function definition syntax confuse in Typescript

Question

All:

[UPDATE] I figure that out: One post in SO explain this: In TypeScript how do I declare an array of functions that accept a string and return a string?
it turns out:

A function type literal of the form

( ParamList ) => ReturnType

is exactly equivalent to the object type literal

{ ( ParamList ) : ReturnType }

So, Colon: with Bracket{} <=> Arrow =>

I am pretty new to Typescript, there is one example confuses me with its syntax:

var sayHello: (input: string) : string = function (s: string) {
    return "Hello " + s;
}
var stringUtils: { (input: string): string; }[];
stringUtils.push(sayHello);

Could anyone help to give a little explanation what this code do? Especially what the first part definition does?

var sayHello: (input: string) : string = function (s: string) {
    return "Hello " + s;
}

The example says sayHello is a function variable, but when I run it, I got "=>" expected error, does this mean this syntax belong to old typescript but not exist now?

Thanks

I'm actually not even sure that's valid TypeScript...? It doesn't seem to work when I paste it here: typescriptlang.org/Playground — Katana314
– Katana314, Commented Dec 2, 2015 at 21:41
@Katana314 Thanks, neither on my side, so basically it could be a typo? — Kuan
– Kuan, Commented Dec 2, 2015 at 21:42
Could this be the intention? var sayHello: (input: string) => string = function (s: string) { return "Hello " + s; }. It would be good to get confirmation before delving into explanation — Katana314
– Katana314, Commented Dec 2, 2015 at 21:45
@Katana314 not sure, but I guess so. This is from a book called "Typescript Revealed"(Pub. 2013), it uses this example to show why we need interface — Kuan
– Kuan, Commented Dec 2, 2015 at 21:47
If the book you're reading keeps having syntax errors in it, you should find a different book...? — Ryan Cavanaugh
– Ryan Cavanaugh, Commented Dec 2, 2015 at 21:50

basarat · Accepted Answer · 2015-12-02 23:04:19Z

1

Could anyone help to give a little explanation what this code do? Especially what the first part definition does var sayHello: (input: string) : string = function (s: string) {

Should be:

var sayHello: (input: string) => string = function (s: string) {

We are saying that sayHello is a function that takes a string and returns a string : (input: string) => string . Then we assign it to such a function = function (s: string) {

edited Dec 2, 2015 at 23:04

answered Dec 2, 2015 at 22:58

basarat

278k61 gold badges475 silver badges528 bronze badges

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

3 Comments

Kuan Over a year ago

Thanks, I am reading your suggested gitbook. One thing gets me confused is the definition of interface, I wonder how to define an interface with a string member and a function member with one string parameter and return string?

Kuan Over a year ago

I read from this post: blogs.msdn.com/b/typescript/archive/2013/01/24/… when specify a function inside a interface, I am not sure why it can use colon this time? (in Basic section: it use interface Greetable { greet(message: string): void; } )

basarat Over a year ago

@Kuan that difference is covered here : stackoverflow.com/questions/32043487/… Its just a property shorthand

Collectives™ on Stack Overflow

function definition syntax confuse in Typescript

1 Answer 1

3 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

3 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related