The keyword let lets you declare a variable with a limited scope. It can only be accessed within the block it was declared.

The keyword const also declares a variable with a limited scope but the variable’s value cannot be changed.

The keyword var is used to declare variables with a global scope and its value can be changed.

An example code in JavaScript would look like this:

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var globalVariable = "This can be used anywhere!"

function myFunction() {
    let localVariable = "This can only be used inside myFunction!"
    const constantVariable = "This has a fixed value!"
}

You can call a function within a function by passing the inner function as a parameter to the outer function. For example, in Javascript, you can do something like this:

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function outerFunction(innerFunction) {
  innerFunction();
}

function innerFunction() {
  console.log("This function is called inside the outer function.");
}

outerFunction(innerFunction);

The innerFunction will be called when you invoke outerFunction.

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let city = prompt("Enter a city: ");

fetch(`http://api.openweathermap.org/data/2.5/weather?q=${city}&APPID=YOUR_APP_ID`)
  .then(response => response.json())
  .then(data => {
    alert(`It is currently ${data.main.temp - 273.15}°C (${(data.main.temp - 273.15) * 9/5 + 32}°F) in ${city} with a humidity of ${data.main.humidity}%.`);
  });

This error is caused when the value you are trying to convert to lower case is null or undefined. To fix the error, make sure the value you are trying to convert is defined and has a value. Also, since toLowerCase() is a method on the string prototype, make sure the value is a string. For example:

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let str = 'Hello'; 
str.toLowerCase(); // hello

let str2;
str2.toLowerCase(); // Uncaught TypeError: Cannot read property 'toLowerCase' of undefined

You can use JavaScript to create events that can execute code in response to user interactions. These events lie at the core of interactive websites. Here is a basic example of an event listener, waiting for a click event on an HTML element:

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document.getElementById("myButton").addEventListener("click", function () {
  alert("Button clicked!");
});

Other events can be used in JavaScript and you can read more about them in the MDN web docs article on events.

You can use the JavaScript fetch() API to request data from a weather API. For example, this code snippet would return the forecasted temperature data for the next 5 days in the fiveDayForecast variable:

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let fiveDayForecast;

fetch('https://api.weather.com/forecast/daily?days=5&units=metric&key=[YOUR API KEY HERE]')
    .then(response => response.json())
    .then(data => {
        fiveDayForecast = data.forecast.forecastDay;
    });

More information on the fetch() API can be found in the MDN Fetch API Documentation.

To show the word length in JavaScript, you can use the .length property. For example:

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let word = "Shecodes"
let wordLength = word.length;
console.log(wordLength); 

This code would output 8, as that is the length of the word.

You can use the Date.now() function to get the current time in JavaScript and display it on a webpage. This code snippet demonstrates how it can be used to display current time:

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let now = Date.now();
let timeInMs = now - Date.parse("01 Jan 1970 00:00:00 GMT");    
document.body.innerHTML = new Date(timeInMs).toUTCString();

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let randomNumber = Math.random();
console.log(randomNumber);

In JavaScript, an arrow function (also called a fat arrow function) is a concise way to write a function expression. It has a more concise syntax than a standard function expression and does not bind its own this, arguments, super, or new.target.

Syntax:

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(param1, param2, …, paramN) => { statements }
(param1, param2, …, paramN) => expression
// equivalent to: (param1, param2, …, paramN) => { return expression; }

// Parentheses are optional when there's only one parameter name:
(singleParam) => { statements }
singleParam => { statements }

// A function with no parameters requires parentheses:
() => { statements }

Source: MDN

This can be done with JavaScript by creating a prompt, storing the value in a variable, and then creating a alert with the variable.

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let result = prompt("What do you want to be alerted");
alert(result);

To convert Fahrenheit (°F) to Celsius (°C) you can use the following formula: °C = (°F - 32) * 5/9. To convert Celsius (°C) to Fahrenheit (°F), use the following formula: °F = °C * 9/5 + 32.

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if (condition) {
  // code
} else {
  // code
}

You can use the toLocaleTimeString method to print 12-hour time instead of 24-hour time in JavaScript. This method takes a locales and options parameters that you can set to localize to your timetable settings.

For example, the following code prints 12-hour time instead of 24-hour timer in JavaScript:

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let currentTime = new Date();
let options = { timeStyle: 'short', hour12: true };
let timeString = currentTime.toLocaleTimeString('en-US', options);
console.log(timeString); // 9:41 PM

You can use the toLocaleString() function to get a localized version of a date as a string. This can be used to produce output strings with ordinal numbers, depending on the provided locale.

For example, in JavaScript you can get a localized version of a Date object that includes ordinal numbers for the day, like this:

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var date = new Date();
var localized = date.toLocaleString('en-US', { day: 'numeric', month: 'short', year: 'numeric' });
console.log(localized); // e.g. 11/Dec/2019

You can use the alert() function to display a message in the form. For example, you can use the following JavaScript code to alert the value of the password input when the form is submitted:

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    document.getElementById("myForm").addEventListener("submit", function(event){
      alert("Password: " + document.getElementById("password").value);
      event.preventDefault();
    });

The error cannot read properties of null is usually caused by an attempt to access a null value or undefined property. This usually occurs when trying to manipulate DOM elements that don't exist. In your case, this may be occurring because you are attempting to call the addEventListener method on an element that has not been successfully selected.

To confirm, try viewing the element in the DOM and make sure it is defined and currently available. You can use console.log() or a debugger to view the actual value of the element you are trying to read.

If the element exists, you should check your syntax and any parameters you are passing to the addEventListener method.

You can use the innerHTML property of the HTML element to change the button description. To do this you will need to select the HTML element you want to target and set it's innerHTML property to the new String you want to show.

For example, with JavaScript:

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document.getElementById("myButton").innerHTML = "My new description";

This code will change the description of the button with ID myButton to My new description.

You can use the addEventListener() method in JavaScript to trigger an event when the navlink is clicked. This event could execute code that would close the navbar hamburger menu.

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navlink.addEventListener('click', () => {
  document.getElementById('hamburger-menu').classList.add('closed');
});

In JavaScript, you can add a class to an HTML element with the classList property. You can use the add() method to add a class to the classList property.

For example:

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document.getElementById("myElement").classList.add("myClass")

This code retrieves the element with an ID of "myElement", then adds the class "myClass" to the classList property.

You can remove an item from an array in Javascript using the splice() method. splice() takes two parameters, the starting index and number of items to be removed from the array.

For example:

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var arr = ["apple", "orange", "banana"];
arr.splice(1,1);

This will remove the item at index 1 from the array, resulting in ["apple", "banana"].

You can create a timer using JavaScript. For instance, you can create a countdown function that counts down from 100 seconds to 0 seconds and displays the remaining time. Here is an example:

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let seconds = 100; 
let timer = setInterval(function() { 
  document.querySelector('h1').innerHTML = seconds + ' seconds left'; 

  seconds--;
  if (seconds < 0) {
    clearInterval(timer);
    document.querySelector('h1').innerHTML = 'Ended';
  }
}, 1000);

You can also create a function to stop the time and display it:

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function stopTimer() {
  clearInterval(timer);
  document.querySelector('h1').innerHTML = seconds + ' seconds left';
}

And you can create a function to restart the time and display it:

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function startTimer() { 
  timer = setInterval(function() { 
    document.querySelector('h1').innerHTML = seconds + ' seconds left';

    seconds--;
    if (seconds < 0) {
      clearInterval(timer);
      document.querySelector('h1').innerHTML = 'Ended';
    }
  }, 1000);
}

Declarative programming is a programming style that expresses the logic of a computation without describing its control flow. In JavaScript, a declarative example could be written as follows:

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const add = (x, y) => x + y;
let result = add(2, 3);
console.log(result); // 5

Imperative programming is a type of programming that focuses on specifying how a program should perform an action by giving step-by-step instructions. A simple example of imperative programming in JavaScript is creating a loop:

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for (let i = 0; i < 10; i++) {
  console.log(i);
}

This code loop tells JavaScript to start at 0, run the code inside the loop with the value of i starting at 0, until it reaches the condition where i is no longer less than 10, and increase the value of i by 1 after each iteration. The loop will output the numbers 0-9 to the console.

You can log the time in milliseconds using the Date.now() method in JavaScript.

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console.log(Date.now());