为什么log输出显示r未被定义呢

Question

<script>
        let arr = ['0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'a', 'b', 'c', 'd', 'e', 'f', '7', '8']
        let color = '#'
        let flag = true
        if (flag) {
            for (let i = 1; i <= 6; i++) {
                let random = Math.floor(Math.random() * arr.length)
                color += arr[random]
            }
        } else {
            let r = Math.floor(Math.random() * 256)
            let g = Math.floor(Math.random() * 256)
            let b = Math.floor(Math.random() * 256)
        }
        console.log(color);
        console.log(`rgb${r},${g},${b}`);
    </script>

Answer 1

因为r,g,b只定义在了else中，且使用let定义的变量只在当前代码块中有效，所以在else外输出的时候就会提示undefined，你可以改成var定义或者在外层定义：

let arr = ['0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'a', 'b', 'c', 'd', 'e', 'f', '7', '8']
let color = '#'
let flag = true
let r = 0, g = 0, b = 0

if (flag) {
  for (let i = 1; i <= 6; i++) {
    let random = Math.floor(Math.random() * arr.length)
    color += arr[random]
  }
} else {
  r = Math.floor(Math.random() * 256)
  g = Math.floor(Math.random() * 256)
  b = Math.floor(Math.random() * 256)

  // 或者
  var r = Math.floor(Math.random() * 256)
  var g = Math.floor(Math.random() * 256)
  var b = Math.floor(Math.random() * 256)
}
console.log(color);
console.log(`rgb${r},${g},${b}`);