preface

[1,2,3], but the real answer is [1, NaN, NaN]. Why? Let’s first look at the array.map () function’s definition in MDN

Array.map()

The map() method returns a new array, the result of which is returned when each element in the array calls one of the provided functions

var arr = [1.2.3];
var arr1 = arr.map(x= > x * 1);
console.log(arr1); / / 1, 2, 3
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You can see that the map method accepts a function that iterates over each element in the array

var new_array = arr.map(function callback(currentValue[, index[, array]]) {
// Return element for new_array
}[, thisArg])
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The callback takes three arguments in total: currentValue is the current element being processed in the array, index is the index of the current element being processed in the array, and array is the array from which the map method was called

ThisArg optional, the value of this used when executing the callback function

parseInt(string, radix)

Radix resolves the cardinality of strings, and the cardinality rules are as follows:

  • The interval ranges from 2 to 36 
    parseInt('123'.5) // Treat '123' as a base 5 number and return the decimal number 38 => 1*5^2 + 2*5^1 + 3*5^0 = 38
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  • When the argument is 0, parseInt() is parsed in decimal
  • If ignored, the default cardinality rule is:
    • If a string begins with “0x”, parseInt() parses the rest of the string into a hexadecimal integer 
      parseInt("0xf")  / / 15
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    • If a string begins with a 0, the following characters are resolved to octal or hexadecimal numbers 
      parseInt("08")   // 0 * 8 ^ 1 + 8 * 8 ^ 0 = 8
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    • If a string begins with a number from 1 to 9, parseInt() will parse it as a decimal integer 
       parseInt("88.99 f")   / / 88
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    • Only the first digit in the string is returned 
      parseInt("10.33") / / 10
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    • Opening and closing Spaces are allowed 
      parseInt("69 10")  / / 69
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    • If the first character of the string cannot be converted to a number, NaN is returned 
      parseInt("f")  // NaN  
parseInt("f".16)  / / 15
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[‘1’, ‘2’, ‘3’].map(parseInt)

[‘1’, ‘2’, ‘3’]

['1'.'2'.'3'].map(function(cur,index){
  return parseInt(cur, index)
})
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So what the code does is

parseInt('1'.0); // Radix 0, use the default base 10 -> 1
parseInt('2'.1); // The radix value range is 2-36, unable to resolve, beyond the range -> NaN
parseInt('3'.2); | | | | | | | | | | | | | | | | | | | | | | | |
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So the end result is: [1, NaN, NaN]