During iteration of array, each key is typed as [string,string]. In TypeScript, I was using the Array.prototype.reduce() method to iterate on an array and populate an object based on an asynchronous function, like this: Typescript is super set of javascript with additional features type assertions. We will also discuss how to iterate over Map entries, Array map, clone and merge maps, merge map with an array, Convert Map Keys/Values to an Array, Weak Map… (I thought it was, but I was wrong.). accumulator; currentValue; currentIndex; array; The first time the callback is called, accumulator and currentValue can be one of two values. This typescript tutorial explains TypeScript Map, how we can create a map in typescript, various map properties and methods. I dedicate this article only for these methods because, in Pure Functional Programming , this kind of method is required to perform some operations on an Array. The filter().map() is easier to read. In the previous article, I introduced the main players or RxJS, and now let’s start getting familiar with RxJS operators.In this article well use the operators map(), filter(), and reduce(). The difference here is that reduce only walks the array a single time, whereas the filter and map combo walks the array two times. Key equality This typescript tutorial explains TypeScript Map, how we can create a map in typescript, various map properties and methods. map, filter, reduce, find. For 5 entries, this isn't a big deal. The flatMap() method returns a new array formed by applying a given callback function to each element of the array, and then flattening the result by one level. As the data elements flow from the observable to the observer, you can apply one or more operators, transforming each element prior to supplying it to the observer. The syntax for the reduce array method in JavaScript is: let newArray = arr.filter(callback(currentValue, accumulatedValue) { // return the accumulated value, given the current and previous accumulated value }, initialValue[, thisArg]); map creates a new array by transforming every element in an array, individually. The reduce() method executes the callback once for each assigned value present in the array, taking four arguments:. TypeScript: adjusting types in reduce function with an async callback # typescript # reduce # async # promises. map() is faster than forEach when changing or altering data. I made my code harder to read, for no gain at all. In this case numeros is an array…and you cannot multiply an array as a whole by 3… you can concactenate the array… then loop through and multiply by 3. or you could do a double map…that is do a map on numeros too Those are 3 really powerful array functions: map returns an array with the same length, filter as the name implies, it returns an array with less items than the original array; reduce returns a single value (or object) find returns the first items in an array that satisfies a condition It is identical to a map() followed by a flat() of depth 1, but slightly more efficient than calling those two methods separately. Generally speaking, I turn to the map function if I need to transform some Array of one object type into another Array of a different object type. finally add the array into map While map() returns an entirely new Array - thus leaving the original array unchanged. You can also chaining on other cool methods like ( map(), filter(), reduce(), etc.) Key equality. 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