# .replaceFirst(old,new)

## Description

replaceFirst substitutes only the first occurrence of a substring with a new substring within a string. Unlike .replace() which replaces all occurrences, this method only changes the first match.

## Parameters

replaceFirst needs two parameters:

* old: the substring to search for
* new: the substring to replace it with

## Usage On Strings

```javascript
str = "hello hello world"

log str.replaceFirst("hello", "hi")
// "hi hello world"
// only the first "hello" is replaced

str = "one two one two"
log str.replaceFirst("one", "1")
// "1 two one two"
// subsequent matches remain unchanged

str = "aaa"
log str.replaceFirst("a", "b")
// "baa"
// only first character is replaced

str = "no matches here"
log str.replaceFirst("xyz", "abc")
// "no matches here"
// if the old substring isn't found, the original string is returned unchanged
```


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