myEach(array, from, to, callback, thisArg) {
  if (typeof from === "function") {
    thisArg = callback;
    callback = to;
    to = from;
    from = 0;
  }
  if (typeof to === "function") {
    thisArg = callback;
    callback = to;
    to = array.length;
  }
  for (var n = from; n < to; ++n) {
    callback.call(thisArg, array[n], n, array);
  }
}

// Usage:
var arr = ["zero", "one", "two", "three", "four", "five", "six", "seven"];
console.log("*** From 3:");
myEach(arr, 3, function(e) {
  console.log(e);
});
console.log("*** From 3 (inclusive) to 5 (exclusive):");
myEach(arr, 3, 5, function(e) {
  console.log(e);
});
console.log("*** All:");
myEach(arr, function(e) {
  console.log(e);
});
console.log("*** Check thisArg handling on 0-2:");
var o = {answer: 42};
myEach(arr, 0, 2, function(e) {
  console.log(e + " (this.answer = " + this.answer + ")");
}, o);

.as-console-wrapper {
  max-height: 100% !important;
}

Answer 6

forEach does not have that capability available. You have a few options to consider:

Using a simple for loop
Choosing to ignore indexes that you do not want to handle (refer to Kind user's answer)
Utilizing slice method (found in Nina's answer)
Creating your own function

Below is an example of #4 as an extension to the Array.prototype (non-enumerable to prevent breaking existing code). In case extending the Array.prototype is not suitable, there is also a standalone version provided:

// Defining the function
Object.defineProperty(Array.prototype, "myEach", {
  value: function(from, to, callback, thisArg) {
    if (typeof from === "function") {
      thisArg = callback;
      callback = to;
      to = from;
      from = 0;
    }
    if (typeof to === "function") {
      thisArg = callback;
      callback = to;
      to = this.length;
    }
    for (var n = from; n < to; ++n) {
      callback.call(thisArg, this[n], n, this);
    }
  }
});

// Implementation:
var arr = ["zero", "one", "two", "three", "four", "five", "six", "seven"];
console.log("*** From 3:");
arr.myEach(3, function(e) { console.log(e); });
console.log("*** From 3 (inclusive) to 5 (exclusive):");
arr.myEach(3, 5, function(e) { console.log(e); });
console.log("*** All:");
arr.myEach(function(e) { console.log(e); });
console.log("*** Check thisArg handling on 0-2:");
var o = {answer: 42};
arr.myEach(0, 2, function(e) {
  console.log(e + " (this.answer = " + this.answer + ")");
}, o);

.as-console-wrapper {
  max-height: 100% !important;
}

Please note that this is a non-enumerable property, which is crucial when extending Array.prototype to avoid breaking existing code.

If you are creating a library to be used by others, it would be better to offer a standalone function instead:

// Defining the function
function myEach(array, from, to, callback, thisArg) {
  if (typeof from === "function") {
    thisArg = callback;
    callback = to;
    to = from;
    from = 0;
  }
  if (typeof to === "function") {
    thisArg = callback;
    callback = to;
    to = array.length;
  }
  for (var n = from; n < to; ++n) {
    callback.call(thisArg, array[n], n, array);
  }
}

// Usage:
var arr = ["zero", "one", "two", "three", "four", "five", "six", "seven"];
console.log("*** From 3:");
myEach(arr, 3, function(e) {
  console.log(e);
});
console.log("*** From 3 (inclusive) to 5 (exclusive):");
myEach(arr, 3, 5, function(e) {
  console.log(e);
});
console.log("*** All:");
myEach(arr, function(e) {
  console.log(e);
});
console.log("*** Check thisArg handling on 0-2:");
var o = {answer: 42};
myEach(arr, 0, 2, function(e) {
  console.log(e + " (this.answer = " + this.answer + ")");
}, o);

.as-console-wrapper {
  max-height: 100% !important;
}

Answer 7

Answer №4

Considering the input from @NinaScholz, it might be worthwhile to utilize variables for any modifications rather than altering the loop itself.

function addToArray(item, newArray){
  newArray.push(item);
}

var originalArray = [1,2,3,4,5,6,7,8,9,10];
var modifiedArray = [];

var startIdx = 1;
var endIdx = 5;
var idx = 0;

for (idx = startIdx; idx < endIndex; idx++){
     addToArray(originalArray[idx], modifiedArray);
}

alert(modifiedArray.join(' '));

Answer 8

Considering the input from @NinaScholz, it might be worthwhile to utilize variables for any modifications rather than altering the loop itself.

function addToArray(item, newArray){
  newArray.push(item);
}

var originalArray = [1,2,3,4,5,6,7,8,9,10];
var modifiedArray = [];

var startIdx = 1;
var endIdx = 5;
var idx = 0;

for (idx = startIdx; idx < endIndex; idx++){
     addToArray(originalArray[idx], modifiedArray);
}

alert(modifiedArray.join(' '));

Answer 9

Answer №5

To incorporate the iterator design pattern, you can create a custom Iterator object with functions to iterate over a list and maintain the state of the last iteration.

var CustomIterator = function (list, position) {
  return {
    hasNext: function () {
      return position + 1 < list.length;
    },
    isDefined: function () {
      return (position < list.length && position >= 0);
    },
    getElement: function () {
      return list[position];
    },
    getPosition: function () {
      return position;
    },
    moveNext: function () {
      if (this.hasNext()) { return CustomIterator(list, position + 1); }
      return CustomIterator([], 0);
    }      
}

CustomIterator.forEach = function (action, iterator, length) {
  var counter = 0;
  while (counter < length && iterator.isDefined()) {
    counter++;
    action(iterator.getElement(), iterator.getPosition());
    iterator = iterator.moveNext();
  }

  return iterator;
}

Create an instance of the CustomIterator to traverse through the list and store the last iteration state.

var list = [2, 4, 6, 8, 10];
var customIterator = CustomIterator(list, 0);

customIterator = CustomIterator.forEach(function (element, index) {
  console.log(element, index);
}, customIterator, 3);

//2 0
//4 1
//6 2

CustomIterator.forEach(function (element, index) {
   console.log(element, index);              
}, customIterator, 5);

//8 3
//10 4

Answer 10

To incorporate the iterator design pattern, you can create a custom Iterator object with functions to iterate over a list and maintain the state of the last iteration.

var CustomIterator = function (list, position) {
  return {
    hasNext: function () {
      return position + 1 < list.length;
    },
    isDefined: function () {
      return (position < list.length && position >= 0);
    },
    getElement: function () {
      return list[position];
    },
    getPosition: function () {
      return position;
    },
    moveNext: function () {
      if (this.hasNext()) { return CustomIterator(list, position + 1); }
      return CustomIterator([], 0);
    }      
}

CustomIterator.forEach = function (action, iterator, length) {
  var counter = 0;
  while (counter < length && iterator.isDefined()) {
    counter++;
    action(iterator.getElement(), iterator.getPosition());
    iterator = iterator.moveNext();
  }

  return iterator;
}

Create an instance of the CustomIterator to traverse through the list and store the last iteration state.

var list = [2, 4, 6, 8, 10];
var customIterator = CustomIterator(list, 0);

customIterator = CustomIterator.forEach(function (element, index) {
  console.log(element, index);
}, customIterator, 3);

//2 0
//4 1
//6 2

CustomIterator.forEach(function (element, index) {
   console.log(element, index);              
}, customIterator, 5);

//8 3
//10 4

Answer 11

Answer №6

array.values() function provides an iterator that can be used with .next().

let array = [1, 2, 3, 4]

var _;
for (let value of (_ = array.values(), _.next(), _)) {
    console.log(value)
}

Answer 12

array.values() function provides an iterator that can be used with .next().

let array = [1, 2, 3, 4]

var _;
for (let value of (_ = array.values(), _.next(), _)) {
    console.log(value)
}

Executing a forEach loop beginning at a specific index in JavaScript

Answer №1

Answer №2

Answer №3

Answer №4

Answer №5

Answer №6

Similar questions

How to split a string by multiple delimiters in PHP and store in a multi-dimensional array

I am encountering an issue with my map function on my array component in React. It seems to be undefined when I attempt to declare it

ERROR702: The requested action cannot be completed as the object does not have support for the specified property or method 'attr'

Opencart: The Key to Your Website's Success

Combine a specific number of elements in an array using JavaScript's comma-separated join without relying on jQuery

Having trouble with getStaticProps serialization error in Next.js?

What is the most common method for creating a dropdown menu for a website's navigation?

Using JavaScript/Angular to borrow a function and add additional parameters

Triggering target selection based on the href attribute consistently executing

Injecting unpredictable values to individual numbers in a vertical list within a spreadsheet

The "as" property in NextJS Link does not properly reload the page when opened

Automatically scroll to each div or element on the page

Look for a specific value within an array and delete the elements that come before and after it

The state is not being processed properly

Exploring characteristics within a Three.Js environment in a React application

Problem with Next.js router language settings

The ViewChild from NgbModalModule in @ng-bootstrap/ng-bootstrap for Angular 6 is causing the modal to return as

Expectable JavaScript array shuffling

Managing shared states across different components in React

Receiving a NoSessionIdError issue when using Webdriver.io