Javascript Hoisting - What Moves and What Stays

JavaScript's improvement mechanism is like the setting of stage play. Before the code is running (in the "creation stage"), JavaScript will raise all the declaration to the top of its scope, just like the stage staff move the props to the specified position before the curtain is raised. The only difference is that only the statement is raised, and the initialization has not been improved.

According to the ECMAScript specification, this behavior is actually part of the creation of the so -called "lexical environment" in the creation stage of JavaScript. But let us not be too technical, just remember that JavaScript will "scan" it before running the code.

improvement

var

In this example, the

console.log(x); // 输出：undefined
var x = 5;
console.log(x); // 输出：5

statement is promoted to the top of the scope, but the initialization

has not been improved. This is why the first output var x instead of throwing an error. Variables exist, but it has not been assigned. x = 5 console.log(x) and undefined The temporary dead area (TDZ) x

In this example, the let statement was improved, but it was located in a temporary dead area (TDZ) before initialization. Try to access const before initialization will lead to

. This behavior is different from ,

console.log(y); // 抛出ReferenceError: Cannot access 'y' before initialization
let y = 10;
console.log(y); // 输出：10

will only return

in similar circumstances. let y y ReferenceError This also includes any content using or var declaration, of course, also includes functions. var undefined

Temporary Dead Area (TDZ)

Speaking of temporary dead areas, this is where let and const declare before the initialization. Different from when

returns

before the initial access, these modern statements will be misunderstood. The ECMAScript specification refers to this behavior as "temporary dead region semantics", but I call it "JavaScript makes us honest."

A slightly difficult example

let const What happened to see here? Functional declaration has been improved, so we can call it in advance. But what about ? It still stays in its place. This is why visit var will give us undefined, and we try to read it before the script is ready.

The trap of the claim declaration

function setupEventHandler() {
  handleClick(); // 可运行！

  const config = {
    debug: true,
  };

  function handleClick() {
    if (config?.debug) {
      // 未定义！
      console.log("Debug mode");
    }
  }
}

setupEventHandler();

The way to improve the class is not the same.

handleClick config Although the class is improved, they are still in the "temporary dead zone" before their definitions are evaluated. This means that you cannot access them before you declare them. handleClick config undefined Remember, the statement was moved to the top, but the initialization remained unchanged.

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