TypeScript constraints are used in generic type declarations to restrict the types that can be used as type arguments for the generic. Constraints allow you to specify that a generic type parameter must meet certain criteria, such as implementing a particular interface, having a specific method, or extending a specific class. This ensures type safety and enhances code predictability.

Syntax:

function functionName<T extends ConstraintType>(param: T): ReturnType {
      // Function implementation
}

Parameters:

• T is the generic type parameter.
• extends is used to specify the constraint.
• ConstraintType is the type or interface that T must extend or implement.
• param is the parameter that must be of type T.
• ReturnType is the return type of the function.

Example 1: In this example, we are getting some parameters and we are restricting that parameter, which means the parameter value can be anything other than a number. and it returns the output according to their length method.

Output:

"x is smaller than y" 
"x is larger than y"

Example 2: In this example, we are giving numbers as a parameter and it is throwing error as numbers do not have length method. this is how we can limit the type.

Output:

"x is smaller than y"
"x is larger than y"
Argument of type 'number' is not assignable to parameter of type '{ length: number; }'.