Demo Menu

Truthy Or Falsy

Immediate Post To Server
Page Form
<form class="d-inline" method="post">
    <div class="form-check form-check-inline">
        <input class="form-check-input" type="checkbox" asp-for="Checkbox"
            onclick="this.form.action = 'TruthyOrFalsy?handler=UpdateCheckbox'; this.form.submit();" />
        <label class="form-check-label" asp-for="Checkbox">
            @Html.DisplayNameFor(model => model.Checkbox)
    <div class="form-check form-check-inline form-switch">
        <input class="form-check-input" type="checkbox" asp-for="Switch"
            onclick="this.form.action = 'TruthyOrFalsy?handler=UpdateSwitch'; this.form.submit();" />
        <label class="form-check-label" asp-for="Switch">
            @Html.DisplayNameFor(model => model.Switch)
    <div class="form-check form-check-inline form-switch switch-big">
        <input class="form-check-input" type="checkbox" asp-for="BigSwitch"
            onclick="this.form.action = 'TruthyOrFalsy?handler=UpdateBigSwitch'; this.form.submit();" />
        <label class="form-check-label" asp-for="BigSwitch">
            @Html.DisplayNameFor(model => model.BigSwitch)
public bool Checkbox { get; set; }
public bool Switch { get; set; }
[Display(Name = "Big Switch")]
public bool BigSwitch { get; set; }

public void OnGet()
    Checkbox = true;
    BigSwitch = true;

public IActionResult OnPostUpdateCheckbox()
    StatusMessage = $"Checkbox updated to {Checkbox}.";
    return Page();
public IActionResult OnPostUpdateSwitch()
    StatusMessage = $"Switch updated to {Switch}.";
    return Page();
public IActionResult OnPostUpdateBigSwitch()
    StatusMessage = $"Big Switch updated to {BigSwitch}.";
    return Page();
Big Switch CSS
.form-switch.switch-big .form-check-input {
    width: 3em;
    margin-left: -2.5em;
    height: 1.5em;
.form-switch.switch-big .form-check-label {
    margin-top: 0.25rem;
    margin-left: 0.5rem;

When you pass a c# bool property from the PageModel to JavaScript, you throw a JavaScript Uncaught ReferenceError.

public bool TrueBoolean { get; } = true;
Razor Page JavaScript
let modelBoolean = @Model.TrueBoolean;
JavaScript Error
Uncaught ReferenceError: True is not defined

The c# bool property uses the TrueString and FalseString fields which are title case and JavaScript Boolean values are lower case. Research for this demo found a lot of ways to mitigate the issue but none as simple as declaring True and False variables. See StackOverflow Answer - Trying to pass in a boolean C# variable to a javascript variable and set it to true.

let True = true, False = false;
Truthy and Falsy Values in JavaScript

As well as a type, each value also has an inherent Boolean value, generally known as either truthy or falsy. Some of the rules are a little bizarre, so understanding the concepts and effect on comparison helps when debugging JavaScript applications. See Truthy and Falsy: When All is Not Equal in JavaScript.

A single value can therefore be used within conditions. To demonstrate how seemingly different values equate to truthy or falsy, I developed JavaScript key/value object to evaluate the values. I tried using an array filter but results were different than the if conditional method.

Values To Test
let testItems = {
    "01": true,
    "02": false,
    "03": True,
    "04": False,
    "05": 0,
    "06": '',  
    "07": null, 
    "08": undefined,
    "09": NaN,
    "10": '0', 
    "11": 'false',
    "12": [],
    "13": {},
    "14": function() {} 
Evaluation Script
// Convert testItems to a key/value array
let testArray = Object.entries(testItems), truthyKeys = [], falsyKeys = [];
testArray.forEach(function(item) {
    if(item[1]) truthyKeys.push(item[0]);
    else falsyKeys.push(item[0]);
DOMContentLoaded Event
document.querySelector('.truthy-keys').textContent = truthyKeys.toString();
document.querySelector('.falsy-keys').textContent = falsyKeys.toString();
Truthy Keys
Falsy Keys
Key To Evaluate
function evalutateTruthy()
    let key = document.querySelector('input[name="RadioButtonGroup"]:checked').value;
    if(testItems[key]) showMessageModal(`${key} is truthy.`, 'alert-success');
    else showMessageModal(`${key} is falsy.`, 'alert-danger');