String.prototype.replace supports replacement patterns
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Today I came across a blog post (it's in German though) written by Peter Kröner and learned something very astonishing.
The article describes the not so well known behaviors of the method String
. Using this method is usually very straight forward. It supports regular expressions if you need to, but for most cases, it's defining a matching string and a replacing string.
That's at least what I thought... 🙈
It turns out that if the second argument is a string (it can also be a function) and includes specific character sequences like $&
or $'
"replacer magic" appears.
const msg = 'This is a great message';
msg.replace('great', 'wonderful');
// "This is a wonderful message"
//
// -> 'great' is replaced by 'wonderful'
msg.replace('great', '$&-$&');
// "This is a great-great message"
// '$&' represents the matched substring
//
// -> 'great' is replaced by 'great-great'
msg.replace('great', '$`');
// "This is a This is a message"
// '$`' represents the string before the matched string
//
// -> 'great' is replaced by 'This is a '
msg.replace('great', `$'`)
// "This is a message message"
// `$'` represents the string after the matched string
//
// -> 'great' is replaced by ' message'
Oh my..., this behavior can lead to very hard to spot bugs!
If you want to read more about it, have a look at the replace
docs on MDN or this overview on replacement values.
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