In this article, you will go over the Map and Set objects, what makes them similar or different to Objects and Arr someArray.map(obj => { //this is a code block, not an object definition } ) the curly braces enclose a code block, not an object literal. JavaScript Converting array of objects into object of arrays; ... How to convert array into array of objects using map() and reduce() in JavaScript. You can use for...of loop to iterate Map in the insertion order, meaning what you first inserted will be the first element to show up in the loop and so on. Make sure you are not still returning langSel, as this is a java Map and will give that output, you should be returning the json object – cowls Dec 17 '12 at 12:06 Thanks cowls, Its working fine. clear() – removes all elements from the map object. Javascript Web Development Front End Technology Object Oriented Programming. The same code works in Javascript as this uses Latest Es6 classes and syntax. delete(key) – removes an element specified by the key. Acutually, I created a JSONObject by new JSONObject() function and then added each map entry to jsonObj by converting the second string to Boolean. Just as Array have length property, Map comes with a size property. When we want to save the contents of a JavaScript Map structure in a file, we must transform it into a string. Map and object: converting nested objects. Whenever a Map only has strings as keys, you can convert it to JSON by encoding it as an object. Introduced in ECMAScript 2015, Maps in JavaScript are ordered collections of key/value pairs, and Sets are collections of unique values. In contrast; obj[key] = value is simply modifying the reducer sum value, rather than recreating it on every loop. entries() – returns a new Iterator object that contains an array of [key, value] for each element in the map object. Convert 2D array to object using map or reduce in JavaScript Javascript Web Development Object Oriented Programming Let’s say, we have a two-dimensional array that contains some data about the age of some people. As such: this.operations.map… If you want an arrow function to return an object, JS requires you to wrap the object literal with parentheses in order to correctly parse your intent. The result is not always what you expect: 5 + null // returns 5 because null is converted to 0 Example 1: This example uses array.from() method to get the keys of the Map in the array. The ES6 spread operator is indeed faster than Array.from(), but is still slower than the for of loop.. TL:DR; crude results below at 100,000 iterations of a 15 element map. Doing a spread operator in the loop {...main, [key]: value} is as inefficient as Object.assign() in the OP. When JavaScript tries to operate on a "wrong" data type, it will try to convert the value to a "right" type. 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