How to separate a substring in a complicate string in JavaScript?

Looks like your string has a structure where there are multiple fields where each field is represented as:

[KEY]=[VALUE];

You can use common string and array methods like split and map to extract what you need. In this case looks like you want the value of the first field:

const str = 'A=sample_text1; B=sample_text2; C=sample_text3;';

const result = str.split(';').map(s => s.split('=').pop().trim()).shift();

console.log(result); //=> 'sample_text1'

https://regexr.com is very useful for creating and testing regex.

const match = "A=sample_text1; B=sample_text2; C=sample_text3;".match(/A=([^;]*)/);
let value = match !== null ? match[1] : undefined;

Would allow you to get the value of A in this case

You could use a regular expression to capture every group surrounded by a = and a ;:

const str = "A=sample_text1; B=sample_text2; C=sample_text3;";
const regexp = "=(.*?);";
const values = [...str.matchAll(regexp)];
const aValue = values[0][1];
console.log(aValue);

It might be an overkill, but to easily access to all the keys / values, you could use Object.fromEntries:

let str = "A=sample_text1; B=sample_text2; C=sample_text3;";

let values = Object.fromEntries(
  str
  // Split all the pairs
  .split(";") 
   // Remove the last empty element
  .slice(0,-1)
  // map to a [key, value] array to pass to Object.fromEntries
  .map(i => i.split("=").map(j => j.trim()))); 

// get a value using a key
console.log(values["A"]) // sample_text1

// check if a key is present
console.log("C" in values) // true
console.log("D" in values) // false

It looks more longer than it is due the comments and the console logs, it can fit in one line.

Notice that this is assume of course that neither the character = or ; can be part of the key or the value.