preface

If you’ve seen scaffolding compiled code, you’ll see that all expressions involving undefined are compiled to void 0, for example

/ / the source code
const a: number = 1
a === undefined

/ / the compiled
"use strict";
var a = 1;
a === void 0;
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Void and undefined

Undefined is a base type and a global property

console.log(window.hasOwnProperty('undefined'))   //true
console.log(window.undefined)  //undefined
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In javascript, global properties are allowed to be overridden, as you can

window.undefined = 1
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Void is an operator (like Typeof) that can only evaluate an expression, not assign it, for example
```
var typeof = 1  //Uncaught SyntaxError: Unexpected token 'typeof'
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```

Security assignment

From the above distinction, it can be seen thatvoid 0To take the place ofundefinedAssignment is actually safer, so almost all scaffolding tools incorporate the ability to convert undefined to void 0
shorter void 0contrastundefinedThe browser transfer process has fewer bytes

A more secure

The fundamental reason is that before ECMAScript 5,undefinedAs a global property, it can be read and written, and some poor quality libraries will overwrite the value of undefined, causing the direct assignment of undefined to be incorrect. After ECMAScript 5,undefinedSet it to read only, and you don’t have the problems described above, and you don’t have much of an issue with compatibility.
shorter

Personally think write source code scenarios must be usedvoid 0Instead ofundefinedReading can bring more mental burden, such as
```
const a = 1
if(a === void 0 || a === 1) {}Copy the code
```
Maintenance and code review costs more than the benefit of saving a few bytes

Void 0 is not recommended for scenarios that require compatibility with older browsers (below Internet Explorer 8)