JS: Array.prototype.map() & parseInt()

Array.prototype.map()方法与parseInt()方法的结合使用的深入反思：基础很重要

1. 写出下面代码的执行结果：

var arr = ["1", "2", "3"].map(parseInt);
console.log(arr);

console print:
Active code page: 65001
ERROR: The process "node.exe" not found.
[ 1, NaN, NaN ]
[Finished in 0.6s]

Array.prototype.map()

The map() method creates a new array with the results of calling a provided function on every element in the calling array.
map()方法创建一个新数组，其结果是在调用数组中的每个元素上调用提供的函数

JavaScript Demo: Array.map()

var array1 = [1, 4, 9, 16];

// pass a function to map
const map1 = array1.map(x => x * 2);

console.log(map1);
// expected output: Array [2, 8, 18, 32]

Syntax

var new_array = arr.map(function callback(currentValue[, index[, array]]) {
// Return element for new_array
}[, thisArg])

Parameters

• callback
  Function that produces an element of the new Array, taking three arguments:
  • currentValue:
    The currents element being processed in the array.
  • index/ Optional
    The index of the current element being processed in the array.
  • array/ Optional
    The array map was called upon.
• thisArg/ Optional
  Value to use as this when executing callback.

Return value

• A new array with each element being the result of the callback function.

Description

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, indexes that have never been set, which have been deleted or which have never been assigned a value).

callback is invoked with three arguments: the value of the element, the index of the element, and the Array object being traversed.

If a thisArg parameter is provided to map, 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.

---

parseInt()

The parseInt() function parses a string argument and returns an integer of the specified radix (the base in mathematical numeral systems).

JavaScript Demo: parseInt()

function roughScale(x, base) {
  var parsed = parseInt(x, base);
  if (isNaN(parsed)) { return 0 }
  return parsed * 100;
}

console.log(roughScale(' 0xF', 16));
// expected output: 1500

console.log(roughScale('321', 2));
// expected output: 0

Syntax

parseInt(string, radix);

Patameters

• string
  The value to parse. If the string argument is not a string, then it is converted to a string(using the ToString abstract operation). Leading whitespace in the string argument is ignored.
• radix
  An integer between 2 and 36 that represents the radix (the base in mathematical numeral systems) of the above mentioned string.

Return value

An integer number parsed from the given string. If the first character cannot be converted to a number, NaN is returned.

Description

The parseInt function converts its first argument to a string, parses it, and returns an integer or NaN. If not NaN, the returned value will be the integer that is the first argument taken as a number in the specified radix (base). For example, a radix of 10 indicates to convert from a decimal number, 8 octal, 16 hexadecimal, and so on. For radices above 10, the letters of the alphabet indicate numerals greater than 9. For example, for hexadecimal numbers (base 16), A through F are used.

If parseInt encounters a character that is not a numeral in the specified radix, it ignores it and all succeeding characters and returns the integer value parsed up to that point. parseInt truncates numbers to integer values. Leading and trailing spaces are allowed.

Because some numbers include the e character in their string representation (e.g. 6.022e23), using parseInt to truncate numeric values will produce unexpected results when used on very large or very small numbers. parseInt should not be used as a substitute for Math.floor().

If radix is undefined or 0 (or absent), JavaScript assumes the following:

• If the input string begins with "0x" or "0X", radix is 16 (hexadecimal) and the remainder of the string is parsed.
• If the input string begins with "0", radix is eight (octal) or 10 (decimal). Exactly which radix is chosen is implementation-dependent. ECMAScript 5 specifies that 10 (decimal) is used, but not all browsers support this yet. For this reason always specify a radix when using parseInt.
• If the input string begins with any other value, the radix is 10 (decimal).

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

For arithmetic purposes, the NaN value is not a number in any radix. You can call the isNaN function to determine if the result of parseInt is NaN. If NaN is passed on to arithmetic operations, the operation results will also be NaN.

To convert number to its string literal in a particular radix use intValue.toString(radix).