JavaScript基础（JavaScript Basic）

First for ++

关于运算符++的规范，思考问题

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let num = 0;
num ++;
let secNum = 0;
++ secNum;
//以上两者会输出什么
//答案是num是0，secNum是1

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let num = "5";
num = num + 1;
let secNum = "5";
secNum ++;
secNum
//以上两者会输出什么
//num的结果是"51"，字符串类型，secNum的结果是6，数字类型

相关文档

13.4.2.1 Runtime Semantics: Evaluation
UpdateExpression : LeftHandSideExpression ++

1. Let lhs be ? Evaluation of LeftHandSideExpression.
2. Let oldValue be ? ToNumeric(? GetValue(lhs)).
3. If oldValue is a Number, then
   a. Let newValue be Number::add(oldValue, 1𝔽).
4. Else,
   a. Assert: oldValue is a BigInt.
   b. Let newValue be BigInt::add(oldValue, 1ℤ).
5. Perform ? PutValue(lhs, newValue).
6. Return oldValue.

13.4.4.1 Runtime Semantics: Evaluation
UpdateExpression : ++ UnaryExpression

1. Let expr be ? Evaluation of UnaryExpression.
2. Let oldValue be ? ToNumeric(? GetValue(expr)).
3. If oldValue is a Number, then
   a. Let newValue be Number::add(oldValue, 1𝔽).
4. Else,
   a. Assert: oldValue is a BigInt.
   b. Let newValue be BigInt::add(oldValue, 1ℤ).
5. Perform ? PutValue(expr, newValue).
6. Return newValue.

6.1.6.2 The BigInt Type
The BigInt type represents an integer value. The value may be any size and is not limited to a particular bit-width. Generally, where not otherwise noted, operations are designed to return exact mathematically-based answers. For binary operations, BigInts act as two’s complement binary strings, with negative numbers treated as having bits set infinitely to the left.

7.1.3 ToNumeric ( value )
The abstract operation ToNumeric takes argument value (an ECMAScript language value) and returns either a normal completion containing either a Number or a BigInt, or a throw completion. It returns value converted to a Number or a BigInt. It performs the following steps when called:

1. Let primValue be ? ToPrimitive(value, number).
2. If primValue is a BigInt, return primValue.
3. Return ? ToNumber(primValue).

以上摘自官方文档，其中，下标𝔽指的是Numbers，下标ℤ指的是BigInts，通过官方文档，表明了无论++ num还是num ++都会进行数字转换再进行相加，而不是单纯的num = num + 1，再一次验证结果如下：

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let num = "e";
num = num + 1;
console.log(num);
let secNum = "e";
secNum++;
console.log(secNum);
//其中的结果是，num为e1，secNum为NaN

另外，从之中可以看出，++n是返回新的结果值，n++返回的是旧的结果值
而num = num + 1只进行了加法运算