Typescript -- preliminary study of variable declaration

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Preface

Get to know TypeScript The basic types in , Next, of course, is the study of variable declaration .

Declare multidimensional arrays

If there is such a statement ：

let arr3: number[][][]; Copy code 

Want to know arr3 Specific types of , The analysis steps are as follows ：

You can see , Type disassembly is from right to left , Corresponding to the specific variable value, it is disassembled layer by layer from outside to inside .

Here's just an example , A person with a little experience can judge at a glance arr3 It's a three-dimensional array . however , Teaching a man to fish is better than giving him a fish , This analysis method is applicable to the analysis of most complex data types .

Property rename

In the presence of ES6 After deconstruction assignment , We can declare variables like this ：

let personInfo1 = {    name: " The samadhi of programming ",    age:22}let {name: nameP, age: ageP} = personalInfo1; Copy code 

The above code declares nameP and ageP Two variables . It should be noted that ： Variable declaration , The colon is not followed by a data type , Instead of the original value of the object .

If in TypeScript in , To specify nameP and ageP Data type of , It needs to be written like this ：

let { name: nameP, age: ageP }: { name: string; age: number } = personInfo1; Copy code 

In fact, this type designation can be placed in the object declaration , Specify the type of each attribute value of the object ：

let personInfo1: { name: string; age: number } = {    name: " The samadhi of programming ",    age: 22};personInfo1.name = 12; // Error  You can't type “number” Assign to type “string”// let {name: nameP, age: ageP} = personInfo1;let { name: nameP, age: ageP } = personInfo1;nameP = 12; // Error  You can't type “number” Assign to type “string” Copy code 

let and const Temporary dead zone of

The following code is TypeScript You can't make a mistake , However, errors will be reported in actual operation .

function foo(): number {    return a1;}foo();let a1: number = 12; Copy code 

Actual operation error ：

My own understanding is ： Static compilation is only responsible for detecting whether the data type is correct , The dynamic runtime will detect syntax and logic errors , It happens that the temporary deadband is generated at runtime .

summary

That's what I'm learning TypeScript Notes and summary recorded during variable declaration .

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~ The end of this paper , Thank you for reading ！

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