Updated February 10, 2023
Introduction to JavaScript Test Undefined
The test undefined in JavaScript is defined as a property of a global object that can specify a variable that has not been assigned a value or has not been declared at all. Because it is a non-configurable, non-writable property, the beginning of it is a primitive value in all provisional browsers. A variable that has not been assigned a value is of type undefined, and the method and statements return an undefined one. If a variable being evaluated does not have an assigned value or if a value has not been reported, the function will return an undefined one.
Key Takeaways
- JavaScript can allocate a primitive value ‘undefined’ when a variable does not have a definite allocated value.
- JavaScript has a unary operator void that can assess the expression and give back undefined.
- Also, undefined means the variable is declared, but the datatype of it is not surviving because it is unassigned.
What is JavaScript Test Undefined?
When we try to execute JavaScript, it can check whether the undefined value is present or not, which is so simple. When we declare the variable, we must assign a value but not always; sometimes variable may be defined, but the value has not been allocated. Hence JavaScript will spontaneously show the word ‘undefined,’ which may be confused with null, as both can indicate an empty phase.
JavaScript can always come up with new potentials that are simple to check. So, it has various methods for it in which users can proclaim the ‘typeof’ because of its safety, and will get fewer errors while checking for undefined. Others can proclaim the void 0, it is also easy and fast, and void and ‘typeof’ are much quicker than a direct comparison.
How to Check JavaScript Test Undefined?
In JavaScript, an undefined variable is also known as a variable without a value because it always returns ‘undefined.’ However, it is not the same as the null, which suggests an empty state after declaration. Commonly, we can assign a value to a variable, but not regularly.
When a variable proclaims without any value, JavaScript can impulsively show the ‘undefined,’ which can be done as given below.
Code:
let abc;
console.log (abc);When we try to approach the value, then an array or object will not be available in which it can flip the ‘undefined.’
Let us see an example:
Code:
let user = {
name: “Jim jam”,
age: 25
};
console.log(user.play);JavaScript Test Undefined Properties
Undefined object property: It has two meanings first, it has never been described in the object, and second, it has an undefined value.
Let us consider the following:
Code:
var o = {p: undefined}The o.p is the undefined value, so that o.q can also be undefined; then, we can say that there is no property ‘q.’
Code:
typeof o.p == 'undefined'
typeof o.q == 'undefined'
o.p === undefined
o.q === undefined
'p' in o
'q' in oWe can say that ‘typeof obj. prop = = ‘undefined’. If we have to make a spelling mistake, it would be considered a string constant.
In JavaScript, by utilizing the ‘typeof’ operator, we can scrutinize the object property in which it can give back a string that can tell them which type of operand it is. It cannot utilize the parenthesis.
Code:
const list = []
const count = 3
typeof record
typeof count
typeof "string"
typeof colorIn the above example, the record is the object, the count is the number, and ‘string’ is the string that can be returned from the type. If the value is not defined, it will give back the undefined.
JavaScript Test Undefined Methods
Given below are the JavaScript test undefined methods:
1. pqr === undefined
Users try to use the pqr === undefined because it is understandable. On the other hand, others can try using the ‘typeof.’ Because it is secure and can make fewer errors while checking the undefined JavaScript values.
Code:
let buyer = {
name: "Jim",
age: 24
};
if (buyer.interest === undefined) {
console.log("This is undefined");
}2. pqr === void (0)
It is somewhat faster than the ‘typeof.’ Because utilizing the ‘void 0’ is straightforward and speedy. So, we can utilize any value instead of ‘0’; it will continually assess the undefined. It can give the value of undefined. If void ‘0’ does not have, we can say that void and ‘typeof’ are more rapid than direct comparison over undefined.
Code:
console.log (void 0);3. typeof pqr === ‘undefined’
The ‘typeof’ does not always give back the error, and it works when a variable has been proclaimed. By using typeof, we can check whether the object property is undefined. The ‘typeof’ can give back an undefined string that can decide the operand type.
The typeof operator can return the undefined. Hence, we can scrutinize the undefined value by utilizing the ‘typeof’ operator and the === operator with ‘typeof’ for scrutinizing the null values.
Code:
const car = {
model: 'Maruti'
}
if (typeof car.color === 'undefined') {
}Conclusion
In this article, we conclude that the test undefined is the property of JavaScript, which can describe a variable globally. We have also discussed how to check the JavaScript test undefined, its properties, its method, key takeaways, and FAQs of it. So this article will help to understand the concept.
FAQ
Given below are the FAQs mentioned:
Q1. How to determine if a variable is undefined or null in JavaScript?
Answer: The equality operator (==) has been utilized to check whether the variable is undefined or not.
Q2. What is the undefined error?
Answer: When we proclaim a variable in the code but do not allocate a value to it before impressing the variable, then it can be considered an undefined error.
Q3. Is NULL === undefined JavaScript?
Answer: It means that null is equal to undefined but not similar. When we try to substitute a variable for undefined, we have to assume that the variable is not survived.
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