Variable Number of Arguments in TypeScript

JavaScript has always supported functions with variable number of arguments, although the syntax required some getting used to. In TypeScript and ECMAScript 6 rest parameters and spread syntax provide a more convenient alternative that also works with arrow functions.

The JavaScript Way

There is an arguments object available in every JavaScript function as a local variable. It contains the array of arguments that where passed to the function:

function varArgs() {
  console.log(arguments.length);
}

To pass the array to another function accepting a variable number of arguments, the apply function can be used:

function varArgs() {
  console.log.apply(console, arguments);
}

If you try to use the arguments object inside an arrow function, it won't work:

The 'arguments' object cannot be referenced in an arrow function in ES3 and ES5. Consider using a standard function expression.

Instead of using a function and manually handling this value, you can now use rest parameters and spread syntax.

The TypeScript / ECMAScript 6 Way

Rest parameters can be used to capture variable number of arguments into an array in a way similar to other C-like languages:

function varArgs(...args: any[]) {
  console.log(args.length);
}

To pass the array to a function accepting a variable number of arguments, spread syntax can be used:

function varArgs(...args: any[]) {
  console.log(...args);
}

This approach works just as well with arrow functions:

let varArgs = (...args: any[]) => {
  console.log(...args);
}