JavaScript Check If Variable Is Undefined

JavaScript Check If Variable Is Undefined | Since a null variable can pass a check for undefined in JavaScript if it is not expressed properly, determining if a variable is undefined can be a little tricky. Because of this, defined values can pass through and vice versa. Make careful to check if a value is equivalent to undefined using rigorous equality ===.

JavaScript Check If Variable Is Undefined Example

let x;
const y = null;

console.log(x == null && y == undefined); // true
console.log(x === null || y === undefined); // false

console.log(x == null && typeof x == 'object'); // false
console.log(x === undefined && typeof x == 'undefined'); // true

Output:-

true
false
false
true

Checking to see whether typeof x === “undefined” is another option. The main distinction between these two methods is that typeof does not raise a ReferenceError if x has not yet been declared, whereas x === undefined does.

JavaScript determines if x is a declared variable that is absolutely equal to undefined when it uses the expression x === undefined. We can use typeof x === ‘undefined’ to determine whether x is precisely equal to undefined irrespective of whether it has been declared or not.

 x === undefined; // Throws a ReferenceError
 typeof x == 'undefined'; // true

Identifying Undefined Object Properties

The ability to access a property that does not exist in the object and have JavaScript report its value as undefined rather than raising a ReferenceError makes it tricky to determine whether an object property is undefined.

const obj = { answer: 2, question: undefined };

console.log(obj.answer); // 2
console.log(obj.question); // undefined
console.log(obj.notInObject); // undefined

Output:-

2
undefined
undefined

In JavaScript, the expression obj.propName === undefined is valid if any obj has a property named “propName” and whose value is strictly equivalent to undefined, or obj doesn’t. Use the in operator to determine whether an object has a property if the property is strictly equal to undefined.

const obj = { answer: 20, question: undefined };

console.log('question' in obj && obj.question === undefined); // true
console.log('notInObject' in obj && obj.notInObj === undefined); // false

Output:-

true
false

If a variable is declared in JavaScript but not given a value, it is automatically given the value undefined. Therefore, the word “undefined” will be shown if you attempt to display the value of such a variable. The null, on the other hand, is a special assignment value that may be used to represent no value by assigning it to a variable.

In plain English, a null value denotes the absence of a value, while an undefined variable is one that has been declared but has not yet been given a value.

You can use the equality operator == or strict equality operator === to determine whether a variable is undefined or null (also called the identity operator).

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