Freewind Freewind - 1 year ago 138
TypeScript Question

How to use underscore lib from DefinitelyTyped with typescript?

DefinitelyTyped has provided a underscore declaration file, which defines a

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

// <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
fit the interface

Where is wrong, and how to fix it?

Answer Source

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;
Recommended from our users: Dynamic Network Monitoring from WhatsUp Gold from IPSwitch. Free Download