Day16 – Why must there be block-level scope

365 days of clocking

🔥 creation is not easy, we help uncle B stack of a key three

• How to use closures to complete library encapsulation 📺 Billbill video 📚 Gold Digging Manuscript 🐱 Github
• [Talk about closures and real-time functions] 📺 Billbill video 📚 Nugget manuscript 🐱 Github
• 📺 Billbill video 📚 Digger manuscript 🐱 Github
• The relationship between closures and curated, partial application functions 📺 Billbill video 📚 Denver Manuscript 🐱 Github
• How to make lazy functions with closures? 📺 Billbill video 📚 Nuggets Script 🐱 Github
• What is a closure and how to create one? 📺 Billbill video 📚 Nuggets Manuscript 🐱 Github
• [new a constructor if the function returnsreturn {} 、 return null ， return 1 ， return trueWhat happens? ]📺 Billbill video📚 Gold digging manuscript🐱 making
• What happens to a new function? 📺 Billbill video 📚 Nuggets Script 🐱 Github
• What are the ways to determine data types? 📺 Billbill video 📚 Nuggets Script 🐱 Github
• [How much storage space does Number() have? What if the background sends a Number that exceeds the maximum limit?] 📺 Billbill video 📚 Nuggets Script 🐱 Github
• [0.1 + 0.2 === 0.3? Why? How to solve?] 📺 Billbill video 📚 Nuggets Script 🐱 Github
• How are JS integers represented? 📺 Billbill video 📚 Nuggets transcript

Knowledge point explanation

Block-level scopes were introduced in Javascript to address an important design flaw in Javascript.

The variable promotion feature leads to a lot of counterintuitive code

What is variable promotion?

(function() {
  console.log(v)
  var v = 123
})()
​
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Js promotes variables declared by var and functions declared by keyword functions to the top of the current scope. The assignment statement waits for assignment in place.

The advantage of variable promotion is to reduce the difficulty of writing programs.

Why say so, in fact, the original program is written when there is a clear order requirements.

For example, let’s look at a prehistoric language called Pascal

Complete program structure

It’s like Honor of Kings not having to go home to buy equipment to make it easier to operate.

Counterintuitive

But there are problems with this approach. After all, you’ve arbitrarily modified the execution logic.

It is mainly divided into two aspects: line and plane

The inner variable overrides the outer variable

var tmp = new Date();
​
function f() {
  console.log(tmp);
  if (false) {
    var tmp = 'hello world';
  }
}
​
f(); // undefined
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The above program, the global definition of the variable, because the program behind the definition of the same name variable program, unexpectedly disappeared.

Variables that should have been destroyed were not destroyed

This is actually known as the circular trap problem. I didn’t say it in detail the other day.

var s = 'hello'; for (var i = 0; i < s.length; i++) { console.log(s[i]); } console.log(i); / / 5Copy the code

The interview guide

When it comes to the effect of block-level scope, some people will talk about the phenomenon first, not the essence. It is suggested to analyze this problem from the perspective of perfecting language defects.