出现 undefined 的几种情况

来源：互联网发布：unity3d 场景资源编辑：程序博客网时间：2024/06/13 05:48

1、变量未定义：

<!DOCTYPE html><html lang="en"><head><meta charset="UTF-8"><title>Document</title></head><body><script>console.log(a);  // undefined</script></body></html>

2、变量定义了但未赋值：

<!DOCTYPE html><html lang="en"><head><meta charset="UTF-8"><title>Document</title></head><body><script>var a;console.log(a);  // undefined</script></body></html>

3、函数未传参：

<!DOCTYPE html><html lang="en"><head><meta charset="UTF-8"><title>Document</title></head><body><script>function show(a){};console.log(show());   // undefined</script></body></html>

4、只有 return 没有值：

<!DOCTYPE html><html lang="en"><head><meta charset="UTF-8"><title>Document</title></head><body><script>function show(){return;};console.log(show());   // undefined</script></body></html>

5、函数没有返回值：

<!DOCTYPE html><html lang="en"><head><meta charset="UTF-8"><title>Document</title></head><body><script>function show(){};console.log(show());  // undefined</script></body></html>

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