Develop Reference

How javascript function body can be an object instead of a series of lines of code?

I've been reviewing how an old AngularJS solution works and in one of the controllers I've found the following code:
Array.prototype.update = function (newArray) {
loop: for (var newIndex1 = 0; newIndex1 < newArray.length; newIndex1++) {
var newItem = newArray[newIndex1];
for (var oldIndex1 = 0; oldIndex1 < this.length; oldIndex1++)
if (this[oldIndex1].Id === newItem.Id)
continue loop;
this.push(newItem);
}
loop1: for (var oldIndex2 = 0; oldIndex2 < this.length; oldIndex2++) {
var oldItem = this[oldIndex2];
for (var newIndex2 = 0; newIndex2 < newArray.length; newIndex2++) {
if (newArray[newIndex2].Id === oldItem.Id)
continue loop1;
}
this.splice(oldIndex2, 1);
}
};
Basically, an "update" function is added to standard Javascript Array that does some adding and removal of the items.
But how come the function body that is defined in curly braces is not a series of lines of javascript code (expressions), but is a javascript object with two properties loop and loop1.
As this seems to be valid javascript, where can I read about this style of definition to learn what is the logic of execution of this?

In the function code above, loop and loop1 are labeled block statements, as kindly noted by James in the comments to the original question.

What is a statment prefix in Javascript? [duplicate]

what does the following code meaning? (it is not json - it is code which does not generate error by js interpreter)
foo: 5
The reason for the question is as follows. In the arrow function examples there is one that shows the confusion between json and code block syntax:
var func = () => { foo: 1 };
The func() returns undefined and the above code does not fail. I tried to put just the foo: 5 code as the only code in a js module - and it works... I do not know about a ':' operator neither about labels in js.

It's a JavaScript label: documentation here.
You can use a label to identify a loop, and then use the break or continue statements to indicate whether a program should interrupt the loop or continue its execution.
Note that JavaScript has NO goto statement, you can only use labels with break or continue.
Example usage (from MDN)
var itemsPassed = 0;
var i, j;
top:
for (i = 0; i < items.length; i++){
for (j = 0; j < tests.length; j++) {
if (!tests[j].pass(items[i])) {
continue top;
}
}
itemsPassed++;
}

What happens if you put an iteration count of a for loop as a variable?

I want to make a program in javascript in which a person inputted the iteration count for a for loop(they could input x++, or y--), but I don't know if I am using the right method.
Here is my code:
var x = prompt("iteration count")
// x should equal something like, i++, or x--
for(var i = 0; i < 10; x){
document.write(i)
}
But when the code ran the program kept crashing.
Why is it crashing and how do I fix this?
Please Help

you need to parse the int value of x because it's a string and use it to increment i
var x = parseInt(prompt("iteration count"))
for (var i = 0; i < 10; i += x) {
document.write(i)
}
EDIT :
based on the question edit and the comments, you can use eval(), but :
Do not ever use eval!
eval() is a dangerous function, which executes the code it's passed with the privileges of the caller.
So before you use it, read the MDN page and check : eval isnt evil it's just misunderstood
where there's this comment from Spudley :
From a security perspective, eval() is far more dangerous in a server
environment, where code is expected to be fully trusted and hidden
from the end user.
In a browser, the user could eval any code they wanted at any time
simply by opening dev tools, so as a developer you can't get away with
having anything on your client code that could be insecure against
eval anyway.
to test the snippet below, type i++ in the prompt
var x = prompt("iteration count");
for (var i = 0; i < 10; eval(x)) {
console.log(i)
}
an alternative to eval() would be new Function or check the answers here : Programatically setting third statement of for loop

var input = 'i++';//Or whatever condition user passing in
var conditionProgramatically = () => new Function(input)() ;
for (var i = 0; i < 10; conditionProgramatically()) {
console.log(i)
}
For for-loop, third statement will be invoked/executed on every iteration, and hence we set a function call, and in that function, we execute whatever user passing in as you've mentioned i++

That is an endless loop because the variable i never incremented. Try this one.
var x = prompt("iteration count")
for(var i = 0; i < x, i++){
document.write(i)
}

You forgot to increment the index variable, it result to endless loop and maximum stack error, you can also use + for parseInt shorcut.
var x = +prompt("iteration count")
for(var i = 0; i < x;i++){
document.write(i)
}

You have to parse the input value and then make it as a condition to stop iterating after the given value.
var x = parseInt(prompt("iteration count"))
for (var i = 0; i < x; i++) {
document.write(i);
}

JavaScript: setInterval and for loop explanation

i searched around for a couple of questions related to the use of the for loop and the setInterval function in JavaScript but i couldn´t find a concrete answer on why this snippet doesn´t work. Could someone explain please what´s happening under the hood and why this code doesn´t print anything at all?
for (let i = 0; i++; i < 10) {
window.setInterval(function () {
console.log('Test');
} , 100)
}

Your for loop is not correct. The condition needs to be the second statement in the for loop.
Following code should work.
for (let i = 0; i < 10 ; i++; ) {
window.setInterval(function () {
console.log('Test');
} , 100)
}
Expected Syntax for loop. You can read more here
for ([initialization]; [condition]; [final-expression])
statement
EDIT 1:
Though all answers (including mine) mentioned that condition needs to be second statement in the for loop which is correct. There is one more additional important behavior.
The for loop for (let i = 0; i++; i < 10) is actually correct in terms of grammar and even the javascript runtime executes this code.
But, as in your case, if the condition is evaluating to falsy value then it would exit the loop.
Breaking your for loop for (let i = 0; i++; i < 10) into each seperate construct
Initialization: let i = 0; This statement initializes the value of variable i to 0.
Condition: i++; Javascript evaluates the statement to check if the statement is true or not. In case of i++ the runtime firstly checks for the current value of i which is 0 . Since 0 is considered a falsy value the condition evaluates to false and hence the statement is not executed. Also, i++ statement is a post increment which basically increments i and then result the original value of i.
So, if you would have written loop like below, using the intiliaztion of i=1, then it would have worked, though it would be running infinitely untill the browser/Server crashes as the condition i++ would always evaluate to true. I hope that makes sense.
for (let i = 1; i++; i < 10) {
// Statements would run
}
Or
for (let i = 0; ++i; i < 10) { **// Pre increment**
// Statements would run
}
Or
for (let i = 0; i=i+1; i < 10) { **// increment i by assigment
// Statements would run
}
Douglas Crockford in his book Good Parts mention about the usage of ++ & -- and how it can confuse readers.

your for loop syntax is wrong, should be
for (let i = 0; i < 10; i++)
your setInterval code will run every 100 milliseconds for each iteration of the loop (so 10 times every 100 milliseconds)

Nothing to do with setInterval, you simply malformed your for loop:
This:
for (let i = 0; i++; i < 10)
Should be this:
for (let i = 0; i < 10; i++)
First declare the initial state of the loop, then the terminating state of the loop, then the incremental change of the loop.
Observe.

How to fix code that returns undefined?

I have this code:
const liked_users = promises[1];
const disliked_users = promises[0];
if (liked_users.length > 0 || disliked_users.length > 0){
for(var i = 0; i < liked_users.length; i++){
for(var j = 0; j < disliked_users.length; j++){
for(var k = 0; k < _USERS.length; k++){
if(_USERS[k].useruid == liked_users[i].likedUseruid || disliked_users[j].dislikedUseruid){
_USERS.splice(i, 1);
i--;
break;
}
basically what is happening is that I access the firebase database and I pull out some data from my objects.
The problem comes where sometimes liked_users is going to be blank and therefore liked_users[i].likedUseruid will return undefined. When they are defined, the code runs fine.
How can I put in some conditional or block of code that allows it to be read in a way that accepts it can be undefined or doesn't run the code until it is defined? I can show more code if it will help.

In JavaScript if you put just variable name in if condition then it will check its available or not.
For example put this block
if(_USERS[k].useruid == liked_users[i].likedUseruid || disliked_users[j].dislikedUseruid){
....
}
in this
if(_USERS[k].useruid && liked_users[i].likedUseruid && disliked_users[j].dislikedUseruid){
....
}
The above condition will check that _USERS[k].useruid is available or not and will continue...
You can make condition as per your need.
I hope this will help you.