Function as an argument - javascript

five = function(parameter1) {
console.log(parameter1)
}
five(function() {
var x = 5;
return x;
})
I have passed an anonymous function as an argument to the five function, why is this function not logging 5?

five = function(parameter1) {
console.log(parameter1())
}
five(function () {
var x = 5;
return x;
})
Change it like this, and it should work.
you did not call the function you just made a reference to it.

five = function(parameter1) {
console.log(parameter1()); // call the passed function
}
five(function() {
var x = 5;
return x;
})

Related

Add values on subsequent call in javascript function.?

Let's consider I have the following function call,
function add(){
x = 0 ;
for(i = 0 i < ##; i++){ // need to run a loop four times
x+=1
}
}
Let's consider I am trying to Implement the function that will add one on each subsequent call, like below
console.log(add()()().getValue()); // 3
console.log(add().getValue()); // 1
console.log(add()().getValue()); // 2
A call to add must return a function which also has a getValue method, and each call to that function must return the same thing. So:
function add() {
var x = 1;
function inner() {
x += 1;
return inner;
}
inner.getValue = function () {
return x;
}
return inner;
}
console.log(add()()().getValue()); // 3
console.log(add().getValue()); // 1
console.log(add()().getValue()); // 2
My guess is they were expecting you to use toString() which is not the greatest way of doing this.
function add(x = 0) {
function next() {
return add(x+1);
}
next.toString = function () {
return x;
};
return next;
}
console.log("example 1", add()()()());
console.log("example 2", add()()()()()()()()());
I think you are trying to emulate the behavior of generator functions. Here is a snippet that illustrates one way you could do it with a generator.
function* adder() {
let x = 0;
while (true) {
yield x + 1;
x++;
}
}
const add = adder();
const firstValue = add.next();
const secondValue = add.next();
const thirdValue = add.next().value;

How to make the return variable hold a new value after Increment/Decrementing it in a function?

Everytime I run this function, the p1_Balance will always reset back to 10 and will not hold the new value of an increment or decrement.
function Balance() {
var p1_Balance=10;
var x= Math.floor(10*Math.random());
if (x<5) {
p1_Balance=p1_Balance-1;
} else {
p1_Balance=p1_Balance+1;
}
return p1_Balance;
}
Pass p1_Balance into the function instead of initializing it each time the function is called with: var p1_Balance = 10;
p1_Balance should be declared outside the scope of the function (meaning not within the function itself). Otherwise, each time the function is called, the initializer that sets the value to 10 runs as well.
var p1_Balance=10;
function Balance(){ ...
You can use Javascript closures to create a function that does what you want, as you can see below:
var Balance = (function() {
var p1_Balance = 10;
return function() {
var x = Math.floor(10 * Math.random());
if (x < 5)
return p1_Balance += 1;
else
return p1_Balance -= 1;
};
})();
for (var i = 0; i < 10; i++)
console.log(Balance());
Alternatively, you will need to define the p1_Balance variable outside the function or pass it as an argument.
There could be several solutions:
one is declaring p1_Balance as a global variable.
var p1_Balance=10;
function Balance(){
var x= Math.floor(10*Math.random());
if (x<5) {
p1_Balance=p1_Balance-1;
}
else {
p1_Balance=p1_Balance+1;
}
return p1_Balance;
}
another is you could pass balance as a function parameter:
function Balance(p1_Balance){
var x= Math.floor(10*Math.random());
if (x<5) {
p1_Balance=p1_Balance-1;
}
else {
p1_Balance=p1_Balance+1;
}
return p1_Balance;
}
.....
value = Balance(10);// value=something that you want to change by that function.

Function that counts how often it call another function

I have a 'twice' function that return 2 of the argument passed into it. I also have another function 'runTwice' that counts the number of times it called the 'twice' function (the idea being that I want the 'twice' function to only run 'twice' no matter how often it is called via the 'runTwice' function). Can you please help?
Functions are given below:
var count = 1;
function twice(num){
return num*2;
}
function runTwice(func){
if (count<3){
count++;
return func;
} else {
return 'Function cannot run!';
}
}
var myFunc = runTwice(twice)
var output = [];
for (var i = 0; i < 3; i++){
output.push(myFunc(i));
}
console.log(output);
I would like the output to be [0, 2, 'Function cannot run!'].
I can make this work if I count the 'twice' function directly but I am looking to understand why this doesn't work as presented above.
Just for fun I'll make a generic expireAfter(invocable[, times[, message]]) function:
function expireAfter(invocable, times = 2, message = 'Function cannot run!') {
return function expires() {
if (times > 0) {
times--;
return invocable.apply(this, arguments);
}
return message;
}
}
function twice(n) {
return n * 2;
}
var myFunc = expireAfter(twice);
console.log(Array(3)
.fill()
.map((_, index) => myFunc(index))
);
The function runTwice should return another function that will decide whether to call the function func (using Function.prototype.apply) or to return a string message instead:
function twice(num){
return num * 2;
}
function runTwice(func){
var count = 0; // this will be trapped in a closure along with func
return function() { // this is the function that gets called
count++; // it increments its version of the count variable
if(count <= 2) // if count is less than 2
return func.apply(this, arguments); // then it calls the function func with whatever arguments passed into it and return the returned value of that call
return "Not available anymore!"; // otherwise (count > 2), then it returns a string
}
}
var myFunc = runTwice(twice);
for (var i = 0; i < 3; i++){
console.log(myFunc(i));
}
Even better:
You can pass in the number of times allowed as well:
function double(num) {
return num * 2;
}
function triple(num) {
return num * 3;
}
function run(func, times){
var count = 0; // this will be trapped in a closure along with func and times
return function() { // this is the function that gets called
count++; // it increments its version of the count variable
if(count <= times) // if count is less than times
return func.apply(this, arguments); // then it calls the function func with whatever arguments passed into it and return the returned value of that call
return "Not available anymore!"; // otherwise (count > times), then it returns a string
}
}
var double2times = run(double, 2); // double2times can only be called 2 times
var triple5times = run(triple, 5); // triple5times can only be called 5 times
for (var i = 0; i < 10; i++){
console.log("Double:", double2times(i));
console.log("Triple:", triple5times(i));
}

How can I get this javascript function to call itself?

As of right now my sum function looks like the code below. It works and returns the sum of the consecutive calls. But how can I make this work without the empty parenthesis at the end? Like so theSum(5)(4)(3) which returns 12.
function theSum(x) {
var total = x;
function rec(y) {
if (y === undefined) return total;
total += y;
return rec;
};
return rec;
}
console.log(theSum(5)(4)(3)()); // 12
Here is a suggestion utilizing a toString method:
function theSum(x) {
var total = x;
function rec(y) {
total += y;
return rec;
};
rec.toString = function() { return total; }
return rec;
}
alert(theSum(5)(4)(3));
console.log(parseInt(theSum(5)(4)(3)));
Note however that you need to convert the returned reference to a string in some way so that you see the result.
This is not possible. A function cannot return a function and an integer. You can make theSum(5, 4, 3) = 12 or theSum([5, 4, 3]) = 12.
Closures and JavaScript duck typing to the rescue:
function NumSumFun(initial){
function NumSumNext(num) {
initial+= num;
return NumSumNext;
}
NumSumNext.valueOf = function () { return initial; }
return NumSumNext;
}
var x = NumSumFun(10)(29); // ==> function 39
x + 1; // ==> 40
So whats happening. It returns a function but the function has a valueOf property that has access to the accumulated value so the function acts as a number when used as a number.

Can I pass parameter like reference

I need function change to change variables and return back to Tst1. I expect to get in console:
5
aaa
but have unchanged ones:
6
bbb
My functions:
function change ( aa,bb )
{
aa=5;
bb="aaa";
}
function Tst1()
{
aa=6;
bb="bbb";
change(aa,bb);
console.log (aa);
console.log (bb);
}
One way is to move change() into the function test(). Then it shares the same variables as the calling scope.
'use strict';
function test() {
function change() {
aa = 6;
bb = 76;
}
var aa = 5,
bb = 6;
change();
document.write(aa + " " + bb);
}
test();
JavaScript is like java in that primitives are never passed by reference but objects are always passed by reference. You need to wrap your data in an object and pass that instead:
function change (aa, bb)
{
aa.value = 5;
bb.value = "aaa";
}
function Tst1()
{
aa = { value: 6 };
bb = { value: "bbb" };
change(aa, bb);
console.log (aa.value); // outputs 5
console.log (bb.value); // outputs aaa
}
or you can play with global variable, but it is not a good practice.
var aa,bb;
function change(){
aa=6;
bb=76;
}
function test(){
aa = 5;
bb = 6;
change();
console.log(aa + " " + bb);
}
test();
Short answer: NO, you can't pass primitive parameters by reference in JS.
One alternative solution to the presented here is to return the result values as array of items:
function change ( aa,bb )
{
aa=5;
bb="aaa";
return [aa, bb];
}
function Tst1()
{
aa=6;
bb="bbb";
result = change(aa,bb);
aa = result[0];
bb = result[1];
document.writeln(aa);
document.writeln(bb);
}
Tst1();

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