TypeScript Functions Optional and Default Parameters
Example
Optional Parameters
In TypeScript, every parameter is assumed to be required by the function. You can add a ? at the end of a parameter name to set it as optional.
For example, the lastName parameter of this function is optional:
function buildName(firstName: string, lastName?: string) {
// ...
}
Optional parameters must come after all non-optional parameters:
function buildName(firstName?: string, lastName: string) // Invalid
Default Parameters
If the user passes undefined or doesn't specify an argument, the default value will be assigned. These are called default-initialized parameters.
For example, "Smith" is the default value for the lastName parameter.
function buildName(firstName: string, lastName = "Smith") {
// ...
}
buildName('foo', 'bar'); // firstName == 'foo', lastName == 'bar'
buildName('foo'); // firstName == 'foo', lastName == 'Smith'
buildName('foo', undefined); // firstName == 'foo', lastName == 'Smith'
