Spread operator in 'new' expressions support for ES5?

[saschanaz]

I've found that #1931 supports this only for ES6, but it seems ES5 also can do this in any way.

var arguments = [1, 2, 3, 4, 5];
new Array(...arguments);

This can be converted to ES5:

// Thanks to: http://stackoverflow.com/a/14378462/2460034
var arguments = [1, 2, 3, 4, 5];
new (Array.bind.apply(Array, [null].concat(arguments)));
// Result: [1, 2, 3, 4, 5]

[RyanCavanaugh]

Seems reasonable enough.

[RyanCavanaugh]

Approved. Be sure to handle the case where the operand to new is a computed expression that needs to be cached in a temporary variable.

[tinganho]

I'm almost done with a PR on this one. Though there are one issue with the type checker:

interface ObjectWithSpreadMethod {
    spreadMethod(x: number, y: number, ...z: string[]);
}

var spread: string[];
var objectWithSpreadMethod: ObjectWithSpreadMethod;

new objectWithSpreadMethod.spreadMethod(1, 2, "string");
new objectWithSpreadMethod.spreadMethod(1, 2, ...spread);
new objectWithSpreadMethod.spreadMethod(1, 2, ...spread, "string"); 

Isn't it correct TS code above? Why can't I do new objectWithSpreadMethod.spreadMethod on a method?

Shall I correct this behavior?

Playground:
http://www.typescriptlang.org/Playground#src=interface%20ObjectWithSpreadMethod%20%7B%0D%0A%20%20%20%20spreadMethod(x%3A%20number%2C%20y%3A%20number%2C%20...z%3A%20string%5B%5D)%3B%0D%0A%7D%0D%0A%0D%0Avar%20spread%3A%20string%5B%5D%3B%0D%0Avar%20objectWithSpreadMethod%3A%20ObjectWithSpreadMethod%3B%0D%0A%0D%0Anew%20objectWithSpreadMethod.spreadMethod(1%2C%202%2C%20%22string%22)%3B%0D%0Anew%20objectWithSpreadMethod.spreadMethod(1%2C%202%2C%20...spread)%3B%0D%0Anew%20objectWithSpreadMethod.spreadMethod(1%2C%202%2C%20...spread%2C%20%22string%22)%3B%20

[RyanCavanaugh]

Seems like it should be typed like this:

interface ObjectWithSpreadMethod {
    spreadMethod: {
        new (x: number, y: number, ...z: string[])
    };
}

Alternatively:

interface ObjectWithSpreadMethod {
    spreadMethod(x: number, y: number, ...z: string[]): void;
}

The code as written has spreadMethod returning any, which is not a valid invocation target for new.

[tinganho]

@RyanCavanaugh Ok thanks, I will land a PR later.

[mhegazy]

thanks @tinganho!

[mdcone]

Does this seem to just be a happy coincidence with Array? Using this technique with my own TS classes doesn't work.

Seems as if it really is impossible. According to MDN --

Apply for new

Example: In ES5 it is not possible to compose new with apply (in ES5 terms apply does a [[Call]] and
not a [[Construct]]). In ES6 the spread syntax naturally supports this.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Spread_operator

Further reading into the ES6 standard:

The Array constructor is the %Array% intrinsic object and the initial value of the Array property of the global object. When called as a constructor it creates and initializes a new exotic Array object. When Array is called as a function rather than as a constructor, it also creates and initializes a new Array object. Thus the function call Array(…) is equivalent to the object creation expression new Array(…) with the same arguments.

Which tells me that using something that does a [[Call]] with an array (.apply() or .call()) also has the same effect as a [[Construct]].

I'm fine with allowing this behavior in this case, so maybe allowing for a warning to be emitted instead of an error, or for special casing the errors to give a hint to the programmer as to why a program isn't behaving as expected might be a useful thing.

[tinganho]

@mdcone could you share your classes? I tested it with TS classes and it seems to work for me.