JavaScript 编译序列如何在这段代码中工作？

Question

为什么将错误代码行放在底部，运行程序的其余部分。但是放在这里不会运行任何单个函数，会导致错误。

const A = "A";
let F;

function doStuff(B) {
  console.log(B);
  const C = "C";
  let H = "H";
  if (1 + 1 === 2) {
    const D = "D";
    H = "something else";
  }

  console.log(H);
  F = "F";
}

console.log(B); // I know it has error

let E = 0;
while (E < 3) {
  E++;
  console.log(A); //But why this
}
doStuff("B"); // and this
console.log(E); // and this is not working unless I remove that line or place that in the bottom of the code.

Answer 1

变量 B 没有在任何地方定义，所以它当然会骂你。

const A = "A";
let F;

function doStuff(B) {
  console.log(B);
  const C = "C";
  let H = "H";
  if (1 + 1 === 2) {
    const D = "D";
    H = "something else";
  }

  console.log(H);
  F = "F";
}

//console.log(B);  <-- What is B, it is not defined anywhere so Javascript will yell at you becasue it doesn't know what it is

let E = 0;
console.log(E); //this works here because E is initialized
while (E < 3) {
  E++;
  console.log(A); //variable A defined at the top of the code can be accessed here so this will work
}
doStuff("B"); // and this
console.log(E); //it will also works here

至于为什么将 console.log(B) 放在底部会运行部分代码，而不是像 C++ 或 C# 那样立即抛出错误，JavaScript 是解释语言，而不是编译语言。这意味着当你运行一段代码时，会有一个解释器会逐行读取每一行代码，然后解释它。 Javascript中根本没有编译步骤。

例如，当浏览器读取第一行，解释它然后运行它时，它仍然会输出“Hello”。它不知道第二行发生了什么

console.log("Hello");
console.log(A)