Freewind Freewind - 1 month ago 16
TypeScript Question

How to use underscore lib from DefinitelyTyped with typescript?

DefinitelyTyped has provided a underscore declaration file, which defines a

List
interface, and used it heavily in the code.

// Common interface between Arrays and jQuery objects
interface List {
[index: number]: any;
length: number;
}

interface UnderscoreStatic {
sortBy(list: List, iterator?: any, context?: any): any;
groupBy(list: List, iterator: any): any;
countBy(list: List, iterator: any): any;
}


I'm trying to use the
countBy
function:

// <reference path="../DefinitelyTyped/underscore/underscore.d.ts" />

declare var _: UnderscoreStatic;

_.countBy([1,2,3], function(item) {
return item%2;
});


When I compile the file, it throws error:

> tsc commons.ts

> E:/commons.ts(5,0): Supplied parameters do not match any signature of call target:
Could not apply type 'List' to argument 1, which is of type 'number[]'


I don't know why this error happened, since
number[]
fit the interface
List
.

Where is wrong, and how to fix it?

Answer

You need to pass an object compatible with the List interface, which is an array with a length:

/// <reference path="underscore.d.ts" />

var list: List;
list[0] = 1;
list[1] = 2;
list[2] = 3;
list.length = 3;

_.countBy(list, function (item) {
    return item % 2;
});

In all honesty, an array technically fulfils this as it has a length property - but the above code compiles.

The shorthand version of this is a bit nasty:

/// <reference path="underscore.d.ts" />

var list = <List><any> [1, 2, 3];

_.countBy(list, function (item) {
    return item % 2;
});
Comments