I am trying to write a memoization function, but keep getting the following error.
Error - "TypeError: getNthFibonacciNo is not a function
at dabebimaya.js:28:38
at https://static.jsbin.com/js/prod/runner-4.1.4.min.js:1:13924
at https://static.jsbin.com/js/prod/runner-4.1.4.min.js:1:10866"
How can I find this error in my code? I have tried googling the error with no avail. Please point out any additional errors too if possible.
function memoize(fn) {
var cache = {};
if (cache[arguments[0]]!==undefined) {
return cache[arguments[0]];
}
else {
var value = fn.apply(this, arguments);
cache[arguments[0]] = value;
return value;
}
}
var getNthFibonacciNo = memoize(function(n){
//1,1,2,3,5,8,13,21,34
if(i<=2)
return 1;
var fib = [0,1,1];
for(var i=3;i<=n;i++) {
fib[i] = fib[i-2]+fib[i-1];
}
return fib[n];
});
console.log(getNthFibonacciNo(7));
Your memoize function isn't returning a function.
function memoize(fn) {
var cache = {};
return function() {
if (cache[arguments[0]]!==undefined) {
return cache[arguments[0]];
}
else {
var value = fn.apply(this, arguments);
cache[arguments[0]] = value;
return value;
}
}
}
now returns a function so that it can be called multiple times.
Usage
function test(a) {
console.log('calling test', a);
return a + 1;
}
const memoized = memoize(test);
memoized(1); // prints calling test and returns 2
memoized(1); // returns 2
memoized(2); // prints calling test and returns 3
I managed to fix my code after suggestions by AnilRedshift. Below is the fixed code.
function memoize(fn) {
var cache = {};
return function() {
var key = JSON.stringify(arguments);
if (cache[key]) {
console.log('cache used');
return cache[key];
}
else {
var value = fn.apply(this, arguments);
cache[key] = value;
console.log('cache not used');
return value;
}
};
}
var fibonacciMemoized = memoize(function(n) {
//1,1,2,3,5,8,13,21,34
if(i<=2)
return 1;
var fib = [0,1,1];
for(var i=3;i<=n;i++) {
fib[i] = fibonacciMemoized(i-2)+fibonacciMemoized(i-1);
}
return fib[n];
});
console.log(fibonacciMemoized(7));
console.log(fibonacciMemoized(9));
Related
I have a function, which calls another function and so on like
var fn1 = function() { return 'bar'; };
var fn2 = function() {
return fn1;
};
var fn3 = function() {
return fn2;
};
now this can keep on going.
Therefore I would like to recursively call a method, till I get a string/value i.e. the innermost return statement.
I tried like this:
function p (val){
var res = val;
while(typeof(res)=="function"){res = p()}
return res;
}
and calling it like p(fn2); but it doesn't seem to work. What am I missing.
You need to reassign the res (or val) inside the loop, then return it (don't return the p) at the end:
var fn1 = function() {
return 'bar';
};
var fn2 = function() {
return fn1;
};
var fn3 = function() {
return fn2;
};
function p(val) {
while (typeof val === "function") {
val = val();
}
return val;
}
console.log(p(fn3));
You can try this:
var fn1 = function() {
return 'bar';
};
var fn2 = function() {
return fn1;
};
var fn3 = function() {
return fn2;
};
function p(val) {
if (typeof val === "function") {
return p(val())
}
return val;
}
console.log(p(fn3));
Why this is working:
var a = () => {
var print = function(i) { console.log(i); return this; }
var print2 = function(i) { console.log(i); return this; }
return { print:print , print2:print2 }
}
a().print(5).print2(5);
this is also working:
var b = () => {
var print = (i) => { console.log(i); return this; }
return { print:print}
}
b().print('Arrow function works');
while this is not working:
var b = () => {
var print = (i) => { console.log(i); return this; }
var print2 = function(i) { console.log(i); return this; }
return { print:print , print2:print2 }
}
b().print(5).print2(5);
https://jsfiddle.net/Imabot/1gt2kxfh/14/
It's all due to arrow functions behavior(docs)
Step by step explanation:
var b = () => {
// 1
var print = (i) => { console.log(i); return this; }
var print2 = function(i) { console.log(i); return this; }
return { print:print , print2:print2 }
}
const res = b()
// 2
const secondRes = res.print(5)
// 3
secondRes.print2(5);
here print function saves this reference from the outer scope, so this can't be reassigned anymore
now print function is not using this reference that comes from res variable, because this has already been attached to print function above
as a result secondRes is not going to reference to the object that was returned by b function. But it will use this reference that is attached to print function. And finally because secondRes doesn't have print2 property - it throws
Hope it helps <3
In a non-arrow function, the value of this depends on how the function is called. If the function is called as a member of an object, this refers to this object:
someObj.myFunction() // inside myFunction this will point to someObj
In contrast, the arrow functions do not affect this. So whithin an arrow function the value of this is whatever it is in the enclosing scope.
The answer from Lex82 gives the why. If you want to return the functions, so you can use function chaining:
var b = () => {
var print = (i) => { console.log(i); return { print:print , print2:print2 }; }
var print2 = function(i) { console.log(i); return this; }
return { print:print , print2:print2 }
}
b().print(5).print2(5);
function modifyFunction(f) {
return function () {
var returnValue = f.apply(this, arguments);
console.log(returnValue);
if (returnValue == undefined) {
return this;
} else {
return returnValue;
}
};
}
function modifyMethod(o, m) {
if (o.hasOwnProperty(m)) {
if (o[m] instanceof Function) {
o[m] = modifyFunction(m);
}
}
}
var o = {
num: 0,
add: function (x) {
return this.num += x;
},
sub: function (x) {
return this.num -= x;
}
};
modifyMethod(o, "add");
o.add(2).add(4);
console.log(o.num); // o.num = 6
modifyMethod(o, "sub");
o.sub(1).add(3).sub(5);
console.log(o.num); // o.num = 3
How would I make it so that in modifyMethod function inside of the "if(o[m] instanceof Function)" would be equal to what returns by the modifyFunction function, when sending it o[m]? I am trying to make it so that it is chainable, yet I am having a very difficult time getting my head around this.
To make o.add(2).add(4); equal to o.add(2); o.add(4); We can observe that what o.add(2) returns should be o. So your modifyFunction should return a function that :
call the passed in function with given parameters.
Return the caller.
So instead of return that returnValue(which is o.num, a number), you should always return this.
Another point is that in your modifyMethod, after you checked if o[m] is a function, you should pass in that function, not m which is just the key. So it should be o[m] = modifyFunction(o[m]);
function modifyFunction(f) {
return function () {
var returnValue = f.apply(this, arguments);
// Return the object that call this function,
// so it becomes chainable.
return this;
};
}
function modifyMethod(o, m) {
if (o.hasOwnProperty(m)) {
if (o[m] instanceof Function) {
// Pass o[m], not m, m is a string.
o[m] = modifyFunction(o[m]);
}
}
}
var o = {
num: 0,
add: function (x) {
return this.num += x;
},
sub: function (x) {
return this.num -= x;
}
};
modifyMethod(o, "add");
o.add(2).add(4);
console.log(o.num); // o.num = 6
modifyMethod(o, "sub");
o.sub(1).add(3).sub(5);
console.log(o.num); // o.num = 3
I have a sealed object with an array member on which I want to prevent direct pushes.
var myModule = (function () {
"use strict";
var a = (function () {
var _b = {},
_c = _c = "",
_d = [];
Object.defineProperty(_b, "c", {
get: function () { return _c; }
});
Object.defineProperty(_b, "d", {
get { return _d; }
});
_b.addD = function (newD) {
_d.push(newD);
};
Object.seal(_b);
return _b;
}());
var _something = { B: _b };
return {
Something: _something,
AddD: _b.addD
};
}());
myModule.Something.c = "blah"; // doesn't update = WIN!!
myModule.AddD({}); // pushed = WIN!
myModule.Something.d.push({}); // pushed = sadness
How can I prevent the push?
UPDATE:
Thanks for all the thoughts. I eventually need the JSON to send to the server. It looks like I might need to use an object for the array then figure out a way to generate and return the JSON needed, or change _something to use .slice(). Will play and report.
you could override the push method:
var _d = [];
_d.__proto__.push = function() { return this.length; }
and when you need to use it in your module, call Array.prototype.push:
_b.addD = function (newD) {
Array.prototype.push.call(_d, newD);
};
I haven't done any performance tests on this, but this certainly helps to protect your array.
(function(undefined) {
var protectedArrays = [];
protectArray = function protectArray(arr) {
protectedArrays.push(arr);
return getPrivateUpdater(arr);
}
var isProtected = function(arr) {
return protectedArrays.indexOf(arr)>-1;
}
var getPrivateUpdater = function(arr) {
var ret = {};
Object.keys(funcBackups).forEach(function(funcName) {
ret[funcName] = funcBackups[funcName].bind(arr);
});
return ret;
}
var returnsNewArray = ['Array.prototype.splice'];
var returnsOriginalArray = ['Array.prototype.fill','Array.prototype.reverse','Array.prototype.copyWithin','Array.prototype.sort'];
var returnsLength = ['Array.prototype.push','Array.prototype.unshift'];
var returnsValue = ['Array.prototype.shift','Array.prototype.pop'];
var funcBackups = {};
overwriteFuncs(returnsNewArray, function() { return []; });
overwriteFuncs(returnsOriginalArray, function() { return this; });
overwriteFuncs(returnsLength, function() { return this.length; });
overwriteFuncs(returnsValue, function() { return undefined; });
function overwriteFuncs(funcs, ret) {
for(var i=0,c=funcs.length;i<c;i++)
{
var func = funcs[i];
var funcParts = func.split('.');
var obj = window;
for(var j=0,l=funcParts.length;j<l;j++)
{
(function() {
var part = funcParts[j];
if(j!=l-1) obj = obj[part];
else if(typeof obj[part] === "function")
{
var funcBk = obj[part];
funcBackups[funcBk.name] = funcBk;
obj[part] = renameFunction(funcBk.name, function() {
if(isProtected(this)) return ret.apply(this, arguments);
else return funcBk.apply(this,arguments);
});
}
})();
}
}
}
function renameFunction(name, fn) {
return (new Function("return function (call) { return function " + name +
" () { return call(this, arguments) }; };")())(Function.apply.bind(fn));
};
})();
You would use it like so:
var myArr = [];
var myArrInterface = protectArray(myArr);
myArr.push(5); //Doesn't work, but returns length as expected
myArrInterface.push(5); //Works as normal
This way, you can internally keep a copy of the interface that isn't made global to allow your helper funcs to modify the array as normal, but any attempt to use .push .splice etc will fail, either directly, or using the .bind(myArr,arg) method.
It's still not completely watertight, but a pretty good protector. You could potentially use the Object.defineProperty method to generate protected properties for the first 900 indexes, but I'm not sure of the implications of this. There is also the method Object.preventExtensions() but I'm unaware of a way to undo this effect when you need to change it yourself
Thank you, dandavis!
I used the slice method:
var myModule = (function () {
"use strict";
var a = (function () {
var _b = {},
_c = _c = "",
_d = [];
Object.defineProperty(_b, "c", {
get: function () { return _c; }
});
Object.defineProperty(_b, "d", {
get { return _d.slice(); } // UPDATED
});
_b.updateC = function (newValue) {
_c = newValue;
};
_b.addD = function (newD) {
_d.push(newD);
};
Object.seal(_b);
return _b;
}());
var _something = { B: _b };
return {
Something: _something,
AddD: _b.addD
};
}());
myModule.Something.c = "blah"; // doesn't update = WIN!!
myModule.AddD({}); // pushed = WIN!
myModule.Something.d.push({}); // no more update = happiness
This allows me to protect from direct push calls enforcing some logic.
validation works fine if validation properties are placed after "HasError" property in VM.
In the case that the property placed before HasError I will get "parameters.hasError" as undefined. I think it's because the property "HasError" is not defined to that time.
Is there any solution without changing the order of the properties inside VM to make it work.
Thanks!
self._BusTypeDefault = function(param) {
var ret = param.BusType;
if(typeof(ret)==='undefined') {
ret = '';
}
else if(ko.isObservable(ret)) {
ret = ret.peek();
}
return ret;
};
self.BusType = ko.observable(self._BusTypeDefault(init)).extend({maxLength: {message: $Resources.PCIBUSError(), maxFieldLength: 255,hasError: self.HasError }});
self._HasErrorDefault = function(param) {
var ret = param.HasError;
if(typeof(ret)==='undefined') {
ret = false;
}
else if(ko.isObservable(ret)) {
ret = ret.peek();
}
return ret;
};
self.HasError = ko.observable(self._HasErrorDefault(init)).extend({errorAggregation: {}});
ko.extenders.maxLength = function (target, parameters) {
//add some sub-observables to our observable
target.hasMaxLengthError = ko.observable();
target.validationMessageMaxError = ko.observable();
//define a function to do validation
function validate(newValue) {
var preValue = target.hasMaxLengthError();
if (newValue.length >= parameters.maxFieldLength) {
target.hasMaxLengthError(true);
target.validationMessageMaxError(parameters.message || "This field is required");
}
else {
target.hasMaxLengthError(false);
target.validationMessageMaxError("");
}
if (parameters.hasError != null && target.hasMaxLengthError() !== preValue && typeof preValue !== 'undefined') {
parameters.hasError(target.hasMaxLengthError());
}
}
//initial validation
validate(target());
//validate whenever the value changes
target.subscribe(validate);
//return the original observable
return target;
};
You can use a function to delay the interpretation of hasError:
this.myObservable = ko.observable(1).extend({ myExtender : { hasError: function () { return self.hasError } } });
Then in the extender you'll need to call the function to actually get the observable behind:
ko.extenders.myExtender = function (target, params) {
function validate(newValue) {
alert("New Value: " + newValue + " ; Has Error: " + params.hasError()());
}
target.subscribe(validate);
}
See this example: http://jsfiddle.net/7ywLN/