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In Node.js documentation, API's are listed like a format I am not understanding.

For Example:

  1. fs.writeFile(file, data[, options], callback)
  2. fs.writeSync(fd, buffer[, offset[, length[, position]]])

I can understand the arguments file and callback but what does it means by data[, options], I want to know the details meaning by data[, options] or buffer[, offset[, length[, position]]]) or like this format.

Primer Cielo
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  • Anything in square brackets is optional – Phil Jul 14 '22 at 04:25
  • Does this answer your question? [How to interpret function parameters in software and language documentation?](https://stackoverflow.com/questions/10925478/how-to-interpret-function-parameters-in-software-and-language-documentation) – Phil Jul 14 '22 at 04:25
  • @Phil Why is it attached with `data[, ]` or `buffer[, ]` with a comma? – Primer Cielo Jul 14 '22 at 04:28
  • Each is independently optional. See the linked post – Phil Jul 14 '22 at 04:28
  • @Phil Can you please describe these two example in answer? – Primer Cielo Jul 14 '22 at 04:31
  • Okay, you said all are independently optional, what if I pass `fs.writeSync(fd, buffer, position)`, means skip previous arguments? – Primer Cielo Jul 14 '22 at 04:55
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    I didn't say _"all"_, I said _"each"_ as in each optional parameter. Keep in mind that JavaScript only offers positional parameters so you cannot skip any – Phil Jul 14 '22 at 04:59

1 Answers1

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When you see this:

fs.writeFile(file, data[, options], callback)

It means you can use it in either of these ways:

fs.writefile(file, data, options, callback)
fs.writefile(file, data, callback);

In other words, the options parameter is optional. You can pass it if needed or leave it out if not needed. The fs.writeFile() code will look at the type and quantify of the arguments you passed and determine which of these forms you are using and adapt as necessary.

When you see this:

fs.writeSync(fd, buffer, offset[, length[, position]])

It means that you can use any of these forms:

fs.writeSync(fd, buffer, offset)
fs.writeSync(fd, buffer, offset, length)
fs.writeSync(fd, buffer, offset, length, position)

You cannot use something like:

fs.writeSync(fd, buffer, position)

because the code would have no way of knowing that the third argument was supposed to be position instead of offset as both are just numeric values. So, if you want to use the position argument, you have to pass all five arguments or use one of the other forms below.

Note that for fs.writeSync(), there are several additional forms that are allowed in nodejs v18. It offers:

fs.writeSync(fd, buffer, offset[, length[, position]])
fs.writeSync(fd, buffer[, options])
fs.writeSync(fd, string[, position[, encoding]])

The second option there is the most flexible as you can fill in the options argument with whichever arguments you want to pass and the others will assume default values.

jfriend00
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  • `fs.writeSync(fd, buffer, position)` will it work? – Primer Cielo Jul 14 '22 at 05:00
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    @PrimerCielo - More has been added to the answer about `fs.writeSync()`. – jfriend00 Jul 14 '22 at 05:02
  • I can not find `fs.writeSync(fd, buffer[, options])` API in Nodejs docs? and did you mean `fs.writeSync(fd, buffer[, offset[, length[, position]]])` instead of `fs.writeSync(fd, buffer, offset[, length[, position]])`? – Primer Cielo Jul 14 '22 at 05:21
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    @PrimerCielo - I'm seeing it [here](https://nodejs.org/api/fs.html#fswritesyncfd-buffer-options). It looks like it's a nodejs v18.3.0 thing, not present in earlier versions. – jfriend00 Jul 14 '22 at 05:23
  • Exactly. I am seeing 16.16.0 LTS. It's okay now. – Primer Cielo Jul 14 '22 at 05:26
  • One thing, is there any study for this understanding? what rules are following in this syntax/format? Because, how can a developer/programmer understand this format without prior knowledge? – Primer Cielo Jul 14 '22 at 05:32
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    @PrimerCielo - Well, each `[tag]` means an optional argument. When they are nested as in `[tag1[, tag2]]`, then you can have either `()` `(tag1)` or `(tag1, tag2)`. I don't know of any formal place where this format is written or taught. For me, it's just been a documentation syntax I became familiar with over time. – jfriend00 Jul 14 '22 at 05:34