In JavaScript, const is a keyword that is used to declare a constant variable. A constant is a value that cannot be re-assigned or re-declared once it has been defined. This means that the value of a const variable remains the same throughout the execution of a program.

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const PI = 3.14159;

In the example above, PI is a constant variable that has been assigned the value of 3.14159. Since it is declared with const, the value of PI cannot be changed later in the program. If you try to re-assign a value to a const variable, you will get an error.

It is important to note that const does not make an object or array immutable. The variable itself is constant, but the properties or elements within the object or array can still be modified.

To declare a CSS variable, you can use the -- prefix followed by the variable name. Here is an example:

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:root {
  --main-color: blue;
}

In this example, we are declaring a CSS variable named --main-color and setting its value to blue.

To make a 2D array an argument of a function in C++, you can declare the function with the array parameter using square brackets for both dimensions.

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void myFunction(int myArray[][3]) {
  // function body here
}

In this example, myFunction() takes a 2D integer array as an argument. The size of the second dimension must be specified, while the size of the first dimension can be left empty or specified. In this case, the second dimension has a size of 3.

Here's an example of calling this function with a 2D array argument:

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int main() {
  int array2D[2][3] = {{1,2,3}, {4,5,6}};
  myFunction(array2D);
  return 0;
}

In this example, main() declares a 2D integer array called array2D, which is then passed as an argument to myFunction(). The function can then access or modify the elements of the array as needed.

You can declare variables using the let keyword in JavaScript, for example:

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let num1 = 5;
let num2 = 10;
let sum = num1 + num2;
console.log(sum); // Output: 15

In this example, we declared three variables num1, num2, and sum using the let keyword. Then we assigned values to num1 and num2 and assigned the result of adding those two numbers to sum. Finally, we logged sum to the console which outputted 15.

You can also use string or boolean variables in the same way as the number variables in the example above.

This means that the code tried to access a variable called city before the variable was initialized with a value. This error occurs when you try to access a variable before it has been declared or assigned a value. To solve this error, make sure the area in your code where you're trying to access city is after the line where it is first declared. For example, in JavaScript this could look like:

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//This code throws an Uncaught ReferenceError
console.log(city);
let city = 'London';

//This code does not throw an error
let city = 'London';
console.log(city);

In many programming languages, including JavaScript, Python, and Java, an array is a structured data type that stores a group of objects (items or values) of the same kind. To create an array, you must declare it first. For example, in Java, you would write the following code to declare an array of integers:

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int[] myArray = new int[4];

This declares an array with 4 elements of type int. You can assign values to each of the elements using the array's index, like this:

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myArray[0] = 40;
myArray[1] = 25;
myArray[2] = 18;
myArray[3] = 33;

In JavaScript, null and undefined are two distinct types that represent different values. By definition, undefined means a variable has been declared but has not yet been assigned a value, whereas null is an assignment value, meaning that a variable has been declared and given the value of null.

To illustrate the distinction between the two, consider the following example:

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let x;
console.log(x); // logs 'undefined'

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let y = null;
console.log(y); // logs 'null'

Another way to distinguish the two is through the typeof operator. typeof will always return undefined for an undefined variable and object for a null variable. For example:

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console.log(typeof z); // logs 'undefined'

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console.log(typeof a); // logs 'object'

For more information, please check out the MDN web docs.

To declare value with a prompt in JavaScript, you'll need to use the prompt() method. This method takes in a string of text which will be used as the prompt to request input from the user.

Example code:

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let age = prompt("What is your age?");
console.log("You are "+age+" years old.");

Learn more about prompt() here.