I don't have prettier extension installed, but after reading the possible duplicate answer I've added from scratch in my User Setting (UserSetting.json, Ctrl+, shortcut):
"prettier.singleQuote": true
A part a green warning (Unknown configuration setting) the single quotes are no more replaced.
I suspect that the prettier extension is not visible but is embedded inside the Vetur extension.
Answer from attdona on Stack Overflowcannot format doubleQuotes to singleQuote in single javascript file
singleQuote config is not respected
eslint - Prettier doesn't work for changing double quote to single quote in visual code - Stack Overflow
"prettier.singleQuote: true" not being honored for vanilla javascript files
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I don't have prettier extension installed, but after reading the possible duplicate answer I've added from scratch in my User Setting (UserSetting.json, Ctrl+, shortcut):
"prettier.singleQuote": true
A part a green warning (Unknown configuration setting) the single quotes are no more replaced.
I suspect that the prettier extension is not visible but is embedded inside the Vetur extension.
Well, like @user2982122 mentioned but instead of File go to Code -> Preferences -> Settings, then look for Quote, select Prettier and check both boxes

As you already have singleQuote: true set in your prettier config, I suspect that you're seeing your single quotes converted to double quotes specifically inside your JSX tags. This is because Prettier has a separate rule for JSX:
"jsxSingleQuote": true
Without this, even with "singleQuote": true, Prettier will still convert single quotes within JSX to double.
According to Prettier, double quotes in JSX is the default because it's descended from HTML, where double quotes are more common.
Set "jsxSingleQuote": true wherever you have your Prettier config, and that should do the trick.
This only happens if you have ' in your string e.g. ('it\'s a thing').
Prettier has this issue and there is apparently no solution to this.
My workaround is, using `` (string template syntax) for such strings and single quote for all other strings.
`it's a thing`
You don't have to escape this.
I'm working with vue, but this keeps happening to all my .vue files. I have the vetur extension + prettier + eslint. Eslint fixes the double quotes to single quotes but I think either prettier or vetur is overwriting it. I may be missing one of the webpack files but I have no idea which one, or if I actually AM missing a file.
I've tried to add:
{
"vetur.format.defaultFormatter.js": "prettier-eslint",
"eslint.validate": [
{ "language": "vue", "autoFix": true },
{ "language": "javascript", "autoFix": true } ],
"eslint.autoFixOnSave": true
}
in a .prettierrc file as suggested by one of the solutions I found but it doesn't work for me. Idek what else to do, I'm pretty new to vue.
Command Palette -> "Format Document with... " - this will list the available formatters (with the default denoted) and will allow you to set the default. If you can't find the command, there's only one formatter available for this context.
Omg i think it worked. Idk HOW it worked cus I had tried that before but it worked now so thank you so much.