Map and reduce in JavaScript

Today, while watching the JS tutorial "official website of Liao Xuefeng", I saw the map and reduce. One of the exercises is: Do not use JS built-in parseInt() functions, using the map and reduce operations to implement a string2int() function (the string is converted to an array, and then converted to a number from arr[0] to Arr[arr. (length-1)]. For example: [1, 3, 5, 7, 9] transform into an integer 13579.

My solution:

function tonum (str)        {            var arr=[];              for (var i=0;i<str.length;i++)            {                Arr.push (str[i]);            }             var num=arr.reduce (function(x, y)            {                return x * + y;            })            alert (num);        }       Tonum (' 123 ');

But the above results pop up with "10203". After the test, there are several reasons:

1, the ARR array for the For loop, gets [' 1 ', ' 2 ', ' 3 '] instead of [All-in-one]. That is, each element in the array is a string, not a number.

function Tonum (str)    {    var arr=[];    for (Var i=0;i<str.length;i++)    {    arr.push (str[i]);    }    return arr;    }       Tonum (' 123 ');//Return ["1", "2", "3"]

2, strings and numbers are subtraction to get a number, string and string addition, is the way strings are concatenated, the resulting string is still.

"1" *10+2;//return 12 "1" *10+ "2";//Return "102"

Correction Scheme one: subtract the string from the first array in the For loop and 0, get an array that is a number, and then apply the reduce function to the array.

function Tonum (str)    {    var arr=[];    for (Var i=0;i<str.length;i++)    {    arr.push (str[i]);    }    var arrn=arr.map (function (x) {return x-0})    var num=arrn.reduce (function [x, y)    {    return x * + y;    })    alert (num);    }       Tonum (' 123 ');

To simplify again, take advantage of the split method of the string:

function Tonum (str)    {    var arr=str.split ('); Each element in the array returned by the//split method is still a string    var arrn=arr.map (function (x) { Return x-0})    var num=arrn.reduce (function (x, y)    {    return x * + y;    })    alert (num);    } Tonum (' 123 ');

Exercise two: "The user entered the non-standard English name, the first letter capitalized, other lowercase canonical name." Input: [‘adam‘, ‘LISA‘, ‘barT‘] , Output: [‘Adam‘, ‘Lisa‘, ‘Bart‘] . ”

My solution:

function Norm (arr) {var arrn=[];    for (Var i=0;i<arr.length;i++)    {    Arrn.push (    arr[i][0].touppercase () +    arr[i].substring (1). toLowerCase ());            }    alert (ARRN);} Norm ([' A dam ', ' Lisa ', ' Bart ']);

　　

　　

Supplement: About substring () and slice ().

SUBSTRING (): the substring () method is used to extract the character of a string intermediary between two specified subscripts.

Syntax:stringobject. substring (start,stop). The substring returned by the substring () method includes the character at start , but does not include the character at the stop .

Slice (): the Slice () method returns the selected element from an existing array .

Syntax: Arrayobject.slice (start,end). Returns a new array containing elements from start to end ( excluding the element) from the Arrayobject.

　

Map and reduce in JavaScript