5 ways to create variables in JS

Traditional solution

var declares a variable
function declares a function

Scheme in ES6

let declare a variable
const declares a variable that cannot be reassigned

Module scheme

import

9 data types in JS

Basic data type

number
string
boolean
null
undefined
symbol
bigint

Reference data type

object
{} ordinary object
[] array object
/^\d+$/ regular object
new Data() date object
function
Ordinary function
Arrow function
Constructor

Data type detection

type of
instanceof
constructor
Object.prototype.toString.call()

阿里巴巴最新面试题

Calculate the results of the following program operation and analyze the reasons

let arr=[10.18,0,10,25,23];
arr=arr.map(parseInt);
console.log(arr);

map(item,index) method: iterate five times, map will return item*10;
so after the first sentence is executed
arr=[101.8,0,100,250,230];

parseInt(value,radix)
1. Start the search from the first character on the left side of the value string and find all the values that match the radix hexadecimal (stop searching until a non-radix hexadecimal value is encountered)
2. The found value As a radix base, finally converted to decimal
3. radix disdain or write 0, the default value is 10 (the default radix is 16 for strings starting with 0x)
4. The value range of radix is between 2–36, not Within this range, the final processing result is all NaN

Convert an N-ary value to a decimal number
1. Each digit in the value *N^bit weight
2. Finally, the result of each digit calculation is accumulated
3. The single digit weight value is 0, ten digits 1, hundred's place 2

Equivalent statement	analysis	result
parseInt(‘10.18’,0)	0 defaults to 10, find the decimal point ending '10' decimal number is itself	10
parseInt(‘0’,1)	1 is not in the range, it is directly NaN	NaN
parseInt(‘10’,2)	'10' binary number converted to decimal to 2	2
parseInt(‘25’,3)	The ternary number 5 does not match, so only the '2' ternary to decimal is still 2.	2
parseInt(‘23’,4)	'23' quaternary to decimal 2*4+3	11

Code execution and result comparison

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <script>
    let arr=[10.18,0,10,25,23];
    arr=arr.map(parseInt);
    console.log(arr);

    </script>
</body>
</html>

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[Zero-based dive front end] Variables in JavaScript and data types in 9 (with parseInt latest interview questions)

Variables in JavaScript and data types in 9