js关于操作符

Question

直接上代码var num = '123'; console.log( ++num );输出124我能理解console.log( ++'123' );报错Uncaught ReferenceError: Invalid left-hand side expression in prefix operation为什么只是一个是存入变量就不会报错 一个是直接暴露出来 就报错

Answer 1

你搞错了自增/自减符号的运算意义
它们的共同点是，把自身+1或者是-1之后的值赋值给自己，注意，是要把结果赋值给自己的。
那么你弄个常量来进行自增自减怎么可能不报错……

规范在这里，你可以参考一下：

UnaryExpression : ++ UnaryExpression

1. Let expr be the result of evaluating UnaryExpression.

2. Let oldValue be ToNumber(GetValue(expr)).

3. ReturnIfAbrupt(oldValue).

4. Let newValue be the result of adding the value 1 to oldValue, using the same rules as for the + operator (see 12.7.5).

5. Let status be PutValue(expr, newValue).

6. ReturnIfAbrupt(status).

7. Return newValue.

PostfixExpression : LeftHandSideExpression ++

1. Let lhs be the result of evaluating LeftHandSideExpression.

2. Let oldValue be ToNumber(GetValue(lhs)).

3. ReturnIfAbrupt(oldValue).

4. Let newValue be the result of adding the value 1 to oldValue, using the same rules as for the + operator (see 12.7.5).

5. Let status be PutValue(lhs, newValue).

6. ReturnIfAbrupt(status).

7. Return oldValue.