Check whether a given string is a valid ABN (Australian Business Number), written in Typescript.

isValidABN.ts

// The following formula can be used to verify the ABN.
// To verify an ABN:
// 1. Subtract 1 from the first (left) digit to give a new eleven digit number 
// 2. Multiply each of the digits in this new number by its weighting factor 
// 3. Sum the resulting 11 products
// 4. Divide the total by 89, noting the remainder
// 5. If the remainder is zero the number is valid

isValidABN(value: string) :boolean {
    var weights : number[] = [10, 1, 3, 5, 7, 9, 11, 13, 15, 17, 19];
    value.replace(" ", "");

    // check length is 11 digits
    if (value.length != 11) return false;

    // Multiply each digit by its weighting factor
    var sum :number = 0;
    for (var i = 0; i < weights.length; i++) {
      // The first digit is reduced by 1
      var digit :number = parseInt(value[i]) - (i==0?1:0);
      sum += digit * weights[i];
    }

    // If the the sum is divisible by 89 the ABN is valid
    return (sum % 89) == 0;
    
}