如何将“[object Object]”转换为数字

Question

I have been set a task, using an external IFFE file, to create an app that works out the cost of fuel for a fleet of cars using the functions inside the IFFE. Then display it in a box.

The IFFE can be see Here , i have been told by the people that set me the task that the following code should work to call the correct function

var fuel = new fuelCalc();


// On the button press
miles = 10000;
var cost = fuel.cost(miles);

Then doing the following should put it into the text box,

document.getElementById("TotalCost").value = cost;

However, all it does is put the following in the text box

[object Object]

What do i need to add to make it show the calculated amount?

Answer 1

您需要指定一年，例如document.getElementById("TotalCost").value = cost[2013];

Answer 2

Based on your code, it looks like your function fuelCalc.cost is actually returning an object though.

Do you need to do further calculations within the function, maybe:

fuelCalc.prototype.cost = function ( annualMileage, start, end ) {
    var costPerLitre = this.costPerLitre;
    var averageMPG = this.averageMPG;

    var startYear = start || 1989;
    var endYear = end || 2020;
    var totalYears = endYear - startYear;

    var data = {};
            var totalCost = 0;

    for (var year = startYear, t = endYear; year <= t; year += 1) {
        data[year] = this.costIn( year, annualMileage );
                    totalCost += data[year]
    }

    return totalCost;
}

Answer 3

The reason you are seeing [object Object] is because fuel.cost(miles) does not return the cost value but a hash or instance of cost class maybe.

Try debugging this javascript on browser's javascript console, by setting a breakpoint at the line:

document.getElementById("TotalCost").value = cost;

At this moment in Console, just print the cost variable. It will list the object structure and you can extract relevant data from that.