typescript-2.1-json.ts

type Json = null | boolean | string | number | Array<{}> | {}

function tArray<T>(f:(t:any) => t is T) {
    return function (a:any): a is Array<T> {
        if (!Array.isArray(a)) return false;
        for (var k = 0; k < a.length; ++k)
            if (!f(a[k])) return false;
        return true;
    }
}

function tString(t: any): t is string { return typeof t === 'string' }

function tBoolean(t: any): t is boolean { return typeof t === 'boolean' }

function tObject<T>(checker: {[p in keyof T]: (t:any) => t is T[p]}) {
  return function(obj: any): obj is {[p in keyof T]: T[p] } {
    for (var key in checker) if (!checker[key](obj[key])) return false;
    return true;
  }
}

var isTodoList = tObject<{items: Array<{text: string, completed: boolean}>, showCompleted: boolean}>({
  items: tArray(tObject<{text: string, completed: boolean}>({text: tString, completed: tBoolean})),
  showCompleted: tBoolean
})



function test(j:Json) {
    if (isTodoList(j)) {

    }
}