Js Basics – (Data types and direct conversions to various data types)

Js basic types and their conversion rules

Primitive value types refer to data types, eight basic data types that are easy to overlookBigInt

There are two special data types: null and undefined.

Conclusion: null and undefined are null and undefined except for themselves and each other.

Implicit conversion scenarios are common in js

1. ifconditional

if(xxx){};
switch(xxx){
    case 1:
    break;    
    default;
}
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In general condition judgment, condition XXX will be converted to Boolean type, and then the judgment of the two sides of the equation

2. = =Comparison operation> = < = 等

== comparison: If the two sides of an expression have different data types, they are implicitly converted to the same type and then compared

>= <= both strings will not convert the two-sided expression to Number, eg: “2” > “10” // true

• Object == string (in this case, objects are converted to strings before comparison)
• null == undefined trueNote: null, undefined, and other values are not equal if there are three equal signs.
• NaN == NaN true// If so= = =It’s not equal
• Symbol == Symbol false
• Other cases [e.g. : object == number, string == Boolean…] They have to be converted into numbers and then compared

Here’s an example:

! [] = =false // true[] = =false // true
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Why does it appear! [] and [] == true?

! [] === false Boolean([])) == Number(false) 2. [] === false

3. The plus sign +With a minus sign-

• + In addition to implicitly converting to Number, there are alsoString splicingThe function of

Common conversions to rules:

• Common difference between n++ and ++n

• +n converts to a number

• ++n converts to a number and adds up to 1

• N++ converts to a number and then adds up to 1

Three chestnuts, please:

CASE1: the following three expressions I ++, I += 1; Is I equal to I plus 1 the same?

So: 3 expressions are not equal, and only I +=1 is equal to I = I +1

CASE2: + has an object on one side

let n = 10;
{} + n // 10, {} will be treated as fast code, do not participate in the operation, only +n
n + {} // "10[object object]" string splicing
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Important point of CASE3: Not all objects are directly converted to strings and then concatenated, but rather

• Go fetch the object firstSymbol.toPromitiveProperty value if there is no property
• To call the object’svalueOfGets the original value, if not the original value
• To call the object’stoStringConvert to a string (if you want to convert to a Number)

console.log(10 + [10.20])  // 1010,20 string concatenation
/ / 1. [10, 20] [Symbol. ToPromitive]
/ / 2. [10, 20]. The valueOf ()
// 3. Finally convert to string
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Two test questions: What does the following expression output?

let i = 10;
console.log(5 + (i++)) // 15, ++ does not take effect the first time
console.log(5 + (++i)) // 17, I = 11 after ++, I = 12

console.log({} + i) ; // 12, {} will be used as a code block
console.log(i + {}); //"12[object Object]"
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How do I change 10 + {a: 10} to 20

let obj ={
    a:10[Symbol.toPrimitive](hint){
        console.log("hint",hint) // "default", 'string', 'number'
        return this.a; }}console.log(10+obj) // This can be done using several CASE3 apis or setters
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What are the built-in object types

What are the methods of data type detection

Three ways to detect data types

Use the Typeof checklist

• The underlying mechanism of Typeof is to determine that these binary, 000 starts with objects and excludes functions

• Typeof variables that are not declared do not report errors

Computer Science: Computer principles, base conversion, computer networks, data structures and algorithms

Characteristics 1:

Typeof null -> "object" typeof null -> "object" Typeof new RegExp(/b/g) : 'object' typeof new RegExp(/b/g) : 'object'Copy the code

Character 2:

typeofObject - >"object" && typeofFunction - >"function"
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Feature 3: Typeof undeclared variables -> “unfedined” : How to use this feature (Scenario example: Exposed API in plug-in packaging)

(function () {
    let utils = {

    };
    // What's wrong with this? If a variable does not exist, typeof can be used to determine
    if (typeof window! = ='undefined') window.utils = utils;
    // common.js specification export
    if (typeof module= ='object' && typeof module.exports === 'object') moudule.exports = utils; }) ()// Browser only
// utils.xxx()
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Fast method for detecting number types

A special data typeSymbolData type if get?

• How does Symbol get unique values

let n = Symbol('AA');
let m = Symbol("AA");
console.log(n == m) //false

let key = Symbol(a);let obj = {
    [key]:100
}
console.log(obj[key]);
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• What are the common APIS of Symbol?

Symbol.hasInstance Symbol.iterator Symbol.toPrimitive Symbol.toStringTag

Convert other types to Object([value])

Rule: Raw conversion is directly enclosed in quotes [bigint removes n]