The map() method createsa new array populated with the results of calling a provided function on every element in the calling array.
Function that is called for every element of arr. Each time callbackFn executes, the returned value is added to newArray.
The function is called with the following arguments:. The current element being processed in the array.
The index of the current element being processed in the array.
The array map was called upon.
Value to use as this when executing callbackFn.
A new array with each element being the result of the callback function. map calls a provided callbackFn function once for each element in an array, in order, and constructs a new array from the results.
callbackFn is invoked only for indexes of the array which have assigned values (including undefined).
It is not called for missing elements of the array; that is:. indexes that have never been set;.
indexes which have been deleted.
Since map builds a new array, using it when you aren't using the returned array is an anti-pattern; use forEach or for..of instead.
You shouldn't be using map if:. you're not using the array it returns; and/or. you're not returning a value from the callback.
callbackFn is invoked with three arguments: the value of the element, the index of the element, and the array object being mapped.
If a thisArg parameter is provided, it will be used as callback's this value.
Otherwise, the value undefined will be used as its this value.
The this value ultimately observable by callbackFn is determined according to the usual rules for determining the this seen by a function.
map does not mutate the array on which it is called (although callbackFn, if invoked, may do so).
The range of elements processed by map is set before the first invocation of callbackFn.
Elements which are assigned to indexes already visited, or to indexes outside the range, will not be visited by callbackFn.
If existing elements of the array are changed after the call to map, their value will be the value at the time callbackFn visits them.
Elements that are deleted after the call to map begins and before being visited are not visited.
Warning: Concurrent modification of the kind described in the previous paragraph frequently leads to hard-to-understand code and is generally to be avoided (except in special cases).
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 following code takes an array of numbers and creates a new array containing the square roots of the numbers in the first array.
The following code takes an array of objects and creates a new array containing the newly reformatted objects.
The following code shows how map works when a function requiring one argument is used with it.
The argument will automatically be assigned from each element of the array as map loops through the original array.
This example shows how to use map on a String to get an array of bytes in the ASCII encoding representing the character values:.
This example shows how to iterate through a collection of objects collected by querySelectorAll.
This is because querySelectorAll returns a NodeList (which is a collection of objects). In this case, we return all the selected options' values on the screen:.
An easier way would be the Array.from() method. (inspired by this blog post). It is common to use the callback with one argument (the element being traversed). Certain functions are also commonly used with one argument, even though they take additional optional arguments.
These habits may lead to confusing behaviors. While one might expect [1, 2, 3], the actual result is [1, NaN, NaN].
parseInt is often used with one argument, but takes two. The first is an expression and the second is the radix to the callback function, Array.prototype.map passes 3 arguments:.
The third argument is ignored by parseInt—but not the second one!
This is the source of possible confusion.
Here is a concise example of the iteration steps:.
Then let's talk about solutions.
One alternative output of the map method being called with parseInt as a parameter runs as follows:.
When undefined or nothing is returned:. BCD tables only load in the browser. Full project: CodePen. If you are gonna build a web app, you will probably have to display some data of the user.To make it easier for users to access the relevant data, we can sort and filter their data.In this article, I will show you how you can do this with Vue.
So let’s say you are trying to make a web app to display a bunch of different recipes.These recipes all have different properties.For a recipe, it’s important to know what ingredients you will need and how much time you need to cook.To improve the user experience, we will add some filters for these values and we’ll add a search function to look for a specific recipe.
First, we’ll need to create an array of recipes.So let’s add the following four recipes to our data. Every object in this array has the following variables:.
ingredients (array). In the CodePen I added some HTML and CSS to display our array of objects as cards.
To loop over the information in our array, we use v-for. The HTML elements we will need for sorting, filtering, and searching in our array is the following:.