The solution to my test

TestSolution.js

function assertEquals(result, expected) {
    if (result !== expected) {
      throw new Error(`result: ${result} expected: ${expected}`);
    }
  }
  
  // No validations needed
  // Assume that the string is composed only by (positive) numbers
  // NOTE: cannot use parseInt and Number constructor
  function parseCoolInteger(value) {
    const numbers = {
        "0": 0, "1": 1, "2": 2, "3": 3, "4": 4, "5": 5, "6": 6, "7": 7, "8": 8, "9": 9,
    }
    let finalNumber = 0;
    const valueLength = value.length;
    
    for (let i=0; i < valueLength; i++) {
        let number = numbers[value[i]]
        // Calculate the exponent of 10, whether 10, 100, 1000 etc.
        let exponent = valueLength - ( i + 1 );

        // Multiply the number by the exponent and add it to the result for instance
        // In 1234, when i === 0, the line below will be 0 += (1 * (10^1000)) which gives 1000
        finalNumber += (number * (10**exponent))
    };
    
    return finalNumber;
  }

  assertEquals(parseCoolInteger("1234"), 1234);
  assertEquals(parseCoolInteger("11"), 11);
  assertEquals(parseCoolInteger("123456789"), 11);