This question already has answers here:
Function with forEach returns undefined even with return statement
(5 answers)
Closed 3 years ago.
So I have a little bit of confusion, I'm solving a challenge on freeCodeCamp.
The challenge reads as follows
Everything Be True
Check if the predicate (second argument) is truthy on all elements of a collection (first argument).
It's solved but I don't understand why I had to take an extra step. My code is as such:
function truthCheck(collection, pre) {
collection.forEach(function(element) {
for (key in element) {
if (!element.hasOwnProperty(pre)) {
return false;
} else if (key === pre) {
if (!Boolean(element[key])) {
return false;
}
}
}
});
return true;
}
truthCheck([
{"user": "Tinky-Winky", "sex": "male"},
{"user": "Dipsy"},
{"user": "Laa-Laa", "sex": "female"},
{"user": "Po", "sex": "female"}
], "sex");
so in this instance it should fail because the 2nd element within collection does not have the sex property. Also you will receive a fail if the pre argument, or in this case sex is not a truthy value.
When these get hit (Which they are, I'm able to tell through console logs) but I figure it would break out of the loop and return from the truthCheck function.....but it doesn't, and it will eventually return true.
I was able to circumvent this by defining a variable and then setting that value to false and then returning the variable at the end. Is there a better way? It seems like these returns should break out of the truthCheck function? Am I missing something?
As the other answers explain, this is meaningless:
collection.forEach(function () {
// do something
return false;
});
because array#forEach simply does not care for the return value of its worker function. It just executes the worker function for each array element.
You could use the worker function to set an outer variable:
function truthCheck(collection, pre) {
var allAreTruthy = true;
collection.forEach(function (elem) {
// if this ever flips allAreTruthy to false, it will stay false
allAreTruthy = allAreTruthy && elem[pre];
});
return allAreTruthy;
}
But there are better ways to express this.
Check if the predicate (second argument) is truthy on all elements of a collection (first argument).
Could be paraphrased as "Every element of the collection has a truthy value at a particular key."
function truthCheck(collection, pre) {
return collection.every(function (elem) { return elem[pre]; });
}
Could be paraphrased as "None of the elements of the collection have a falsy value at a particular key (or are missing the key entirely)."
Or, since an Array#none method does not actually exist, "There aren't some elements of the collection that have a falsy value at a particular key."
function truthCheck(collection, pre) {
return !collection.some(function (elem) { return !elem[pre]; });
}
The advantage of using Array#some is that it stops iterating the array as soon as the condition it seeks for is fulfilled. If your array had many elements this would mean improved performance. For short arrays there's not a lot of difference to using Array#every or Array#forEach.
The above is semantically equivalent to
function truthCheck(collection, pre) {
var i;
for (i = 0; i < collection.length; i++) {
if (!collection[i][pre]) return false;
}
return true;
}
Since JS objects simply return undefined when you access a key that has not been set, a check for hasOwnProperty is superfluous here.
You can't return anything from a ForEach loop. It will return undefined by default.
As the official documentation, Array.prototype.forEach() - JavaScript | MDN, says:
There is no way to stop or break a forEach() loop other than by throwing an exception. If you need such behavior, the forEach() method is the wrong tool, use a plain loop instead. If you are testing the array elements for a predicate and need a Boolean return value, you can use every() or some() instead.
So you can use a very simple for..in loop, for example:
for(var c in collection){
// Do whatever you want
}
[collection].forEach of javascript does not work like an ordinary loop. There is no way to prematurely end it not unless you make it throw an exception.
The behavior that you are expecting is what you would expect out of a javascript for loop but since forEach uses callback functions for each looped object, then you are only exiting the callback function instead of the forEach. Also it is worth noting that in your code, the you have a for loop which has a return in it. The return block in this loop only breaks this loop and not the forEach (which I have mentioned earlier, cannot be prematurely terminated unless otherwise)
As you can see forEach is mostly meant to iterate all elements instead of conditional checks per iterated element.
function truthCheck(collection, pre) {
return collection.every(function (person) { return !!person[pre]; });
}
you execute a function on each element of the collection. This function checks that element an returns something. But that returned value does not affect the result of the outer function. Since the outer function does not depend on the inner function your result is allways true.
if you define a variable, set this to false and return that variable at the end would works but it would be inefficient. Lets think of the following scenario. You found one element which does not has the target key. So right now you should return but you can't. You have to working yourself through the whole collection. The forEach loop does not give you the chance to exit without a mess. Thus a better idea is the for loop. Your can exit exit the for loop if found what you were looking for
a slightly easier way would be:
function truthCheck(collection, pre) {
//iterate thrugh your collection
for (var c in collection){
//get keys of element as an array and check if pre is in that array
if( Object.keys(collection[c]).indexOf(pre) == -1){
// pre was not found
return false;
}
}
return true;
}
I have a variable 'fieldCount' that is equal to 5 (the number of fields I have). How can I write the following out without specifically stating each index:
if (
fields[0].checkValidity() && fields[1].checkValidity() && fields[2].checkValidity()
&& fields[3].checkValidity() && fields[4].checkValidity()
) {
fieldsPass = true;
}
You can use a loop, or some fairly standard built-in browser functions.
Loop
Here's how to do it with a loop. This should be the fastest code, but you probably don't need the fastest code unless you are checking a huge number of fields on a huge number of items. I recommend instead the "correct, clear, concise, fast" priorities, so think you should start with the built-in browser functions. But here it is for reference:
var fieldsPass = true;
var i, l = fields.length;
for (i = 0; i < l; i += 1) {
if (fields[i].checkValidity()) continue;
fieldsPass = false;
break;
}
Note that declaring the i variable outside of the loop and capturing the field length outside the loop are optional.
The first I did because many people don't know about hoisting, and the fact that for (var i ... does NOT create a variable only available inside the for loop, it is the same as having declared var i at the top of the function, and this behavior can lead to bugs.
The second I did out of habit, though as I said you can put it inside the loop check. See this discussion. If you do use the loop method, you're probably looking for better performance, so might want to use the captured length way for the best possible performance. (And if it really matters that much, you can do the var i = fields.length; while (i--) { } method.)
Browser Function Array.Every
You can use Array.prototype.every() (from ECMAScript 2015 6th edition):
Mozilla Developer Network
The every() method tests whether all elements in the array pass the test implemented by the provided function.
fieldsPass = fields.every(function(field) {
return field.checkValidity();
});
It returns truewhen the passed-in function, when run on every item in the array, returns true for all of them. If any one returns false, it stops and returns false. In some languages, they call the same concept all, if you're familiar with that.
Alternately, it might be better to declare your checkValidity function once, instead of putting it on every field. This may not be possible, depending on how you implemented it (perhaps it has access to private variables?). Notice that the first argument of the callback function you provide (see the documentation in the links above) is the currentValue of the iteration, the field you want to check. If your function thus looks like this, it will work:
function checkValidity(field) { /* check validity */ }
fieldsPass = fields.every(checkValidity);
Browser Function Array.Some
You can use also Array.prototype.some() (from ECMAScript 2015 6th edition):
Mozilla Developer Network
The some() method tests whether some element in the array passes the test implemented by the provided function.
fieldsPass = !fields.some(function(field) {
return !field.checkValidity();
});
Notice that it is basically just the inverse of every, as "ALL VALID" is the same as "NOT (ANY INVALID)". It just means that it checks for any one item in the array that passes the function, and if so, returns true. In some languages, they call the same concept any, if you're familiar with that.
General Browser Compatibility Notes
Note that for these two functions, browser compatibility is pretty good. If you don't care about IE below version 9 then you're pretty much safe. If you do, then you would want to use a polyfill, which is available on the above-linked MDN pages. You would include that code in the global scope of your javascript file, and then would be able to use it as normally in IE 8 and below. (I'm talking about the code block starting like this:)
if (!Array.prototype.every) {
Array.prototype.every = function(callbackfn, thisArg) {
...
}
You have an array of fields, and you want to iterate them, and invoke a validity check method on each of them. Only if all of them pass, the flag fieldsPass would be true.
This is exactly the behavior of Array#every. According to MDN:
The every() method tests whether all elements in the array pass the
test implemented by the provided function.
Use Array.every to invoke checkValidity() on each field. If all fields are valid, the result would be true. If one field fails checkValidity(), the loop would return false immediately, without checking the other fields.
var fieldPass = fields.every(function(field) {
return field.checkValidity();
})
Like you said, use a loop.
var fieldsPass = true;
for (var i = 0; i < fields.length; i++) {
// Exit early if one of them fails
if (!fields[i].checkValidity()) {
fieldsPass = false;
break;
}
}
You can use the .some() method and reverse the logic to tell if all passed like so:
var allPassed = !fields.some(function (field) {
return !field.checkValidity();
});
You can do it like this:
fieldsPass = true;
for (i = 0; i < fieldCount; i++)
{
if(!fields[i].checkValidity())
{
fieldsPass = false;
break;
}
}
I want to search a string in a nested JSON object. If the string found in an object, I need to return that object.
I am using a recursive function to achieve this. The problem is, the function is recursing until the end and not returning the object found.
Please see the entire code in jsfiddle
function search(obj, name) {
console.log(obj["name"], ",", name, obj["name"] == name);
if (obj["name"] == name) {
return obj; //NOT RETURNING HERE
}
if (obj.children || obj._children) {
var ch = obj.children || obj._children;
//console.log(ch);
ch.forEach(function(val) {
search(val, name)
});
}
return -1;
}
search(myJson, "VM10-Proc4")
I am not sure what is going wrong.
You need to stop looping over children when it finds one that matches.
function search(obj, name) {
console.log(obj.name, ",", name, obj.name == name);
if (obj.name == name) {
return obj;
}
if (obj.children || obj._children) {
var ch = obj.children || obj._children;
for (var i = 0; i < ch.length; i++) {
var found = search(ch[i], name);
if (found) {
return found;
}
}
}
return false;
}
FIDDLE demo
The correct return value is getting lost in the chain of recursive function calls. After the correct value is found, any additional searches made will return incorrect values from that point on.
A couple ways to handle this:
1. Cancel the search
When the correct value is found, immediately return it all the way up the recursive stack, without searching any more of the current arrays or the nested arrays. In other words, cancel the rest of the search.
#Barmer's answer is an example of this. The key part of his code is the use of for loops rather than the each method to iterate through the arrays, since it's much easier to interrupt a for loop.
2. Store the value somewhere safe
When the correct value is found, store it somewhere safe, allow the rest of the search to continue, and after the initial function call has finished access the value. The simplest way is to store the correct value in a global variable, though that's not a good practice as it violates the encapsulation of the function.
#shyam's answer presents a cleaner solution: Passing a reference to a global variable as a function parameter, setting the parameter when the correct value is found, and then accessing the global variable after the initial function call has finished.
Choosing between the two
In laymen's terms, the intended logic of the function could be summed up as follows: When you find what you're looking for, stop, and let me know what it is immediately. The only reason to continue searching would be if multiple pieces of data needed to be found. I'm assuming that's not the case here.
Of the two approaches, #2 is a quick-fix workaround that should work fine but will further confuse anyone who's trying to understand the intended logic of the function. Why is the search continuing if it's only looking for a single piece of data that's already been found?
#1 is a refactoring of the function so that it behaves more consistently with the intended logic, which would make the function easier to understand. The function stops searching when it finds what it needs.
I know this is an old post, but I can see 2 issues with this:
1) the recursive call isn't returning the result up the call stack i.e.
search(val, name)
Should be
return search(val, name)
2) it looks like you are using array.forEach(). The documentation states:
There is no way to stop or break a forEach() loop other than by throwing an exception. If you need such behavior, the forEach() method is the wrong tool. Use a plain loop instead. If you are testing the array elements for a predicate and need a Boolean return value, you can use every() or some() instead. If available, the new methods find() or findIndex() can be used for early termination upon true predicates as well.
What this means is, effectively, when you find the result you are looking for you want to send it back up the call stack. Using array.forEach will continue to recursively look through the hierarchy and return all values, not just the one you are interested in. Therefore the last returned value maybe one you don't expect, like undefined! Therefore use a different method of iterating i.e.
for (var i = 0; i < ch.length; i++) {
var val = ch[i];
return search(val, name, ret);
}
The accepted answer does spoon feed you part of this answer but doesn't explain why. Hence this answer
Since you are recursing the return might be nested too deep for you to get a meaningful result. Instead you can try passing an extra argument to collect the results.
function search(obj, name, ret) {
console.log(obj["name"], ",", name, obj["name"] == name);
if (obj["name"] == name) {
ret.push(obj);
return
}
if (obj.children || obj._children) {
var ch = obj.children || obj._children;
ch.forEach(function(val) {
search(val, name, ret);
});
}
}
var result = [];
search(myJson, "VM10-Proc4", result)
Here is a solution using object-scan
// const objectScan = require('object-scan');
const myJson = {"name":"UCS - San Jose","type":"Minor","children":[{"name":"VM1","type":"Clear","children":[{"name":"VM1-Proc1","type":"Clear","children":[{"name":"VM1-Proc1-child1","type":"Clear"}]},{"name":"VM1-Proc2","type":"Clear"},{"name":"VM1-Proc3","type":"Clear"},{"name":"VM1-Proc4","type":"Clear"},{"name":"VM1-Proc5","type":"Clear"},{"name":"VM1-Proc6","type":"Clear"},{"name":"VM1-Proc7","type":"Clear"},{"name":"VM1-Proc8","type":"Clear"},{"name":"VM1-Proc9","type":"Clear"},{"name":"VM1-Proc10","type":"Clear"}]},{"name":"VM2","type":"Clear","children":[{"name":"VM2-Proc1","type":"Clear"},{"name":"VM2-Proc2","type":"Clear"},{"name":"VM2-Proc3","type":"Clear"},{"name":"VM2-Proc4","type":"Clear"},{"name":"VM2-Proc5","type":"Clear"},{"name":"VM2-Proc6","type":"Clear"},{"name":"VM2-Proc7","type":"Clear"},{"name":"VM2-Proc8","type":"Clear"},{"name":"VM2-Proc9","type":"Clear"},{"name":"VM2-Proc10","type":"Clear"}]},{"name":"VM3","type":"Clear","children":[{"name":"VM3-Proc1","type":"Clear"},{"name":"VM3-Proc2","type":"Clear"},{"name":"VM3-Proc3","type":"Clear"},{"name":"VM3-Proc4","type":"Clear"},{"name":"VM3-Proc5","type":"Clear"},{"name":"VM3-Proc6","type":"Clear"},{"name":"VM3-Proc7","type":"Clear"},{"name":"VM3-Proc8","type":"Clear"},{"name":"VM3-Proc9","type":"Clear"},{"name":"VM3-Proc10","type":"Clear"}]},{"name":"VM4","type":"Minor","children":[{"name":"VM4-Proc1","type":"Clear"},{"name":"VM4-Proc2","type":"Clear"},{"name":"VM4-Proc3","type":"Minor"},{"name":"VM4-Proc4","type":"Clear"},{"name":"VM4-Proc5","type":"Clear"},{"name":"VM4-Proc6","type":"Minor"},{"name":"VM4-Proc7","type":"Clear"},{"name":"VM4-Proc8","type":"Clear"},{"name":"VM4-Proc9","type":"Clear"},{"name":"VM4-Proc10","type":"Clear"}]},{"name":"VM5","type":"Clear","children":[{"name":"VM5-Proc1","type":"Clear"},{"name":"VM5-Proc2","type":"Clear"},{"name":"VM5-Proc3","type":"Clear"},{"name":"VM5-Proc4","type":"Clear"},{"name":"VM5-Proc5","type":"Clear"},{"name":"VM5-Proc6","type":"Clear"},{"name":"VM5-Proc7","type":"Clear"},{"name":"VM5-Proc8","type":"Clear"},{"name":"VM5-Proc9","type":"Clear"},{"name":"VM5-Proc10","type":"Clear"}]},{"name":"VM6","type":"Minor","children":[{"name":"VM6-Proc1","type":"Clear"},{"name":"VM6-Proc2","type":"Clear"},{"name":"VM6-Proc3","type":"Minor"},{"name":"VM6-Proc4","type":"Clear"},{"name":"VM6-Proc5","type":"Clear"},{"name":"VM6-Proc6","type":"Clear"},{"name":"VM6-Proc7","type":"Minor"},{"name":"VM6-Proc8","type":"Clear"},{"name":"VM6-Proc9","type":"Clear"},{"name":"VM6-Proc10","type":"Clear"}]},{"name":"VM7","type":"Clear","children":[{"name":"VM7-Proc1","type":"Clear"},{"name":"VM7-Proc2","type":"Clear"},{"name":"VM7-Proc3","type":"Clear"},{"name":"VM7-Proc4","type":"Clear"},{"name":"VM7-Proc5","type":"Clear"},{"name":"VM7-Proc6","type":"Clear"},{"name":"VM7-Proc7","type":"Clear"},{"name":"VM7-Proc8","type":"Clear"},{"name":"VM7-Proc9","type":"Clear"},{"name":"VM7-Proc10","type":"Clear"}]},{"name":"VM8","type":"Clear","children":[{"name":"VM8-Proc1","type":"Clear"},{"name":"VM8-Proc2","type":"Clear"},{"name":"VM8-Proc3","type":"Clear"},{"name":"VM8-Proc4","type":"Clear"},{"name":"VM8-Proc5","type":"Clear"},{"name":"VM8-Proc6","type":"Clear"},{"name":"VM8-Proc7","type":"Clear"},{"name":"VM8-Proc8","type":"Clear"},{"name":"VM8-Proc9","type":"Clear"},{"name":"VM8-Proc10","type":"Clear"}]},{"name":"VM9","type":"Clear","children":[{"name":"VM9-Proc1","type":"Clear"},{"name":"VM9-Proc2","type":"Clear"},{"name":"VM9-Proc3","type":"Clear"},{"name":"VM9-Proc4","type":"Clear"},{"name":"VM9-Proc5","type":"Clear"},{"name":"VM9-Proc6","type":"Clear"},{"name":"VM9-Proc7","type":"Clear"},{"name":"VM9-Proc8","type":"Clear"},{"name":"VM9-Proc9","type":"Clear"},{"name":"VM9-Proc10","type":"Clear"}]},{"name":"VM10","type":"Clear","children":[{"name":"VM10-Proc1","type":"Clear"},{"name":"VM10-Proc2","type":"Clear"},{"name":"VM10-Proc3","type":"Clear"},{"name":"VM10-Proc4","type":"Clear"},{"name":"VM10-Proc5","type":"Clear"},{"name":"VM10-Proc6","type":"Clear"},{"name":"VM10-Proc7","type":"Clear"},{"name":"VM10-Proc8","type":"Clear"},{"name":"VM10-Proc9","type":"Clear"},{"name":"VM10-Proc10","type":"Clear"}]}]};
const search = (obj, name) => objectScan(['**.name'], {
rtn: 'parent',
abort: true,
filterFn: ({ value }) => value === name
})(obj);
console.log(search(myJson, 'VM10-Proc4'));
// => { name: 'VM10-Proc4', type: 'Clear' }
.as-console-wrapper {max-height: 100% !important; top: 0}
<script src="https://bundle.run/object-scan#13.7.1"></script>
Disclaimer: I'm the author of object-scan