runLength.js

// Using the JavaScript language, have the function RunLength(str) take the str parameter being passed and return a compressed // version of the string using the Run-length encoding algorithm. This algorithm works by taking the occurrence of each 
// repeating character and outputting that number along with a single character of the repeating sequence. 
// For example: "wwwggopp" would return 3w2g1o2p. The string will not contain any numbers, punctuation, or symbols. 

// Input = "aabbcde"        Output = "2a2b1c1d1e"
// Input = "wwwbbbw"        Output = "3w3b1w"

function RunLength( str ) {
	var output = '';
	while ( str.length > 0 ) {
		var current = new RegExp( str[0] + '+' );
		var length = str.match( current ).toString().split( '' ).length;
		output += length.toString() + str.match( current )[0][0];
		str = str.replace( str.match( current )[0], '' );
	}
	
	return output;
}

function encode(code) {
    if (!code) return '';
    let encode = '';

    for (let i = 0; i < code.length; i++) {
      let count = 1;
      for (let j = i; j < code.length; j++) {
        if (code[i] !== code[j+1]) break;
        count++;
        i++;
      }
      encode += count === 1 ? code[i] : count + code[i];
    }

    return encode
  }
  
 encode("WWWWWWWWWWWWBWWWWWWWWWWWWBBBWWWWWWWWWWWWWWWWWWWWWWWWB"); // "12WB12W3B24WB"
 encode("AABBBCCCC"); // "2A3B4C"
 encode(""); // ""

https://gist.github.com/odykyi/05b31fb476dbd5bf7326736f19010c60