Detecting an undefined object property

In JavaScript, you can use the typeof operator to check if an object property is defined. The typeof operator returns the type of a variable or expression. If the property is defined, the typeof operator will return the type of the property (e.g. “string”, “number”, “object”, etc.). If the property is not defined, the typeof operator will return “undefined”. Here’s an example

let obj = {
  name: "John"
};

if (typeof obj.age === "undefined") {
  console.log("The 'age' property is not defined.");
}

Another way of detect an undefined property on an object is using the in operator. This operator checks if an object has a property with a given name. You can check if an object property is undefined or not by checking if the property is in the object or not. Like this:

let obj = {
  name: "John"
};

if (!('age' in obj)) {
  console.log("The 'age' property is not defined.");
}

You can also use the hasOwnProperty() method, it checks if an object has the specified property as its own property, not inherited from its prototype

let obj = {
  name: "John"
};

if (!obj.hasOwnProperty('age')) {
  console.log("The 'age' property is not defined.");
}

It is important to note that undefined values are different from null values, so if you want to check if a property has a null value, you should use strict equality operator === to check it.

let obj = {
  name: "John",
  age: null
};

if (obj.age === null) {
  console.log("The 'age' property is null");
}

It’s important to notice that if the property does not exist on the object and you use the obj.property notation it will return undefined, but if the object doesn’t exist at all, trying to access its properties will cause a ReferenceError.

Get the current URL with JavaScript?

You can use the window.location object in JavaScript to get the current URL of a web page. The location object is a property of the window object and contains information about the current URL.

Here’s an example of how you can use it to get the current URL:

var currentUrl = window.location.href;
console.log(currentUrl);

This will return the entire URL of the current page, including the protocol (e.g. “http” or “https”), the domain, and any query parameters or hash fragment.

location.origin will only give the protocol and the domain.

var originUrl = window.location.origin;
console.log(originUrl);

You can also use location.pathname to get only the path of the current page

var pathUrl = window.location.pathname;
console.log(pathUrl);

You can use location.search to get any query parameters in the current page

var searchUrl = window.location.search;
console.log(searchUrl);

It’s important to note that the location object is a read-only object, meaning you can’t use it to change the current URL of a page. If you need to change the URL, you can use the window.location object’s assign() method to load a new URL or the replace() method to replace the current URL in the browser’s history.

How do I remove a property from a JavaScript object?

To delete a property from a JavaScript object, you can use the delete operator.

For example:

let obj = {
  key1: 'value1',
  key2: 'value2'
};

delete obj.key1;

console.log(obj);  // { key2: 'value2' }

This will remove the key1 property from the obj object.

Note that the delete operator only works on own properties of an object, and not on properties inherited from the object’s prototype.

If you want to remove a property from an object and all of its prototypes, you can use the delete operator in combination with the in operator, like this:

let obj = {
  key1: 'value1'
};

let obj2 = Object.create(obj);

delete obj2.key1;  // This will delete the property from obj2, but not from obj

console.log(obj2.key1);  // undefined
console.log(obj.key1);   // 'value1'

delete obj2.key1 in obj;  // This will delete the property from obj and all of its prototypes

console.log(obj2.key1);  // undefined
console.log(obj.key1);   // undefined

How do I get the current date in JavaScript?

There are a few ways to get the current date in JavaScript, depending on what you’re trying to do.

The simplest way is to use the Date object, which is built into JavaScript. Here’s an example of how you can use it to create a new Date object representing the current date and time:

var currentDate = new Date();
console.log(currentDate);

You can also pass in a date string to create a Date object representing a specific date. For example:

var birthday = new Date("January 1, 1990");
console.log(birthday);

If you want to format the date in a specific way, you can use the various methods provided by the Date object.

Here’s an example of how you can use the toDateString() method to format the date as a string:

var currentDate = new Date();
console.log(currentDate.toDateString());

which will output date in string form : Mon Jan 10 2022

You can also use other methods like toUTCString() or toISOString() to format the date and time in a specific way.

You can also use getDate(),getMonth() and getFullYear() method to get day, month and year respectively

var currentDate = new Date();
console.log(currentDate.getDate());
console.log(currentDate.getMonth());
console.log(currentDate.getFullYear());

Keep in mind that the Date object is based on the number of milliseconds that have passed since January 1, 1970, so the output of the Date object may be affected by time zone differences.

What is the difference between “let” and “var”?

In JavaScript, let and var are both used to declare variables, but they have some important differences in terms of scope and behavior.

• The var keyword was used to declare variables before the introduction of let and const in ECMAScript 6 (ES6). Variables declared with var are function-scoped, which means they are only accessible within the function in which they were declared, or if they were declared outside any function, they are accessible throughout the global scope. If a variable is declared with var keyword inside a function, it is still accessible throughout the function, even if it’s declared inside an if-statement or for loop, for example.

function varFunction() {
  if (true) {
    var x = 5;
  }
  console.log(x); // Output: 5
}
varFunction();
console.log(x); // Output: 5

• The let keyword was introduced in ECMAScript 6 (ES6) as an alternative to var, and it is block-scoped. Variables declared with let are only accessible within the block in which they were declared.

function letFunction() {
  if (true) {
    let x = 5;
  }
  console.log(x); // ReferenceError: x is not defined
}
letFunction();

Why let and var bindings behave differently using setTimeout function?

In JavaScript, setTimeout is a function that runs a callback function after a specified amount of time has passed. However, the behavior of var and let bindings can be different when used in conjunction with setTimeout due to their different scoping rules.

When a variable is declared with var inside a function, it’s accessible throughout the entire function, regardless of whether it’s inside a block or not. This means that when a setTimeout callback function is invoked, it has access to the variables declared with var in the outer scope.

Here’s an example that shows how var behaves with setTimeout:

function varExample() {
  for (var i = 0; i < 3; i++) {
    setTimeout(function() {
      console.log(i);
    }, 0);
  }
}
varExample(); // Output: 3 3 3

In the above example, the output is 3 three times because by the time the callback function is invoked, the for-loop has already completed and the variable i has the final value of 3.

On the other hand, variables declared with let are block-scoped, which means that they’re only accessible within the block in which they were declared. let bindings can be captured by closures, so they can be used inside the callback function of setTimeout without any problem

function letExample() {
  for (let i = 0; i < 3; i++) {
    setTimeout(function() {
      console.log(i);
    }, 0);
  }
}
letExample(); // Output: 0 1 2