Replace file content without creating a new file with Node.js

Question

I'm playing with Node.js fs.writeFile() flags to find the right file for my problem: I want to replace the content of a file but throw an error if the file does not exist.

My first try was with r+ but I have issues if the new content is shorter than the old one:

fs.writeFileSync('test', '11111111111');
> 111111111111
fs.writeFileSync('test', '22', {flag: 'r+'})
> 221111111111

Removing the flag solve the problem (give 22) but create a new file if test doesn't exist.

Is it doable with a flag or do I need to detect file existence before (not very found of that)?

gelliott181 · Accepted Answer · 2016-11-14 17:58:42Z

2

You can't do this in a single step: opening a file for writing will create the file if it does not exist, while opening the file with r+ mode presents the problem you mention.

I see two options:

Test for existence using fs.stat(), then fs.writeFile() in w mode.
Open the file in r+ mode, write to it, then fs.ftruncate() the file to the desired size.

I recommend the first approach. The code will be easier to read and reason about.

edited Nov 14, 2016 at 17:58

gelliott181

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answered Nov 14, 2016 at 17:48

salezica

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2 Comments

NorTicUs Over a year ago

I was always told testing the file before is an anti-pattern because it won't guarantee the file status when you'll write. I may have concurrent access on this file so isn't the 2nd better? Thank you

NorTicUs Over a year ago

Since I want to override the file, I found out the best method is indeed delete it, watch for error (if it doesn't exist), then write it with default flag. Thank you

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