Javascript reduce object array

The reduce function reduces the array values into a single value. The reduce function never modified the original array. The reduce function operates the array values from left to right.

The resultant of reduce function will be stored in a variable called the accumulator. If suppose array can not contain any values then reduce function will not be executed. Real-Time Application: When we need to perform the constant operation on array values sequentially like addition, subtraction, multiplication, etc.

Observe below syntax carefully. We can see it in the above output. Reduce function cannot change the original array elements. This is a guide to reduce Function JavaScript. Here we discuss Syntax JavaScript logic, include, exclude with examples to implement with proper codes and examples.

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16.7: Array Functions: reduce() - Topics of JavaScript/ES6

Free Software Development Course. Login details for this Free course will be emailed to you. Email ID. Contact No.Instead of logging the sum, you could divide the sum by the length of the array before you return a final value. The way to do this is by taking advantage of the other arguments in the reduce method.

The first of those arguments is the index. Much like a for-loop, the index refers to the number of times the reducer has looped over the array. The last argument is the array itself. For example, you could double the total, or half each number before adding them together, or use an if statement inside the reducer to only add numbers that are greater than It will repeat the logic for each amount in the array and then return a single value.

You can reduce an array into a new array. For instance, lets reduce an array of amounts into another array where every amount is doubled. To do this we need to set the initial value for our accumulator to an empty array. The initial value is the value of the total parameter when the reduction starts. You set the initial value by adding a comma followed by your initial value inside the parentheses but after the curly braces bolded in the example below. In previous examples, the initial value was zero so I omitted it.

By omitting the initial value, the total will default to the first amount in the array. By setting the initial value to an empty array we can then push each amount into the total. Then we return the total when there are no more amounts to push. For these examples, it would make more sense to use map or filter because they are simpler to use.

The benefit of using reduce comes into play when you want to map and filter together and you have a lot of data to go over. If you chain map and filter together you are doing the work twice. You filter every single value and then you map the remaining values. With reduce you can filter and then map in a single pass.

javascript reduce object array

Use map and filter but when you start chaining lots of methods together you now know that it is faster to reduce the data instead. Use it when : You have a collection of items and you want to know how many of each item are in the collection.And I still need to google for specific ways to manipulate them almost every time. What if there was an ultimate guide that could always give you the answer? Quite literally. This object represents a car.

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There can be many types and colors of cars, each object then represents a specific car. Now, most of the time you get data like this from an external service. But sometimes you need to create objects and their arrays manually.

Like I did when I was creating this e-shop:. Arrays of objects don't stay the same all the time. We almost always need to manipulate them. So let's take a look at how we can add objects to an already existing array. To add an object in the middle, use Array.

This function is very handy as it can also remove items. Watch out for its parameters:. Let me ask you a question here: Why do you want to loop through an array of objects? The reason I'm asking is that the looping is almost never the primary cause of what we want to achieve. JavaScript provides many functions that can solve your problem without actually implementing the logic in a general cycle.

Let's take a look. The Array. If we want to get all red cars, we need to use Array. This is something we need very often. Transform an array of objects into an array of different objects. That's a job for Array.

Let's say we want to classify our cars into three groups based on their size. But what if we want the car object too? In that case we can enhance the object for a new property size.

This is a good use-case for the Array.Join Stack Overflow to learn, share knowledge, and build your career. Stack Overflow for Teams is a private, secure spot for you and your coworkers to find and share information.

javascript reduce object array

After the first iteration your're returning a number and then trying to get property x of it to add to the next object which is undefined and maths involving undefined results in NaN. The return value of each iteration of []. Clarification : I prefer my method over the other top answer in this thread as I disagree with the idea that passing an optional parameter to reduce with a magic number to get out a number primitive is cleaner. It may result in fewer lines written but imo it is less readable.

Using destructuring. The key to this is setting initial value.

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The return value becomes first parameter of the next iteration. The accepted answer proposes NOT passing the "optional" value. This is wrong, as the idiomatic way is that the second parameter always be included. Three reasons:. Dangerous -- Not passing in the initial value is dangerous and can create side-effects and mutations if the callback function is careless.

For the record, unless you intend to mutate the original object, set the first parameter of Object. Like this: Object. If you don't set the initial value, in many situations it might not be able to guess and you could end up with creepy runtime errors.

In the functional world, there is a standard on how you "fold" or reduce an array. When you fold or apply a catamorphism to the array, you take the values of that array to construct a new type. You need to communicate the resulting type--you should do this even if the final type is that of the values in the array, another array, or any other type.

Let's think about it another way. In JavaScript, functions can be pass around like data, this is how callbacks work, what is the result of the following code?

If initialItem is provided, the reduce function passes the initialItem as the accumulator and the first item of the array as the itemInArray.The reduce method executes a reducer function that you provide on each element of the array, resulting in single output value.

The source for this interactive example is stored in a GitHub repository. Your reducer function's returned value is assigned to the accumulator, whose value is remembered across each iteration throughout the array, and ultimately becomes the final, single resulting value. A function to execute on each element in the array except for the first, if no initialValue is supplied.

The reduce method executes the callback once for each assigned value present in the array, taking four arguments:.

javascript reduce object array

The first time the callback is called, accumulator and currentValue can be one of two values. If initialValue is provided in the call to reducethen accumulator will be equal to initialValueand currentValue will be equal to the first value in the array. If no initialValue is provided, then accumulator will be equal to the first value in the array, and currentValue will be equal to the second.

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Note: If initialValue is not provided, reduce will execute the callback function starting at index 1skipping the first index. If initialValue is provided, it will start at index 0. If the array is empty and no initialValue is provided, TypeError will be thrown. If the array only has one element regardless of position and no initialValue is provided, or if initialValue is provided but the array is empty, the solo value will be returned without calling callback.

It is almost always safer to provide an initialValuebecause there can be up to four possible output types without initialValueas shown in the following example:. The callback would be invoked four times, with the arguments and return values in each call being as follows:. The value returned by reduce would be that of the last callback invocation You can also provide an Arrow Function instead of a full function.

The code below will produce the same output as the code in the block above:. If you were to provide an initialValue as the second argument to reducethe result would look like this:.

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 supply an initialValueso that each item passes through your function. Note: If you are using an environment compatible with Set and Array. Using Array. If you like for loops, you can filter and map while traversing once with Array. Skip to main content Skip to search Array.

It takes four arguments: accumulator The accumulator accumulates callback 's return values. It is the accumulated value previously returned in the last invocation of the callback—or initialValueif it was supplied see below. Starts from index 0 if an initialValue is provided.

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reduce() Function JavaScript

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Convert 2D array to object using map or reduce in JavaScript

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Our flagship business publication has been defining and informing the senior-management agenda since 1964. Article - McKinsey Quarterly - October 2016 Chinese consumers: Revisiting our predictions By Yuval Atsmon and Max Magni Chinese consumers: Revisiting our predictions Article Actions Share this article on LinkedIn Share this article on Twitter Share this article on Facebook Email this article Download this article As their incomes rise, Chinese consumers are trading up and going beyond necessities.

In 2011, we tried our hand at predicting the ways in which, in the decade to come, Chinese consumers would change their preferences and behaviors. This article takes stock of those predictions. Why check in now. Another is a comprehensive new McKinsey survey, which follows nearly ten years of previous research that includes interviews with more than 60,000 people in upward of 60 cities in China. Deeper and more nuanced understanding of Chinese consumers can help reveal fresh opportunitiesfor new entrants and incumbents alikeand signal those areas where established players may need to be more wary.

While geographic differences persist, Chinese consumers are, on the whole, more individualistic, more willing to pay for nonnecessities and discretionary items, more brand loyal, and more willing to trade up to more expensive purchaseseven as their hallmark pragmatism endures. Just as it was then, generalizing about Chinese consumers continues to be almost as difficult (and maybe as foolish) as it is to generalize about European consumers.

We predicted these differences would remainand even grow more significant, especially in the consumption patterns and tastes that relate to discretionary items.

To help companies better tailor their go-to-market approach, we grouped most cities in China into clusters based on their similarities, including their geographic proximity and the transportation infrastructure that connects them. Furthermore, when our latest survey compared the consumers in the Shanghai area to those around Beijing and Hangzhou, certain spending attitudes also showed marked differences.

For example, brand loyalty increased much faster in Shanghai (24 percent increase in three years versus just 7 percent in Beijing and 9 percent in Hangzhou), as did the willingness to pay for better or healthier products.

Despite geographic differences, there are broad similarities among Chinese consumers. These mirror the general trends economists have found among consumers around the world as economies develop. The general tendency is for consumers, as they earn more, to spend a lower percentage of their income on food, a little more on healthcare, and even more on travel and transportation, as well as on recreational activities. It was no great stretch then, in our report five years ago, to predict a significant shift in consumption from necessities and seminecessities into discretionary categories.

Sure enough, our new survey shows Chinese consumers following the anticipated pattern. When we asked how they plan to increase spending as their income increases, dramatically fewer consumers said they will increase it on food (46 percent in the latest survey, compared to the 76 percent who said they would do so three years earlier).

Responses trended slightly up for healthcare products (from 16 percent to 17 percent), and increased for travel (from 14 percent to 23 percent) and leisure (from 17 percent to 25 percent). In our previous predictions, we also argued that as the income of Chinese consumers grew, they would aspire to improve their quality of life by not only spending more on discretionary items, but also by shifting their spending to more expensive items in the same categories.

In necessity categories such as food, for example, we predicted consumers would be willing to spend more for healthier versions of the same productsfor instance, that olive oil would grow much faster than less healthy (and less expensive) oils. In seminecessity categories like apparel, we predicted people would buy more special-occasion and premium brands.

We anticipated that the strongest beneficiaries of these changes would be in the more discretionary and aspirational categories, such as skincare and automotive. So what has happened so far. Premium categories have really accelerated. Comparing cosmetics purchases between 2011 and 2015, 44 percent of consumers have traded up their purchases, compared with 4 percent who traded down.


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