Javascript map Array/Method смотреть онлайн

Javascript map Array/Method , I am shanjai raj and we take a look at the array.map method or the map array works in javascript es6. Difference between map vs forEach method in javascript as well. This is a tutorial for beginners in Javascript and also can be applied as a beginner based tutorial. This shows how the high order function .map() works. This only covers the basics of how the map array or how the map method works in short, so if you want more videos, then let me know by leaving a comment down below?.

How does the map method work -

map calls a provided callback function once for each element in an array, in order, and constructs a new array from the results. callback 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:

1.indexes that have never been set;
2.which have been deleted; or
3.which have never been assigned a value.

When not to use map() -
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?:

1.you're not using the array it returns; and/or
2.you're not returning a value from the callback.

Parameters in Detail

callback 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 callback 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 callback, if invoked, may do so).

The range of elements processed by map is set before the first invocation of callback. Elements which are appended to the array after the call to map begins will not be visited by callback. If existing elements of the array are changed, their value as passed to callback will be the value at the time map visits them. Elements that are deleted after the call to map begins and before being visited are not visited.

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.

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