How can I stub a problematic piece of code [Unit Testing]

Question

New to writing unit tests and the concept of 'mocking' to be more exact. I have a basic function 'addPercentSign' that adds a percent character to a user input if it is between 50-100:

    addPercentSign: function (oEvent, control) {
        var inputVal = oEvent.getParameters().value;
        var inputNumber = parseFloat(inputVal);

        if (inputNumber) {

            if (inputNumber < 50 || inputNumber > 100) {
                return null;
            } else {
                var finalVal = inputNumber.toFixed(1);
                var finalOutput = finalVal + "%";

//Error: cannot setValue of undefined. How can I 'stub' the line below?
                control.learningCurve.setValue(finalOutput);

                return finalOutput;
            };
        }
    }

The Problem

The problem I'm facing is when I write a unit test for this function, I cannot test the returned value(finalOutput) because the line above it returns an error as it relies on the DOM element (control.learningCurve) to set a value. Since this is a unit test, I must isolate this function from any dependencies.

Therefore, I need to 'mock' the 'setValue' call. I figured creating a stub would make sense, but I'm not sure how?

Here is the unit test code that I'd like to test the above function (addPercentSign):

function (formatter, viewControls) {
    "use strict";

    var testEvent = {
        getParameters : function() {
                return {value : 50}
        }
    }

    QUnit.module("Formatter Functions");
    QUnit.test("Add Percent Sign", function (assert) {

    assert.ok(formatter.addPercentSign(testEvent, viewControls) == '50.0%', "Percent Sign Added: Pass");

    });
}

The Question

How can I mock the setter so I can unit test this function without DOM dependencies?:

control.learningCurve.setValue(finalOutput)

Answer 1

Sinon can be used as the test double.

This example is using sinon stub.

sinon.stub(control.learningCurve, 'setValue').returns('value that you need');

Edit:

function (formatter, viewControls) {
"use strict";

    sinon.stub(control.learningCurve, 'setValue').returns('value that you need');

    var testEvent = {
        getParameters : function() {
                return {value : 50}
        }
    }

    QUnit.module("Formatter Functions");
    QUnit.test("Add Percent Sign", function (assert) {

    assert.ok(formatter.addPercentSign(testEvent, viewControls) == '50.0%', "Percent Sign Added: Pass");

    });
}