Access array in if statement

I have JavaScript calculator wherein I have defined two arrays as follows:

var degInc, degArr = []; 
var radInc, radArr = [];
var PI = Math.PI;
var radStart = (-91*PI/2), radEnd = (91*PI/2);

 for (degInc = -8190; degInc <= 8190; degInc+=180) {
       degArr.push(degInc);
    }
 for (radInc = radStart; radInc <= radEnd; radInc+=PI) {
        var radIncFixed = radInc.toFixed(8);
       radArr.push(radIncFixed);
    }

to be used in conjunction with the tangent function (below) so as to display a value of Undefined in an input (HTML below) should the user attempt to take the tangent of these values (I have included other relavent function as well):

Input -

<INPUT NAME="display" ID="disp" VALUE="0" SIZE="28" MAXLENGTH="25"/>

Functions -

function tan(form) {
  form.display.value = trigPrecision(Math.tan(form.display.value));
}

function tanDeg(form) {
  form.display.value = trigPrecision(Math.tan(radians(form)));
}

function radians(form) {
  return form.display.value * Math.PI / 180;
}

with jQuery -

$("#button-tan").click(function(){
        if (checkNum(this.form.display.value)) {
            if($("#button-mode").val() === 'DEG'){
                tan(this.form); // INSERT OTHER 'if' STATEMENT HERE FOR RAD ARRAY
            } 
            else{
                tanDeg(this.form); // INSERT OTHER 'if' STATEMENT HERE FOR DEG ARRAY
            }

        }
});

I would like to incorporate an array check within the .click function such that if the user input is contained in the array (degArr or radArr depending on the mode), the calculator returns Undefined. Now, I know how to display Undefined in the input display ($('#disp').val('Undefined')), but I cannot figure out how to configure an if statement that checks the relevant array. Is there a way to do so within the #button-tan function where I have commented?