open-ended function arguments with TypeScript

Question

IMO, one of the main concerns of the TypeScript language is to support the existing vanilla JavaScript code. This is the impression I had at first glance. Take a look at the following JavaScript function which is perfectly valid:

Note: I am not saying that I like this approach. I am just saying this is a valid JavaScript code.

function sum(numbers) { 

    var agregatedNumber = 0; 
    for(var i = 0; i < arguments.length; i++) { 
        agregatedNumber += arguments[i];
    }

    return agregatedNumber;
}

So, we consume this function with any number of arguments:

console.log(sum(1, 5, 10, 15, 20));

However, when I try this out with TypeScript Playground, it gives compile time errors.

I am assuming that this is a bug. Let's assume that we don't have the compatibility issues. Then, is there any way to write this type of functions with open-ended arguments? Such as params feature in C#?

Just curious, why do you even have a named numbers parameter? You're not doing anything with it. — Justin Morgan
– Justin Morgan, Commented Aug 30, 2013 at 17:49
@JustinMorgan having at least one parameter lets Intellisense at least hint that there should be some arguments. — drzaus
– drzaus, Commented Jan 25, 2015 at 19:44

David Harkness · Accepted Answer · 2014-10-29 18:41:22Z

328

The TypeScript way of doing this is to place the ellipsis operator (...) before the name of the argument. The above would be written as,

function sum(...numbers: number[]) {
    var aggregateNumber = 0;
    for (var i = 0; i < numbers.length; i++)
        aggregateNumber += numbers[i];
    return aggregateNumber;
}

This will then type check correctly with

console.log(sum(1, 5, 10, 15, 20));

edited Oct 29, 2014 at 18:41

David Harkness

36.6k11 gold badges119 silver badges136 bronze badges

answered Oct 2, 2012 at 19:57

chuckj

29.8k7 gold badges57 silver badges56 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

tugberk Over a year ago

Great! thanks. This is definitely the way to go. So, if someone has an existing code as in my question, it will break. Right?

chuckj Over a year ago

Yes. It complains about the call but you can declare sum as accepting multiple parameters of any type by changing the signature to sum(...) instead an it will quiet the error. Please feel free to submit this as a bug on CodePlex.

A. M. Over a year ago

Was looking for a rest parameter example. Ty.

Al.G. Over a year ago

Note that a generic variant of this has been implemented in 2020: github.com/microsoft/TypeScript/pull/39094

Jeroen van Dijk-Jun · Accepted Answer · 2018-08-08 06:27:59Z

11

In addition to @chuckj answer: You can also use an arrow function expression in TypeScript (is kind of a lambda in Java / .NET)

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

edited Aug 8, 2018 at 6:27

answered Mar 19, 2017 at 9:37

Jeroen van Dijk-Jun

1,03810 silver badges22 bronze badges

Comments

Sheo Dayal Singh · Accepted Answer · 2018-05-10 06:56:14Z

6

In Typescript it is the concept of Rest Parameter, it is the parameter which receives multiple values of similar type.If we target the typescript then we have to write the code ECMAScript 6 standard,then typescript transpiler converts it to its equivalent java script code(which is ECMAScript 5 standard).If we use typescript then we have to use three dot(...) preferx with the restparameter variable name, such as function sum(...numbers: number[]), then it would work.

Note: Rest Parameter must be last parameter in the parameter list.likewise function sum(name:string,age:number,...numbers: number[]).

answered May 10, 2018 at 6:56

Sheo Dayal Singh

1,71321 silver badges12 bronze badges

Collectives™ on Stack Overflow

open-ended function arguments with TypeScript

3 Answers 3

4 Comments

Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

4 Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related