How to isset Equivalent in JavaScript

1. isset Equivalent in JavaScript
2. Check if Value Is Set or Not Using the in Operator in JavaScript
3. Check if Value Is Set or Not Using Object.hasOwnProperty() in JavaScript

PHP provides a function isset that determines if a variable is set; this means if a variable is declared and assigned a value other than null. When a variable is assigned to null, isset() will return false. In JavaScript, we need to determine whether a variable is set or not.

In this post, we will learn how to check if a variable is set or not.

isset Equivalent in JavaScript

JavaScript provides the typeof operator. It returns a string indicating the operand type that wasn’t evaluated. This operator is useful when the operand type is unknown, and depending on the type, you can perform the next task.

If the operand type is a number, add the numbers, then it will concatenate the strings.

Syntax:

typeof operand typeof (operand)

It requires an expression representing the primitive or object whose type must be returned. There are eight results depending on the expression passed to it. It can either be undefined, string, symbol, function, object, boolean, number, or object.

For more information, read the documentation of typeof.

const inputObject = {
  id: 42,
  name: 'John Doe'
};

let phone

console.log(typeof inputObject.id !== 'undefined');
if ((typeof phone === 'undefined')) {
  phone = '7878787878';
}
console.log(phone);

In the example, we defined two properties of an object id and name. When you check typeof inputObject.id, it will check if the id property exists in inputObject. To check the opposite condition, add ! before the condition, then it will negate the check.

For a single variable, you can check if it’s set or not. You can add an assigned value to phone if it doesn’t exist.

The output will look like this.

true
"7878787878"

Check if Value Is Set or Not Using the in Operator in JavaScript

The in operator is a built-in method to verify that the specified property belongs to an object or its prototype chain. It repeats the object and returns the Boolean value according to the result.

Syntax:

prop in object

This function takes prop as input in the string format, and it’s a required parameter. This method checks whether the specified property or the array index exists in Object or prototype chain. Since the array is derived from an object, this method can also be called for an array.

For more information, read the documentation of the in operator method.

const inputObject = {
  id: 42,
  name: 'John Doe'
};

console.log('name' in inputObject);
if (!('country' in inputObject)) {
  inputObject.country = 'India';
}
console.log(inputObject);

In the example, we defined two properties of an object id and name. If you pass name in inputObject, it checks whether the name property exists in inputObject or not. Add ! in front of the condition to verify the opposite condition, refusing verification.

You can add a new country property if it doesn’t exist within an object. The output will look like this.

true
{
  id: 42,
  name: "John Doe",
  country: "India"
}

Check if Value Is Set or Not Using Object.hasOwnProperty() in JavaScript

hasOwnProperty is also a built-in method that checks whether the specified property exists in an Object or not. It repeats the object and returns the Boolean value according to the result.

Syntax:

Object.prototype.hasOwnProperty(prop);

This function takes prop as input in the format string and is a required parameter. This method checks whether the specified property exists or not regardless of the value in Object.

Even if the property’s value is null or undefined, it will return true. This method can also be called for an Array, since the array is derived from an object.

For more information, read the documentation of hasOwnProperty() method.

const inputObject = {
  name: 'John Doe'
};

console.log(inputObject.hasOwnProperty('name'));
if (!inputObject.hasOwnProperty('email')) {
  inputObject.email = 'Johndoe@gmail.com';
}
console.log(inputObject);

In the example, we defined two properties of an object id and name. If you pass inputObject.hasOwnProperty ('name'), it will check whether the name property in inputObject exists or not. Add ! in front of the condition to verify the opposite condition, refusing verification.

We can add a new property if it doesn’t exist in an object. The output is shown below.

true
{
  email: "Johndoe@gmail.com",
  name: "John Doe"
}

The difference between hasOwnProperty and in operator is that hasOwnProperty returns false if the property is inherited within the object or not declared. In contrast, the in operator doesn’t validate the object in the specified property - prototype string.

The base class Object has hasOwnProperty as its function, so it returns false, while in operator returns true.