# Questions about callback function parameters in the map method of array

Source: Internet
Author: User

To get to the point, let's look at two examples first.

var arr=[' 1 ', ' 4 ', ' 9 ', ' 16 '];

var r=arr.map (MATH.SQRT);

Guess what the result of R would be?

That's right.

[1,2,3,4]

Let's try another one.

var arr=[' 1 ', ' 4 ', ' 9 ', ' 16 '];

var r=arr.map (parseint);

And guess what the result is?

is [1,2,3,4]?

Console try to see what the result is.

[1,nan,nan,1]!

Not surprised.

Actually, the truth ————

That's the number of arguments!

Let's look at one more example

var arr=[' 1 ', ' 4 ', ' 9 ', ' 16 '];

var r=arr.map (function (x) {return parseint (x)});

r//[1,2,3,4]

The result is not normal!

Actually in the call

Arr.map (parseint);

There are three parameters passed to map

Current value (Currentvaluve)//' 1 ', ' 4 ', ' 9 ', ' 16 '

Index of current value (CURRENTINDEX)//0,1,2,3

and the current array (currentarray)//[' 1 ', ' 4 ', ' 9 ', ' 16 '], each time this

Each time the parseint () function is used, only two values (Currentvalue,currentindex) are passed in.

So the result is:

parseint (' 1 ', 0)//1

The second parameter, if passed after the number function is converted to 0 or `NaN` , is ignored. --mozilla Official documents

parseint (' 4 ', 1)//illegal, NaN

parseint (' 9 ', 2)//illegal, NaN

parseint (' 16 ', 3)//here refers to the note, when parsing the string ' 16 ', found that 6 is greater than or equal to 3, so the subsequent numbers are ignored, leaving only one 1 returned

If `parseInt` a character is encountered that is not part of the `radix` cardinality specified by the parameter, then both the character and the character after it are ignored. --mozilla Official documents

When we finally know the truth, we must pay attention to the problem of the number of callback parameters in the map function!

var r=arr.map (function (x) {return parseint (x)});

Questions about callback function parameters in the map method of array

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