TypeScript Optional and Default Parameters
An optional parameter may be left out at a call. A default parameter supplies a value when its argument is left out.
An optional parameter
The greeting may be left out. Inside the function, an omitted parameter holds undefined.
function greet(name: string, greeting?: string): string {
if (greeting === undefined) {
return "Hello, " + name;
}
return greeting + ", " + name;
}
console.log(greet("Ada"));
console.log(greet("Ada", "Welcome"));A default parameter
The tax rate defaults to 0.1. A call may pass its own rate instead.
function total(price: number, taxRate: number = 0.1): number {
return price + price * taxRate;
}
console.log(total(100));
console.log(total(100, 0.25));One argument too many is refused
The function takes one parameter. A call with two arguments fails the check.
function greet(name: string): string {
return "Hello, " + name;
}
console.log(greet("Ada", "Ben"));Example
function receipt(item: string, quantity: number = 1, note?: string): string {
let line: string = item + " x" + quantity;
if (note !== undefined) {
line = line + " (" + note + ")";
}
return line;
}
console.log(receipt("Notebook"));
console.log(receipt("Pen", 3));
console.log(receipt("Mug", 2, "gift"));Key points
- A question mark after a parameter name makes that parameter optional.
- An optional parameter comes after every required parameter.
- Inside the function, an omitted optional parameter holds the value undefined.
- A default value is written with an equals sign after the parameter type.
- A call that omits a default parameter uses the default value.
- A call that passes a value for a default parameter uses that value instead.
In this example, quantity has a default value of 1 and note is optional. The first call passes only an item, so the quantity is 1 and no note is added. The second call passes its own quantity. The third call also passes a note, so the note joins the line in brackets.
Try it Yourself
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Exercise
What makes a parameter optional in TypeScript?