Если ваш код не работает и / или браузер жалуется, что что-то не определено, убедитесь, что вы правильно указали все имена переменных, имена функций и т. д.

Некоторые общие встроенные функции браузера, которые вызывают проблемы:

Вам нужно убедиться, что вы не помещаете точки с запятой неправильно. Например:

Есть ряд вещей, которые могут пойти не так с функциями.

Одна из наиболее распространённых ошибок - объявить функцию, но нигде ее не вызвать. Например

:

function myFunction() {  alert("This is my function.");}

Этот код ничего не сделает, если вы его не вызовете, например

myFunction();

Область действия

Помните, чтоfunctions have their own scope —вы не можете получить доступ к значению переменной, установленному внутри функции извне функции, если вы не объявили переменную глобально (т. е. не внутри каких-либо функций), илиreturn the value из функции.

Запуск кода после оператора возврата

Помните также, что когда вы возвращаете значение из функции, интерпретатор JavaScript выходит из функции - никакой код после выполнения оператора return не выполняется.

Фактически, некоторые браузеры (например, Firefox) выдадут вам сообщение об ошибке в консоли разработчика, если у вас есть код после оператора return. Firefox даёт вам «недостижимый код после оператора возврата».

Когда вы назначаете что-то в JavaScript, вы используете один знак равенства, например:

var myNumber = 0;

Это не работает вObjects, однако - с объектами, вам нужно отделить имена членов от их значений, используя двоеточия, и разделить каждый элемент запятой, например:

var myObject = {  name: "Chris",  age: 38,};

• What is JavaScript?
• What is a variable?
• What are strings?
• What is an array?
• What is a loop?

• What is a function?
• What is an event?
• What is an object?
• What is JSON?
• What is a web API?
• What is the DOM?

• How do you add JavaScript to your page?
• How do you add comments to JavaScript code?

• How do you declare a variable?
• How do you initialize a variable with a value?
• How do you update a variable's value? (also seeAssignment operators)
• What data types can values have in JavaScript?
• What does 'loosely typed' mean?

• What types of number do you have to deal with in web development?
• How do you do basic math in JavaScript?
• What is operator precedence, and how is it handled in JavaScript?
• How do you increment and decrement values in JavaScript?
• How do you compare values in JavaScript? (e.g. to see which one is bigger, or to see if one value is equal to another).

• How do you create a string in JavaScript?
• Do you have to use single quotes or double quotes?
• How do you escape characters in strings?
• How do you join strings together?
• Can you join strings and numbers together?
• How do you find the length of a string?
• How you find what character is at a certain position in a string?
• How do you find and extract a specific substring from a string?
• How do you change the case of a string?
• How do you replace one specific substring with another?

• How do you create an array?
• How do you access and modify the items in an array? (this includes multidimensional arrays)
• How do you find the length of an array?
• How you add and remove array items?
• How do you split a string into array items, or join array items into a string?

• What are the basic types of error?
• What are browser developer tools, and how do you access them?
• How do you log a value to the JavaScript console?
• How do you use breakpoints, and other JavaScript debugging features?

For more information on JavaScript debugging, seeHandling common JavaScript problems; also seeOther common errors for a description of common errors.

• How do you execute different blocks of code, depending on a variable's value or other condition?
• How do you use if ...else statements?
• How do nest one decision block inside another?
• How do you use AND, OR, and NOT operators in JavaScript?
• How do you conveniently handle a large number of choices for one condition?
• How do you use a ternary operator to make a quick choice between two options based on a true or false test?

• How do you run the same bit of code over and over again?
• How do you exit a loop before the end if a certain condition is met?
• How do you skip to the next iteration of a loop if a certain condition is met?
• How do you use while and do ... while loops?
• How to iterate over the elements in an array
• How to iterate over the elements in a multidimensional array
• How to iterate over the members in an object
• How to iterate over the members of an object nested inside an array

• How do you find functions in the browser?
• What is the difference between a function and a method?
• How do you create your own functions?
• How do you run (call, or invoke) a function?
• What is an anonymous function?
• How do you specify parameters (or arguments) when invoking a function?
• What is function scope?
• What are return values, and how do you use them?

• How do you create an object?
• What is dot notation?
• What is bracket notation?
• How do you get and set the methods and properties of an object?
• What isthis, in the context of an object?
• What is object-oriented programming?
• What are constructors and instances, and how do you create them?
• What different ways are there to create objects in JavaScript?

• How do you structure JSON data, and read it from JavaScript?
• How can you load a JSON file into a page?
• How do you convert a JSON object to a text string, and back again?

• What are event handlers and how do you use them?
• What are inline event handlers?
• What does theaddEventListener() function do, and how do you use it?
• Which mechanism should I use to add event code to my web pages?
• What are event objects, and how do you use them?
• How do you prevent default event behaviour?
• How do events fire on nested elements? (event propagation, also related — event bubbling and capturing)
• What is event delegation, and how does it work?

• What are object prototypes?
• What is the constructor property, and how can you use it?
• How do you add methods to the constructor?
• How do you create a new constructor that inherits its members from a parent constructor?
• When should you use inheritance in JavaScript?

• How do you manipulate the DOM (e.g. adding or removing elements) using JavaScript?