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VisualizaçõesWhy does 0.-5 evaluate to -5?
Suppose I write 0.5 as 0.-5 in unexpected way, but it can still run. What does 0. in 0.-5 do so that it can still run and evaluates to -5?
I also tried alert(0.-5+1) which prints -4, does JavaScript ignore 0. in 0.-5?
3 Respostas
Responde à pergunta0
Trailing digits after a . are optional:
console.log(0. === 0); // trueSo
0.-5
evalutes to
0 - 5
which is just -5. Similarly,
0.-5+1
is
0 - 5 + 1
which is
-5 + 1
or -4.
0
0.-5 could be successfully parsed as 0.[1], - and 5. Below is the abstract syntax tree for the expression 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 . DecimalDigitsopt ExponentPartopt
0
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.