The null values show the non-appearance of any object value. It is usually set on purpose to indicate that a variable has been declared but not yet assigned any value.
This contrasts null from the similar primitive value undefined , which is an unintentional absence of any object value.
That is because a variable that has been declared but not assigned any value is undefined, not null.
Approaches: There are many ways to check whether a value is null or not in JavaScript:
- By equality Operator (===)
- By Object.is() Function
- By the typeof Operator
1. By equality Operator (===): By this operator, we will learn how to check for null values in JavaScript by the (===) operator. This operator only passes for null values not for undefined, false, 0, NaN.
Syntax:
x === y;
Example: The following code snippets show some comparison of objects.
Javascript
<script> const object1 = { key: "value", }; const object2 = { key: "value", }; console.log(object1 === object2); console.log(object1 === object1); </script> |
Output:
false true
2. By Object.is() function: This function checks whether two objects’ values are equal or not. If they are the same the two object’s values are the same if both values are null.
Syntax:
Object.is( a, null );
Example:
Javascript
<script> let maybeNull = null // The following is equivalent to // maybeNull == null // or maybeNull == undefined: console.log(Object.is(maybeNull, undefined) || Object.is(maybeNull, null)) // Compare to the following: console.log(maybeNull == null) console.log(maybeNull == undefined) console.log(maybeNull === null) console.log(Object.is(maybeNull, null)) console.log(maybeNull === undefined) console.log(Object.is(maybeNull, undefined)) maybeNull = undefined console.log(maybeNull === undefined || maybeNull === null) console.log(maybeNull == null) console.log(maybeNull == undefined) console.log(maybeNull === null) console.log(Object.is(maybeNull, null)) console.log(maybeNull === undefined) console.log(Object.is(maybeNull, undefined)) </script> |
Output:
true true true true true false false true true true false false true true
3. By the typeof Operator: Using typeof operator is very useful because if a variable is not declared, it will show ReferenceError. But, the typeof a non-declared value is undefined, so using the typeof is a better way to check for null and undefined variables.
Syntax:
typeof var;
Example:
Javascript
<script> // This will safely check a value to make sure it // has been declared and assigned a value other // than null or undefined: console.log(typeof undeclaredVariable !== "undefined" && (typeof undeclaredVariable !== "object" || !undeclaredVariable)) // Compare to checking for null using ==, // which will only work for declared variables: try { undeclaredVariable == null } catch (e) { console.log(e) } try { undeclaredVariable === null } catch (e) { console.log(e) } try { undeclaredVariable === undefined } catch (e) { console.log(e) } let declaredButUndefinedVariable // Values that have been declared but not // assigned a value will have the value // undefined, which has a typeof undefined console.log(typeof declaredButUndefinedVariable !== "undefined" && (typeof declaredButUndefinedVariable !== "object" || !declaredButUndefinedVariable)) let stringVariable = "Here's Johnny!" // If the variable has been both declared and // assigned a value that is neither null or undefined, // we will get true. console.log(typeof stringVariable !== "undefined" && (typeof stringVariable !== "object" || !stringVariable)) // This can be used to create a function that will return // false for undefined, undeclared, and null values. const isNotNullNorUndefined = (value) => typeof value !== "undefined" && (typeof value !== "object" || !value) console.log(isNotNullNorUndefined(stringVariable)) </script> |
Output
...js:7:7)
at Module._compile (node:internal/modules/cjs/loader:1155:14)
at Object.Module._extensions..js (node:internal/modules/cjs/loader:1209:10)
at Module.load (node:internal/modules/cjs/loader:1033:32)
at Function.Module._load (node:internal/modules/cjs/loader:868:12)
at Function.executeUserEntryPoint [as runMain] (node:internal/modules/run_main:81:12)
at node:internal/main/run_main_module:22:47
ReferenceError: undeclaredVariable is not defined
at Object.<anonymous> (/home/guest/sandbox/55da4d93-5c09-4cd3-8be9-a1459e8ed1b0.js:8:7)
at Module._compile (node:internal/modules/cjs/loader:1155:14)
at Object.Module._extensions..js (node:internal/modules/cjs/loader:1209:10)
at Module.load (node:internal/modules/cjs/loader:1033:32)
at Function.Module._load (node:internal/modules/cjs/loader:868:12)
at Function.executeUserEntryPoint [as runMain] (node:internal/modules/run_main:81:12)
at node:internal/main/run_main_module:22:47
false
true
true