Description

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Beginner · 10 min

Typed functions

Describe function parameters and return values.

Type the inputs

Parameter annotations describe what callers may supply. In strict mode, a parameter without an inferred or explicit type is an error.

function double(value: number): number {
  return value * 2;
}
console.log(double(4));

Output

8

Describe the result

A return annotation helps detect an accidental change in a function result. A default parameter also gives TypeScript information about its type.

function greet(name: string, greeting = "Hello"): string {
  return greeting + ", " + name;
}
console.log(greet("Ada"));

Output

Hello, Ada

Put it into practice

  1. Read the types and predict the output before running the examples.
  2. Complete the task, then check your understanding with the practice questions.

Try it yourself

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Return a numeric total

Make "total" return the price multiplied by the quantity. Keep its return type as number.

Practice