Variadic Functions in TypeScript

Variadic functions can be called with any number of trailing arguments. For example, console.log in JavaScript (and TypeScript) is a common variadic function.

Here’s a function that will take an arbitrary number of integers as arguments.

function sum(...nums: number[]): void {
    console.log(nums, " ");
    let total = 0;
    for (const num of nums) {
        total += num;
    }
    console.log(total);
}

Within the function, the type of nums is equivalent to number[]. We can call nums.length, iterate over it with for-of, etc.

Variadic functions can be called in the usual way with individual arguments.

sum(1, 2);
sum(1, 2, 3);

If you already have multiple arguments in an array, apply them to a variadic function using the spread operator like this.

const nums = [1, 2, 3, 4];
sum(...nums);

To run the TypeScript code, put the code in a .ts file, compile it to JavaScript using tsc, and then run the compiled JavaScript file.

$ tsc variadic-functions.ts
$ node variadic-functions.js

The output should be:

[ 1, 2 ] 3
[ 1, 2, 3 ] 6
[ 1, 2, 3, 4 ] 10

Another key aspect of functions in TypeScript is their ability to form closures, which we’ll look at next.

Next example: Closures.