推入前敲除检查是否在observableArray中存在

Question

I am new to Knockout and I have a little problem stopping me from completing my simple project.
I have an observableArray called loanDeductions that I display in a table with foreach binding.
I have another observableArray called loanDeductionsList which is also from the json data of my first observableArray, I used it in my drop down list which when a value is selected, it will push the selected data to my table. If it didn't make sense as I cannot really explain it clearly, this is my javascript file:

var deductionLine = function (deductionID, deductionName, amount) {
    self = this;
    self.deductionID = ko.observable(deductionID);
    self.deductionName = ko.observable(deductionName);
    self.amount = ko.observable(formatCurrency(amount));
};

function LoanDeductions(deductions) {
    var self = this;
    self.loanDeductions = ko.observableArray(ko.utils.arrayMap(deductions, function (deduction) {
        return new deductionLine(deduction.deductionID, deduction.deductionName, deduction.amount)
    }));
    self.loanDeductionsList = ko.observableArray(ko.utils.arrayMap(deductions, function (deduction) {
        return new deductionLine(deduction.deductionID, deduction.deductionName, deduction.amount)
    }));

    self.selectedDeduction = ko.observable();

    self.selectedDeduction.subscribe(function (data) {
        self.loanDeductions.push({
            deductionID: data.deductionID,
            deductionName: data.deductionName,
            amount: data.amount,
        });
    });
}

Can you help me find a way to make my function selectedDeduction.subscribe() push the data ONLY when the item to be pushed is not existing in my loanDeductions observableArray.
Any help will be greatly appreciated. Thank you!

I am somewhat aware that the way I populate my dropdown list may o may not be the best way to do it, I am open to suggestion of a better way and rewrite my program.

Answer 1

just to share what I did I added this line in my selectedDeduction.subscribe():

self.selectedDeduction.subscribe(function (data) {
        var match = ko.utils.arrayFirst(self.loanDeductions(), function(deduction) {
            return deduction.deductionID() === data.deductionID(); 
        });
        if (match) {
            alert(data.deductionName() + ' already exists!');
        } else {
            self.loanDeductions.push({
                deductionID: data.deductionID,
                deductionName: data.deductionName,
                amount: data.amount,
            });
            }
});