Nullish coalescing operator in Typescript

The nullish coalescing operator (?? or typescript double question mark)  is a logical operator that takes two arguments. It returns the right-hand side operand when its left-hand side operand is null or undefined. otherwise, it returns its left-hand side operand.

Syntax

The syntax of the nullish coalescing operator or typescript double question mark is as follows.

leftExpr ?? rightExpr

If leftExpr is not null or undefined, then returns leftExpr

If leftExpr is either null or undefined, then returns rightExpr

Example

In the following code, since email is undefined, the ?? returns the "No Email Selected".

let email

let selecteditem =  email ?? "No Email Selected";

console.log(selecteditem)   //No Email Selected

If the email already has a value, it returns the email.

let email="[email protected]"

let selecteditem =  email ?? "No Email Selected";

console.log(selecteditem)   //[email protected]

If the email has any value other than null or undefined, it returns it even if it is zero or empty string, etc.

let email=""
let selecteditem =  email ?? "No Email Selected";
console.log(selecteditem)   //""


email="0"
selecteditem =  email ?? "No Email Selected";
console.log(selecteditem)   // 0

Difference with ||

You can also make use of the logical OR operator. Because null & undefined are falsy values.

let email
let selecteditem =  email || "No Email Selected";
console.log(selecteditem)   //No Email Selected 


email="[email protected]"
selecteditem =  email || "No Email Selected";
console.log(selecteditem)   //"[email protected]" 

But, if email has other falsy values like false, 0,-0 , 0n, "", NaN, then the || & ?? produces different outputs.

let email=""
let selecteditem =  email || "No Email Selected";
console.log(selecteditem)   //No Email Selected


selecteditem =  email ?? "No Email Selected";
console.log(selecteditem)   //"" 

Difference with Conditional Operator

We can mimic the ?? using the Ternary Conditional Operator.

let email
let selecteditem =  (email != undefined && email != null ) ? email:  "No Email Selected"
console.log(selecteditem)   //No Email Selected

Assiging Default Values

One of the use cases of the ?? operator is assigning the default value.

We usually use the || to assign a default value. In the following example, we have not given any value to score. Hence it is undefined. Both the || & ?? does not return the score, but prints the message “Please enter your score”

let score;

console.log(score|| "Please enter your score")
 //Please enter your score
console.log(score?? "Please enter your score")
 //Please enter your score

Now, assume that the user entered his score as 0, which is a valid score. But the || operator still returns the message “Please enter your score”, because 0 is false. But the ?? returns the score correctly.

let score=0
console.log(score || "Please enter your score")
 //Please enter your score
console.log(score ?? "Please enter your score")
 //0

Short-circuiting

?? like || & && operators, stops evaluating if the left-hand operand is not null/undefined. Hence the right-hand expression is not evaluated.

For Example, the following code evaluates the getNumber and values of a becomes 11.

let a=10 

function getNumber() {
   a=a+1
   return a;
}

let b = ( null ?? getNumber() )   //getNumber() is invoked
console.log(a);  //11

But, here the ?? returns with the string ‘Hello’ and getNumber() is never invoked. The value of a remains at 10.

let a=10 

function getNumber() {
   a=a+1
   return a;
}


let b = ( "Hello" ?? getNumber() )   //getNumber() is not invoked

console.log(a);  //10

Summary

• The leftExpr ?? rightExpr is the syntax of the nullish coalescing operator or typescript double question mark.
• If leftExpr is not null or undefined, then returns leftExpr else returns the rightExpr.

Reference

1. Expressions & Operators
2. Precedency & Associativity

Read More

1. Complete Typescript Tutorial
2. Typescript Operators
3. Arithmetic Operators
4. Unary plus / Unary minus Operators
5. Increment/Decrement Operators
6. Comparison / Relational Operators
7. Equality Operator / Strict Equality Operators
8. Ternary Conditional Operators
9. Logical Operators
10. Bitwise Operators
11. Assignment Operators
12. Nullish coalescing operator
13. Comma Operator in Typescript
14. Operator Precedence