In TypeScript (and JavaScript), ?? is a syntax known as the Nullish Coalescing Operator. It is used to return the right operand when the left operand is null or undefined, otherwise it returns the left operand. This is particularly useful for handling values that may be null or undefined, especially when setting default values for variables.
Syntax:
let result = value1 ?? value2;
Example:
let name: string | null = null;
let defaultName = "Default Name";
let displayName = name ?? defaultName;
console.log(displayName); // Outputs "Default Name"
In this example, since name is null, displayName will be assigned the value of defaultName, which is "Default Name".
Difference from ||:
Unlike the logical OR operator (||), the Nullish Coalescing Operator only returns the right operand when the left operand is null or undefined, while the logical OR operator returns the right operand when the left operand is a falsy value (such as null, undefined, false, 0, NaN, or "").
let value1 = 0;
let value2 = value1 || 10;
let value3 = value1 ?? 10;
console.log(value2); // Outputs 10, because 0 is considered a falsy value
console.log(value3); // Outputs 0, because 0 is not null or undefined
In this example, value2 is assigned 10 because value1 is a falsy value, while value3 is assigned 0 because value1 is not null or undefined.