undefined 表示未经初始化的变量，null表示已经定义的变量没有赋值，0表示数值0，false表示逻辑上的错误，“”表示空字符串。
null,undefined,0,false,""
| 操作数 | 操作 | 被操作数 | 结果 |
|---|
| null,undefined,0,false,"" | && | null,undefined,0,false,"" | 操作数 |
| null,undefined,0,false,"" | || | null,undefined,0,false,"" | 被操作数 |
| null,undefined,0,false,"" | & | null,undefined,0,false,"" | 0 |
| null,0,false | +/* | null,0,false | 0 |
| null,0,false | +/* | undefined | NaN |
| undefined | +/* | null,0,false | NaN |
|
>,<.<=,>=操作
| 操作数 | 操作 | 被操作数 | 结果 |
|---|
| null,undefined,0,false | > | null,undefined,0,false | false |
| null,undefined,0,false | < | null,undefined,0,false | false |
| null,0,false | <=/>= | null,0,false | true |
| undefined | <=/>= | undefined | true |
| null,0,false | <=/>= | undefined | false |
| undefined | <=/>= | null,0,false | false |
| | | | |
| | | | |
== 与 ===
| 操作数 | 操作 | 被操作数 | 结果 |
|---|
| null,undefined | == | null,undefined | true |
| 0,false | == | 0,false | true |
| null,undefined | == | 0,false | false |
| 0,false | == | null,undefined | false |
| null,undefined,0,false | === | null,undefined,0,false | 当操作数与被操作数相同时为true否则为false |
