Typed functions in TypeScript

Parameter, return, and function-type annotations.

function greet(name: string, greeting = "Hello"): string {
  return `${greeting}, ${name}!`;
}

const square = (x: number): number => x * x;

type Formatter = (value: number, digits?: number) => string;

const currency: Formatter = (value, digits = 2) =>
  `$${value.toFixed(digits)}`;

function sum(...nums: number[]): number {
  return nums.reduce((a, b) => a + b, 0);
}

How it works

1. A default value makes a parameter optional.
2. A type alias can describe a whole signature.
3. Rest parameters type as an array.

Keywords and builtins used here

• Formatter
• const
• function
• number
• return
• string
• type

The run, in numbers

Lines

14

Characters to type

366

Tokens

113

Three-star pace

95 tpm

At the three-star pace of 95 tokens a minute, this run takes about 71 seconds.

Type this snippet

Step 1 of 5 in Functions & tuples, step 16 of 23 in Types & annotations.

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