The range of elements processed by map is set before the first invocation of callback. Examples Mapping an array of numbers to an array of square roots The following code takes an array of numbers and creates a new array containing the square roots of the numbers in the first array. Certain functions are also commonly used with one argument, even though they take additional optional arguments. If IsCallable callback is false, throw a TypeError exception. Any value both objects and may be used as either a key or a value. The source for this interactive example is stored in a GitHub repository. Elements that are deleted after the call to map begins and before being visited are not visited.
If existing elements of the array are changed, their value as passed to callback will be the value at the time map visits them. Description map calls a provided callback function once for each element in an array, in order, and constructs a new array from the results. You can work around this by inserting the following code at the beginning of your scripts, allowing use of map in implementations which do not natively support it. . Returns the value associated to the key, or undefined if there is none. Calls callbackFn once for each key-value pair present in the Map object, in insertion order. If a thisArg parameter is provided to forEach, it will be used as the this value for each callback.
The this value ultimately observable by callback is determined according to. Let Pk be ToString k. These habits may lead to confusing behaviors. The constructor function that is used to create derived objects. Returns a new Iterator object that contains the values for each element in the Map object in insertion order. Objects and maps compared are similar to Maps in that both let you set keys to values, retrieve those values, delete keys, and detect whether something is stored at a key. Returns a new Iterator object that contains the keys for each element in the Map object in insertion order.
Sets the value for the key in the Map object. To count how many elements are in a Map, use. Each key-value pair is added to the new Map; null values are treated as undefined. The Map object holds key-value pairs and remembers the original insertion order of the keys. Returns true if an element in the Map object existed and has been removed, or false if the element does not exist.
Even though every NaN is not equal to itself NaN! Tricky use case It is common to use the callback with one argument the element being traversed. If you'd like to contribute to the interactive examples project, please clone and send us a pull request. Return value A new array with each element being the result of the callback function. Elements which are appended to the array after the call to map begins will not be visited by callback. If a thisArg parameter is provided to map, it will be used as callback's this value. The last repeated key wins.
If thisArg was supplied, let T be thisArg; else let T be undefined. It is not called for missing elements of the array that is, indexes that have never been set, which have been deleted or which have never been assigned a value. Otherwise, the value will be used as its this value. User must explicitly enable this feature. Allows the addition of properties to all Map objects. This is the function by default. The argument will automatically be assigned from each element of the array as map loops through the original array.
Returns a boolean asserting whether a value has been associated to the key in the Map object or not. Let len be ToUint32 lenValue. Due to the algorithm defined in the specification if the array which map was called upon is sparse, resulting array will also be sparse keeping same indices blank. The last repeated key wins. User must explicitly enable this feature. Represents the prototype for the Map constructor.
Since map builds a new array, using it when you aren't using the returned array is an anti-pattern; use or instead. Thus, when iterating over it, a Map object returns keys in order of insertion. Map instances All Map instances inherit from. . . . .