JS interview collection test question 1

// Find the output 
console.log(['1','2','3'].map(parseInt))

 JavaScript map() Function USES

One 、 Concept

map() Method defined in JavaScript Of Array in , It returns a new array , The elements in the array are the values processed by the original array call function .
Be careful ：

• map() Empty arrays are not detected
• map() It doesn't change the original array

Two 、 Parameter description ：

array.map(function(currentValue, index, arr), thisIndex)

function(currentValue, index, arr)： must . For a function , Every element in the array performs this function . Where function parameters ：

1. currentValue： must . The value of the current element .
2. index： Optional . Index of the current element .
3. arr： Optional . The array object to which the current element belongs .

thisValue： Optional . Object as the callback to execute , Pass to function , Used as a "this" Value .

So this problem can be explained as

[
    parseInt('1',0),
    parseInt('2',1),
    parseInt('3',2)
]

Look again parseInt() Definition of function

grammar

parseInt(string, radix);

parseInt(string, radix)   Parses a string and returns a decimal integer with the specified cardinality ,radix  yes 2-36 Integer between , Represents the cardinality of the parsed string .

Parameters

string

The value to be parsed . If the parameter is not a string , Then convert it to a string ( Use  ToString Abstract operations ). Whitespace at the beginning of the string will be ignored .

radix  Optional

from  2  To  36, Represents the cardinality of the string . For example, specify  16  Indicates that the parsed value is a hexadecimal number . Please note that ,10 Not the default ！ The description later in the article explains when the parameter  radix  The specific behavior of this function when it is not transmitted .

Return value

An integer parsed from a given string .

perhaps  NaN, When

• radix  Less than  2  Or greater than  36, or
• The first non space character cannot be converted to a number .

If  radix  yes  undefined、0  Or unspecified ,JavaScript It is assumed that ：

1. If you enter  string  With  0x  or  0X（ One 0, Followed by lowercase or uppercase X） start , that radix Assumed to be 16, The rest of the string is parsed as a hexadecimal number .
2. If you enter  string  With "0"（0） start ,radix  Assumed to be  8（ octal ） or  10（ Decimal system ）. Which one to choose radix It depends on the realization of .ECMAScript 5 Clarified that... Should be used 10 ( Decimal system ), But not all browsers support . therefore , In the use of  parseInt  when , Be sure to specify a radix.
3. If you enter  string  Start with any other value ,radix  yes  10 ( Decimal system ).

If the first character cannot be converted to a number ,parseInt  Returns the  NaN.

The answer for ：

[1, NaN, NaN]