javascript - .toFixed() called on negative exponential numbers returns a number, not a string - Stack Overflow

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I noticed that when calling toFixed against a negative exponential number, the result is a number, not a string.

First, let's take a look at specs.

Number.prototype.toFixed (fractionDigits)

Return a String containing this Number value represented in decimal fixed-point notation with fractionDigits digits after the decimal point. If fractionDigits is undefined, 0 is assumed.

What actually happens is (tested in Chrome, Firefox, Node.js):

> -3e5.toFixed()
-300000

So, the returned value is -3e5. Also, notice this is not a string. It is a number:

> x = -3e5.toFixed()
-300000
> typeof x
'number'

If I wrap the input in parentheses it works as expected:

> x = (-3e5).toFixed()
'-300000'
> typeof x
'string'

Why is this happening? What is the explanation?

I noticed that when calling toFixed against a negative exponential number, the result is a number, not a string.

First, let's take a look at specs.

Number.prototype.toFixed (fractionDigits)

Return a String containing this Number value represented in decimal fixed-point notation with fractionDigits digits after the decimal point. If fractionDigits is undefined, 0 is assumed.

What actually happens is (tested in Chrome, Firefox, Node.js):

> -3e5.toFixed()
-300000

So, the returned value is -3e5. Also, notice this is not a string. It is a number:

> x = -3e5.toFixed()
-300000
> typeof x
'number'

If I wrap the input in parentheses it works as expected:

> x = (-3e5).toFixed()
'-300000'
> typeof x
'string'

Why is this happening? What is the explanation?

• javascript
• node.js
• tofixed

Share Improve this question Follow edited Jun 20, 2020 at 9:12 CommunityBot 111 silver badge asked Apr 19, 2016 at 14:55 Ionică BizăuIonică Bizău 114k9494 gold badges310310 silver badges487487 bronze badges 3

• It's parsed as -(3e5.toFixed()) – Bergi Commented Apr 19, 2016 at 15:05
• Moral of the story: Calling methods on literal numbers is fraught with peril. For instance, 5.toFixed() is a syntax error. Use parens or variables to avoid falling into pitfalls. (You can do the 5.toFixed() thing with 5..toFixed() but...just don't. Parens are nice and reliable: (5).toFixed().) – T.J. Crowder Commented Apr 19, 2016 at 15:05
• 1 @T.J.Crowder Definitely, I was just curious what was going on. :-) – Ionică Bizău Commented Apr 19, 2016 at 16:14

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2 Answers 2

Sorted by: Reset to default 6

I guess this is because of higher precedence of the member ('.') operator pared to the sign operator.

https://developer.mozilla/en/docs/Web/JavaScript/Reference/Operators/Operator_Precedence

What is happening here is the order of operations. Lets break it down:

First what's going to to happen is 3e5 is going to return a number (300000), then toFixed will be called on in, turning it into a string, then the sign operator is going to be executed, coercing the string back to a number.

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