How to replace before first forward slash - JavaScript?

Let's say the following is our string with forward slash ?

var queryStringValue = "welcome/name/john/age/32"

To replace before first forward slash, use replace() along with regular expressions.

Syntax

string.replace(/^[^/]+/, "replacement")

Example

Following is the code ?

var regularExpression = /^[^/]+/
var queryStringValue = "welcome/name/john/age/32"
var replacedValue = queryStringValue.replace(regularExpression, 'index');

console.log("Original value=" + queryStringValue);
console.log("After replacing the value=" + replacedValue);

Original value=welcome/name/john/age/32
After replacing the value=index/name/john/age/32

How the Regular Expression Works

The regular expression /^[^/]+/ breaks down as follows:

• ^ ? matches the start of the string
• [^/] ? matches any character except forward slash
• + ? matches one or more of the preceding characters

Multiple Examples

// Different replacement scenarios
console.log("api/users/123".replace(/^[^/]+/, "v2"));
console.log("home/dashboard".replace(/^[^/]+/, "admin"));
console.log("products".replace(/^[^/]+/, "items"));
console.log("/already/starts/with/slash".replace(/^[^/]+/, "new"));

v2/users/123
admin/dashboard
items
/already/starts/with/slash

Alternative Method Using indexOf

function replaceBeforeFirstSlash(str, replacement) {
    let firstSlash = str.indexOf('/');
    if (firstSlash === -1) return replacement;
    return replacement + str.substring(firstSlash);
}

console.log(replaceBeforeFirstSlash("welcome/name/john", "greeting"));
console.log(replaceBeforeFirstSlash("singleword", "replaced"));

greeting/name/john
replaced

Conclusion

Use the regular expression /^[^/]+/ with replace() to efficiently replace text before the first forward slash. This method works for URL paths, file paths, and similar slash-separated strings.