为什么0.5等于 -5？

假设我以意想不到的方式将 0.5写成 0.-5，但它仍然可以运行。0.-5中的 0.做了什么以至于它仍然可以运行并计算为 -5？

我还试了 alert(0.-5+1)，它打印 -4，JavaScript 是否忽略 0.-5中的 0.？

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最佳答案

Trailing digits after a . are optional:

console.log(0. === 0); // true

0.-5

evalutes to

0 - 5

which is just -5. Similarly,

0.-5+1

0 - 5 + 1

which is

-5 + 1

or -4.

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In JS you can express a number with optional decimal point.

x = 5.;    //5
x = 5. + 6.   //11

And as of Tvde1's comment, any Number method can be applied too.

5..toString()

This syntax let us run the Number functions without parentheses.

5.toString() //error
(5).toString() //good
5..toString() //good
5 .toString() // awesome

See this question to find out why.

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0.-5 could be successfully parsed as 0.^[1], - and 5. Below is the abstract syntax tree for the expression generated by AST explorer:

Parse tree generated by AST explorer

This (in an unexpected way) is valid JavaScript and evaluates to -5.

^[1] According to the grammar for numeric literals the decimal digits and exponent parts are optional:

NumericLiteral ::
  DecimalLiteral
  [...]

DecimalLiteral ::
  DecimalIntegerLiteral . DecimalDigits_opt ExponentPart_opt

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I would think that the real answer is not about the decimal point, but is about the minus sign: isn't that going to be interpreted as an operator if it is preceded by anything that looks like a number?

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console.log(0. - 5)      // -5
console.log(0 - 5)       // -5
console.log('0.' - 5)    // -5
console.log('0' - 5)     // -5
console.log(0.-5 === -5) // true

'0.' or '0' is the same in JavaScript because the type is unique for numbers, called Number. The minus operator is between Numbers, try always to convert what you pass to a Number. In Python is different first is a Float and the second an Integer because it has several types.