what is ... syntax? (found in Angular2)

Question

I don't know if this syntax or symbol or whatever belongs to Angular2 or something else...

but I wonder with,

what ... syntax is and how it works behind the scene?

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

Answer 1

It's the TypeScript rest operator. In this case, it means any number of arguments of any type can occur there; the function will see them as an array of any. (JavaScript recently got rest and spread operators as well, as of ES2015, but the :any[] in your example tells us this is TypeScript.)

E.g.:

ngOnChanges('a', 42, null);

...would show

["a", 42, null]

in the console.

Here's a complete example (live copy):

function foo(...args:any[]) {
    console.log("args.length = " + args.length);
    args.forEach((arg:any, index:Number) => {
        console.log("args[" + index + "]: " + arg);
    });
}
foo('a', 42, null);

outputs

args.length = 3
args[0]: a
args[1]: 42
args[2]: null