javascript - 我应该将变量放在 if 条件中吗？

Question

我想知道，根据良好做法，在这种情况下放置变量最方便的方法是什么:

如果你有一个函数，只有当条件语句为真时它的内容才会被执行，你会把变量放在哪里？

  addMessage (text) {
    let lastMessage = React.findDOMNode(this.refs.messages);
    if (text.length) {
      ChatActions.addMessage(text);
      lastMessage.scrollTop = lastMessage.scrollHeight;
    }
  }

或者像这样:

  addMessage (text) {
    if (text.length) {
      let lastMessage = React.findDOMNode(this.refs.messages);
      ChatActions.addMessage(text);
      lastMessage.scrollTop = lastMessage.scrollHeight;
    }
  }

Answer 1

是的，绝对是

正如许多 JavaScript 风格指南中所述:

---

Airbnb JavaScript Style Guide() {

13.4 Assign variables where you need them, but place them in a reasonable place.

Why? let and const are block scoped and not function scoped.

---

Principles of Writing Consistent, Idiomatic JavaScript

// 2.B.1.4
// const and let, from ECMAScript 6, should likewise be at the top of their scope (block).

// Bad
function foo() {
  let foo,
    bar;
  if ( condition ) {
    bar = "";
    // statements
  }
}
// Good
function foo() {
  let foo;
  if ( condition ) {
    let bar = "";
    // statements
  }
}