JavaScript (||) 中的双管道抛出错误而不是评估为虚假

Question

我读到 JavaScript 中的双管道检查变量是否为假值，undefined 是 JavaScript 中的假值，例如

It means that if the value is falsey (e.g. 0, "", null, undefined (see also All falsey values in JavaScript)), it will be treated as false; otherwise it's treated as true.

所以我试了一下，发现 undefined 确实没有被评估为 falsy 而是抛出一个错误：

let elemContent = document.getElementById('content');

let a = null;
let b = 2;

elemContent.innerHTML += a || 'ok'; // "ok"
elemContent.innerHTML += b || 'ok'; // "2"
elemContent.innerHTML += whatever || 'ok'; // "ERROR: whatever is not defined"

http://jsfiddle.net/ueqo6yko

undefined是不是JavaScript中的虚值，或者如何理解这个矛盾？

Answer 1

因为在您的代码中，whatever 不仅是 undefined，而且 未声明

要避免此错误，您可以执行以下操作：

let elemContent = document.getElementById('content');

let a = null;
let b = 2;

elemContent.innerHTML += a || 'ok'; // "ok"
elemContent.innerHTML += b || 'ok'; // "2"
elemContent.innerHTML += (typeof whatever !== 'undefined' && whatever) || 'ok3'; // "ok3"

Answer 2

undefined 确实是假的，但是在 JavaScript 中使用一个变量在它被声明之前是一个错误。

在某处添加 let whatever = undefined 以查看您期望的行为。

Answer 3

undefined 和 not defined 的错误是不同的。 undefined 是 声明的 变量的值，当 not defined 的错误意味着您的变量未声明。

undefined的例子：

let elemContent = document.getElementById('content');

let a = null;
let b = 2;
let whatever;

console.log(whatever)

elemContent.innerHTML += a || 'ok'; // "ok"
elemContent.innerHTML += b || 'ok'; // "2"
elemContent.innerHTML += whatever || 'ok'; // "whatever is undefined"

<div id="content"></div>

一个有 not defined 错误的例子：

let elemContent = document.getElementById('content');

let a = null;
let b = 2;

elemContent.innerHTML += a || 'ok'; // "ok"
elemContent.innerHTML += b || 'ok'; // "2"
elemContent.innerHTML += whatever || 'ok'; // "ERROR: whatever is not defined"

<div id="content"></div>

这是一个示例，您可以如何检查是否声明了变量并且它是否具有值（不是 undefined）：

let test = "test";

if (typeof test !== undefined) {
  console.log("'test' is declared");
  if(test) {
    console.log("'test' has a value (not 'null', 'false', 'undefined'...)");
  }
}

Answer 4

确认undefined是假的：

var whatever = undefined;
console.log(whatever || "undefined is really falsy");

Answer 5

let elemContent = document.getElementById('content');

let a = null;
let b = 2;
let c;


elemContent.innerHTML += a || 'ok'; // "ok"
elemContent.innerHTML += b || 'ok'; // "2"
elemContent.innerHTML += c || 'ko'; // "ko"
elemContent.innerHTML += whatever || 'ok'; // "ERROR: whatever is not defined"

c 是 undefined。不管是 undeclared

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