javascript reduce method want to add up values by having a condition statement

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I am trying to sum up values by using reduce method, since I am new to JavaScript I need some help with condition statement when adding app all the values, I have a selector method where I select days and then there are movement on each days, I want to add all those movements but not want to include first and last day movements when I calculate sum by using reduce method.

export const getTotalMoveCount = createSelector(
  dailyMoves => dailyMoves.reduce((sum, day) => sum + day.get(MoveTypes.movesTotal), 0)

Each day has totalmoves, and with this method I want to count total movecounts by adding up all days moves, for example if i select days from 10jan to 16jan, then it will add all the movements from 11jan to 15 jan, I want that there will be some condition statement which ignores first and last day of selection, thanks for the help.

Exclude the first and last items using slice() and then reduce() it:

const result = [5, 10, 2, 3, 4].slice(1, -1)
                .reduce((accumulator, currentValue, currentIndex, array) => accumulator + currentValue);


A Guide To The Reduce Method In Javascript​, JavaScript's reduce method is one of the cornerstones of functional programming​. Use it when: You have an array of amounts and you want to add them all up. When the loop starts the total value is the number on the far left (29.76) and the I'll use ES6 syntax for the rest of the examples, since it's more  JavaScript’s reduce method is one of the cornerstones of functional programming. Let’s explore how it works, when you should use it, and some of the cool things it can do. A Basic Reduction. Use it when: You have an array of amounts and you want to add them all up.

Why not just ignore the first and last values of the index then, which is the 3rd parameter of the reduce function?

export const getTotalMoveCount = createSelector(
  (dailyMoves) => {
    return dailyMoves.reduce((sum, day, index) => {
      if (index !== 0 && index !== dailyMoves.length - 1) {
        return sum + day.get(MoveTypes.movesTotal);

      return sum;
    }, 0);

Array.prototype.reduce(), The reduce() method executes a reducer function (that you provide) on Syntax. arr .reduce( callback ( accumulator , currentValue [, index [ Caution: If you need to support truly obsolete JavaScript engines that do not support Object. To sum up, the values contained in an array of objects, you must  The reduce() method reduces the array to a single value. The reduce method executes a provided function for each value of the array (from left-to-right). The return value of the function is stored in an accumulator (result/total). Note: reduce does not execute the function for array elements without values.

While I am not getting into your app details. This is the way to ignore first & last values:

const x = [0, 1, 2, 3, 4].reduce(function(accumulator, currentValue, currentIndex, array) {
  if (currentIndex == 0 || currentIndex == [0, 1, 2, 3, 4].length-1)
    return accumulator
  return accumulator + currentValue;

alert(x) // console.log(x)

Learn & Understand JavaScript's Reduce Function, The reduce() method is used to apply a function to each element in the array to reduce the array to a single value. Here's what the syntax looks like: We want to find the sum of all the values in our array. Using the reduce() method, how would you sum up the population of every country except China? To use the reduce() function, we’ll start with the same simple array of numbers: let arr = [1,2,3,4]; arr is the array we’re going to reduce. We want to find the sum of all the values in our array. To do this, each iteration we will add the current value to our accumulator and return it. This returned value will then be our new accumulator.

Here is a simple and clear code:

const sum = (arr) => {
    return arr.reduce((a, b) => a + b);

console.log(sum([5, 10, 2, 3, 4]));

Understanding Reduce in JavaScript, Reducing an array is a helpful functional programming technique to use when you need to reduce multiple values into a single value. This structure easily captures categories that ended up having a total of 0 at the end which peer group someone was in based on conditional logic related to their age. You need to return the current sum and not undefined if your condition is not matched. Otherwise you aren't returning your accumulator to the reduce function and end up adding unefined + record.value when matched or undefined + undefined when it is not matched. Try this:

Iterating Over and Reducing Data, forEach provides for a basic way to loop through our data set. where we want to coerce our data into the form we want - but it comes together with object in the callback. lodash's clone would be another approach to getting a copy of for me to remember, so let's go over it with the classic example: summing up a value. Jan 29, 2018 · 7 min read. If you’re starting in JavaScript, maybe you haven’t heard of .map (), .reduce (), and .filter (). For me, it took a while as I had to support Internet Explorer 8

JavaScript Array reduce() Method, The return value of the function is stored in an accumulator (result/total). Syntax​. array.reduce(function(total, currentValue, currentIndex, arr), initialValue)  Look at the code above, we have: 1 if/else statement that filter out invalid condition; 3 levels of nested if statement (condition 1, 2 & 3) A general rule I personally follow is return early when invalid conditions found.

Rewriting JavaScript: Sum an Array - Chris Burgin, We want to make sure that numbers is a constant; it's not going to change. This function simply takes two numbers and adds them together. Because we give reduce the function add , the values accumulator and down step by step for each value in our array as it is processed by the reduce statement. Using reduce() we check to see if the current Y value of the array is less than the minimum value stored in the reduce() function. If it is, we return the current Y value using p.y. If it’s not, the saved value of min is returned. Finally, we have an initial value (for the first iteration of the function) set to the first Y in the data set.

  • You can use slice: [1,2,3,4,5].slice(1,-1)
  • if (index !== 0 && index !== dailyMoves.length - 1), it ignores the moves from sum when index !==0 but does not ignore when other condition I mean this seems index !== dailyMoves.length - 1 not working,
  • OP asked to ignore the first and last items of the array in the count. The above code does that.