CODEWITHSUNDEEP
javascript
ASDFGerte
ASDFGerte

Reputation: 5193

Keep trailing or leading zeroes on number

Is it possible to keep trailing or leading zeroes on a number in javascript, without using e.g. a string instead?

const leading  = 003;                 // literal, leading
const trailing = 0.10;                // literal, trailing
const parsed   = parseFloat('0.100'); // parsed or somehow converted

console.log(leading, trailing, parsed); // desired: 003 0.10 0.100

This question has been regularly asked (and still is), yet I don't have a place I'd feel comfortable linking to (did i miss it?).

Fully analogously would be keeping any other aspect of the representation a number literal was entered as, although asked nowhere near as often:

console.log(0x10); // 16 instead of potentially desired 0x10
console.log(1e1);  // 10 instead of potentially desired 1e1

For disambiguation, this is not about the following topics, for some of which I'll add links, as they might be of interest as well:

Upvotes: 1

Views: 2416

Answers (1)

ASDFGerte
ASDFGerte

Reputation: 5193

No. A number stores no information about the representation it was entered as, or parsed from. It only relates to its mathematical value. Perhaps reconsider using a string after all.

If i had to guess, it would be that much of the confusion comes from the thought, that numbers, and their textual representations would either be the same thing, or at least tightly coupled, with some kind of bidirectional binding between them. This is not the case.

The representations like 0.1 and 0.10, which you enter in code, are only used to generate a number. They are convenient names, for what you intend to produce, not the resulting value. In this case, they are names for the same number. It has a lot of other aliases, like 0.100, 1e-1, or 10e-2. In the actual value, there is no contained information, about what or where it came from. The conversion is a one-way street.

When displaying a number as text, by default (Number.prototype.toString), javascript uses an algorithm to construct one of the possible representations from a number. This can only use what's available, the number value, also meaning it will produce the same results for two same numbers. This implies, that 0.1 and 0.10 will produce the same result.

Concerning the number1 value, javascript uses IEEE754-2019 float642. When source code is being evaluated3, and a number literal is encountered, the engine will convert the mathematical value the literal represents to a 64bit value, according to IEEE754-2019. This means any information about the original representation in code is lost4.

There is another problem, which is somewhat unrelated to the main topic. Javascript used to have an octal notation, with a prefix of "0". This means, that 003 is being parsed as an octal, and would throw in strict-mode. Similarly, 010 === 8 (or an error in strict-mode), see Why JavaScript treats a number as octal if it has a leading zero

In conclusion, when trying to keep information about some representation for a number (including leading or trailing zeroes, whether it was written as decimal, hexadecimal, and so on), a number is not a good choice. For how to achieve some specific representation other than the default, which doesn't need access to the originally entered text (e.g. pad to some amount of digits), there are many other questions/articles, some of which were already linked.

[1]: Javascript also has BigInt, but while it uses a different format, the reasoning is completely analogous.

[2]: This is a simplification. Engines are allowed to use other formats internally (and do, e.g. to save space/time), as long as they are guaranteed to behave like an IEEE754-2019 float64 in any regard, when observed from javascript.

[3]: E.g. V8 would convert to bytecode earlier than evaluation, already exchanging the literal. The only relevant thing is, that the information is lost, before we could do anything with it.

[4]: Javascript gives the ability to operate on code itself (e.g. Function.prototype.toString), which i will not discuss here much. Parsing the code yourself, and storing the representation, is an option, but has nothing to do with how number works (you would be operating on code, a string). Also, i don't immediately see any sane reason to do so, over alternatives.

Upvotes: 4

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