21 days proficient in typescript-4 - type inference and semantic check

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https://blog.csdn.net/shangyanaf/category_11822476.html

Type inference

In the coding area , Enter the following code ：

let userName= '123'

When you place the mouse over the string of code , Will see TS Deduce userName The type is string

When you add the following code

userName = 18

At this time, a red wavy line will appear under the string of codes .TS You will be prompted ：Type ‘number’ is not assignable to type ‘string’.

stay JS in , We can assign different types of values to a variable , Like here userName You can assign him a string , It can also be assigned to a number .

This kind of writing that does not pay attention to variable types , It may bring some imperceptible mistakes .

stay TS in , When we initialize a variable with a string type , Later, we assigned him a number type ！TS Specific error messages will be thrown .

This is the same. TS Type inference of . When we initialize a variable , And the assignment .TS This type will be inferred from the assignment . for example ：let userName= '123',TS

Will infer that userName The type of is string .

Of course, in addition to simple types ,TS Type inference also supports Object and Array Type inference .

For example, the following user object ,TS Automatically infer user The type of object is ：{name:" Pinellia ternata "}, And internal properties name The type of can also be inferred .

let user={
    
  name:" Pinellia ternata "
}

The same type inference applies to function returns .

For example, the following example ,result Accepted the return value of the function 0, When the mouse is on result yes ,TS Will prompt result yes number type .

function test() {
    
  return 0;
}
let tesult=test()

Semantic analysis

Baidu Encyclopedia ： Semantic analysis is a logical stage in the compilation process , The task of semantic analysis is to review the context related properties of the structurally correct source program , Conduct type review .

stay TS in , The semantic analysis stage is divided into ：

• Scope analysis
• Type checking

TS One of our goals is to statically analyze code , Know the possible errors in the code in advance during the development process .

For example, the following code , This is an extreme example , No one should write that . stay test Internal function , There is one for loop , unfortunately , We are not careful when cycling , Used const Definition i.

function test(){
    for(const i=0;i<100;i++){
    
     }
}

stay TS In the environment ：

stay JS In the environment ：