What does the question mark (?) on a typescript variable/parameter name denote?

Question mark on a typescript variable marks it as optional. Any optional parameters must follow required parameters.

function sayHello(name: string, salutation?: string) {
    if (salutation)
        return  "Hello " + saluation +"." +  name;
    else
        return "Hello " + name;
}

The optional parameters will have value as undefined when unused.

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What does the question mark (?) on a typescript variable/parameter name denote?

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