CODEWITHSUNDEEP
javascriptnumbers
ocomfd
ocomfd

Reputation: 4020

Why 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?

Upvotes: 90

Views: 11921

Answers (5)

Kolzar
Kolzar

Reputation: 873

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.

Upvotes: 5

CertainPerformance
CertainPerformance

Reputation: 370989

Trailing digits after a . are optional:

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

So

0.-5

evalutes to

0 - 5

which is just -5. Similarly,

0.-5+1

is

0 - 5 + 1

which is

-5 + 1

or -4.

Upvotes: 195

Salman Arshad
Salman Arshad

Reputation: 272236

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 . DecimalDigitsopt ExponentPartopt

Upvotes: 49

Charlie
Charlie

Reputation: 23818

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.

Upvotes: 40

Sean_999
Sean_999

Reputation: 79

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?

Upvotes: 7

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