How is regex stored in javascript. Is not stored like the usual way other var types like string is stored.
var regexOne = /^(regex).*$/gm;
var regexTwo = /^(regex).*$/gm;
var regexThree = /^(regex).*$/gm;
var regexFour = /^(regex).*$/gm;
var searchQuery = [regexOne, regexTwo, regexThree, regexFour];
for(query in searchQuery){
console.dir(query.toString());
}
The above code prints:
'0'
'1'
'2'
'3'
How can i get this working.
When you iterate an Array with for..in loop, the loop variable with have just the current index as string, not the actual value. Quoting MDN documentation on Array iteration and for...in,
The for..in statement iterates over the enumerable properties of an object, in arbitrary order.
....
....
Note: for..in should not be used to iterate over an Array where index order is important.
Array indexes are just enumerable properties with integer names and are otherwise identical to general Object properties. There is no guarantee that for...in will return the indexes in any particular order and it will return all enumerable properties, including those with non–integer names and those that are inherited.
Because the order of iteration is implementation dependent, iterating over an array may not visit elements in a consistent order. Therefore it is better to use a for loop with a numeric index (or Array.forEach or the for...of loop) when iterating over arrays where the order of access is important.
The bold text above says it all. So, you should iterate arrays with one of the following options
normal for loop
for(var i = 0; i < searchQuery.length; i += 1) {
console.dir(searchQuery[i]);
}
Array.prototype.forEach function
searchQuery.forEach(function(currentRegEx) {
console.dir(currentRegEx);
});
for...of, loop (Note: This will work only in environments which implement ECMAScript 6)
for(var currentRegEx of searchQuery) {
console.dir(currentRegEx);
}
for-in, in JavaScript, loops through the enumerable property names of an object. It's not for looping through array entries or array indexes (although with safeguards it can be used for the latter, which is why you're seeing 0, 1, etc. — those property names are array indexes).
For details about looping through arrays, see this answer, which has a thorough list of options and explanations of each of them.
Side note 1:
Your code is falling prey to The Horror of Implicit Globals because you never declare the query variable. (The for-in construct doesn't declare it for you.)
Side note 2:
Unless you need the regexOne and such variables, you can create your array of regexes more concisely:
var searchQuery = [
/^(regex).*$/gm,
/^(regex).*$/gm,
/^(regex).*$/gm,
/^(regex).*$/gm
];
Related
This is my Array
let setOne = [""];
Now, I created a function that takes the element from the array i.e setOne, store that element in one Object as a property name and give that property-name a value of true and prints the object data.
Function is
function checkTheSameBetter(setOne) {
let ObjectShop = {};
for (let indexSetOne = 0; indexSetOne < setOne.length; indexSetOne++) {
ObjectShop[setOne[indexSetOne]] = true;
}
console.log(ObjectShop);
}
The Output which I get is
{ '': true }
No Problem so far
Here comes the main part
when I add another value to an array i.e setOne, consider "1".
let setOne = ["",1];
And then when I execute the function checkTheSameBetter. I get output
{'1': true, '': true }
So my question is, "how did that '1' get stored in the first
position?"
The output I expected was in this order
{'': true, '1': true}
Here is the sandbox Link
https://codesandbox.io/s/cranky-cori-qx116?file=/src/index.js
can anyone please tell me what's happening here?
If you need any clarification I will give it.
Thank You
JS Objects have traditionally been 'unordered' (until ES5). Since ES6, there is a predictable order to the Object properties iteration. It, however, isn't the 'insertion' order. The order of keys will be as follows:
First, all non-negative integer keys less than 232, in ascending order. (eg. '1', '79', etc. basically all valid array indices. Caveat: '05' wouldn't be considered integer key, since the integer parsed from it will yield a different string representation).
Then, all String keys, in the original order of insertion. (Numeric strings not falling within bounds of step one will be considered here.)
Then, all Symbol keys, in the original order of insertion.
Looking at the rules above, it makes sense that '1', being an integer key, appeared before '' (a string).
Please note that this only applies to ES2015 and later. To avoid confusion, and eye rolls from colleagues habituated of viewing objects as unordered, please don't rely on enumeration order of Object properties. If the enumeration order is relevant, you can always use Map which guarantees that insertion order will be maintained.
ES6 defines an order in which own properties of an object are enumerated. Following are the rules according to which own properties of an object are enumerated:
String properties whose names are non-negative numbers are listed first, from smallest to largest. This means that properties of array and array-like objects will be enumerated in order.
After that, all properties with string names are listed in the order they were added in the object. This also includes properties that look like non-negative numbers or floating point numbers.
At last, properties whose names are Symbols are listed in the order they were added in the object.
Following functions list the properties in the above described order, subject to their own constraints.
Object.keys()
Object.getOwnPropertyNames()
Object.getOwnPropertySymbols()
Reflect.ownKeys()
One thing to keep in mind is that enumeration order for for in loop is not as tightly specified as it is for above mentioned enumeration functions but it typically enumerates own properties in the order described above.
As for in loop also enumerates properties in the prototype chain, once own properties have been enumerated, it will then move up the prototype chain, enumerating properties of each prototype object in the same order as described above. Although if a property has already been enumerated, any property with the same name won't be enumerated again. Property won't be enumerated even if a non-enumerable property with the same name has already been considered.
const obj = {};
obj[2] = 2;
obj['-1'] = -1;
obj['1'] = 1;
obj['as'] = 'as';
obj['10'] = 10;
obj['b'] = 'b';
console.log(Reflect.ownKeys(obj));
console.log(Object.getOwnPropertyNames(obj));
for (const key in obj) {
console.log(key);
}
.as-console-wrapper { max-height: 100% !important; top: 0; }
I'm trying to learn Javascript - here's my issue:
In the w3schools.com javascript array examples, they show the sequent example:
var person = [];
person["firstName"] = "John";
person["lastName"] = "Doe";
person["age"] = 46;
document.getElementById("demo").innerHTML =
person[0] + " " + person.length;
An array "person" has been defined, but then they proceed to add some elements whit a "named" index. Then tries to print the HTML document the 0th element and the number of elements of the array, like you would do with a standard array.
The description says:
If you use a named index when accessing an array, JavaScript will
redefine the array to a standard object, and some array methods and
properties will produce undefined or incorrect results.
In fact, person[0] and person.length return respectively "undefined" and "0". Even is person was initially defined as an array, by inserting new named indexes elements, the array should be redefined as an object. But when i try do use the Array.isArray() method for checking it, it returns true:
var person = [];
person["firstName"] = "John";
person["lastName"] = "Doe";
person["age"] = 46;
document.getElementById("demo").innerHTML =
person[0] + " " + person.length;
document.getElementById('test').innerHTML = Array.isArray(person);// returns true
So, why? if, as specified by the tutorial, this has been effectively redefined as a standard object, and the ECMAScript 5 has added the .isArray() method for checking if something is an array and nothing else, shouldn't this return false insted of true?
I'm sure i missed something. If i define person like this:
person = {};
then it returns false, as expected. What is happening here? I just wanted to understand arrays a little bit more, this confuses me. Is this just a broken array, but still an array?
Here's the example (without the Array.isarray() bit, just the default): https://www.w3schools.com/js/tryit.asp?filename=tryjs_array_associative_2
First of all I want to note that the example you took from the w3schools page on arrays, is from the "Associative Arrays" section, which has this important introduction:
Many programming languages support arrays with named indexes.
Arrays with named indexes are called associative arrays (or hashes).
JavaScript does not support arrays with named indexes.
In JavaScript, arrays always use numbered indexes.
This puts the example into context, because it really makes no sense to define a variable as an array and then use string keys. But this was an example to illustrate the point.
Does an Array become an Object?
That JavaScript still considers the variable to be an array is as expected. It becomes an array at the moment of assignment of [], and that does not change by adding properties to that object. Yes, arrays are objects. They just have additional capabilities.
The array did not lose any of its array-like capabilities, but those features just don't work on those string properties, ... only on numerical ones (more precisely, the non-negative integer ones).
You loosely quoted the following statement from w3schools:
If you use named indexes, JavaScript will redefine the array to a standard object.
That is wrong information and leads to your misunderstanding. There is no redefinition happening. When you add properties to any object, then the object does not change "type". It remains an instance of what it was before... An array remains an array, a date object remains a date, a regex object remains a regex, even if you assign other properties to it. But non-numerical properties do not "count" for an array: the length will remain unchanged when you add such properties. The length only reveals something about the numerical properties of the object.
This quote is yet another illustration of what the JavaScript community thinks about w3schools.com, i.e. that it is not the most reliable reference, even though it has its value for learning the language.
Example of adding useful properties to arrays
Having said the above, there are cases where you may intentionally want to make use of such properties on arrays. Let's for example think of an array of words that is sorted:
const arr = ["apple", "banana", "grapefruit", "orange", "pear"];
Now let's add something to this array that denotes that it is currently sorted:
arr.isSorted = true;
We could imagine a function that would allow one to add a value to this array, but which also verifies if the array is still sorted:
function addFruit(arr, fruit) {
if (arr.length && fruit < arr[arr.length-1]) {
arr.sorted = false;
}
arr.push(fruit);
}
Then after having added several values, it would maybe be interesting to verify whether the array needs sorting:
if (!arr.sorted) arr.sort();
So this extra property helps to avoid executing an unnecessary sort. But for the rest the array has all the functionality as if it did not have that extra property.
An object that is set up as an array and then filled as an object becomes a member of both classes. Methods of the Array class will apply to its 'array-ness':
Array.isArray(person);
returns true. Methods of the Object class will apply to its 'object-ness':
typeof(person);
returns object. When it could be either one, the 'array-ness' will prevail, because the variable was first defined as an array:
console.log(person);
will put Array [ ] on the console, because it runs the Array class's logging method. It is displayed as an empty array, since it has no numbered elements, but you could add some:
person[2]=66;
and then console.log would log Array [ <2 empty slots>, 66 ].
I think the polyfill implementation of isArray() will clear your doubt by some extent.
#Polyfill
var xs=[]; for (var i in [1,2,3]) xs.push(i); console.log(xs);
Why ["0", "1", "2"] and not [0, 1, 2]?
I have the hunch that this is due to array internal implementation.
Array in javascript may be seen as {"0":1, "1":2, "2":3}?
EDIT: I write a compiler. I don't care that I don't need to use for in and arrays. I don't care about readability of the line. I'd like to know why the keys of the array object are strings and not numbers. Any statements from specification would be appreciated.
Don't loop with in on an array.
Loop with of :
var xs=[];
for (var i of [1,2,3]) xs.push(i);
console.log(xs);
Basically, in loops on object keys, and of loops on array value. The "0" "1" and "2" are keys.
The problem:
for...in loop is not meant to iterate over an array, but was designed to enumerate the object keys. Per the docs on MDN:
for...in should not be used to iterate over an Array where the index order is important.
If used with the arrays, the loop treats the array like an object, and considers the indices as the keys (so the stringified keys are printed).
The quick fix:
The problem with you code is that the statement for (var i in [1,2,3]) yields the array indices as i in the loop body. You may try a simple fix in the loop body:
var xs=[]; for (var i in [1,2,3]) xs.push([1,2,3][i]); console.log(xs);
You may want to extract [1,2,3] in a variable to get the code more readable.
The recommended fix:
Use Array#forEach instead. It is specifically designed to loop over arrays, and is stable across all environments.
var xs=[]; [1,2,3].forEach(item => xs.push(item)); console.log(xs);
I saw this for the first time (or noticed it for the first time) today during a maintenance of a colleagues code.
Essentially the "weird" part is the same as if you try to run this code:
var arr = [];
arr[0] = "cat"; // this adds to the array
arr[1] = "mouse"; // this adds to the array
arr.length; // returns 2
arr["favoriteFood"] = "pizza"; // this DOES NOT add to the array. Setting a string parameter adds to the underlying object
arr.length; // returns 2, not 3
Got this example from nfiredly.com
I don't know what the technical term for this "case" is so I haven't been able to find any additional information about it here or on Google but it strikes me very peculiar that this "behaviour" can at all exists in JavaScript; a kind of "mix" between Arrays and Objects (or Associative Arrays).
It states in the above code snippet that that Setting a string parameter adds to the underlying object and thus not affect the length of the "array" itself.
What is this kind of pattern?
Why is it at all possible? Is it a weird JS quirk or is it deliberate?
For what purpose would you "mix" these types?
It's possible because arrays are objects with some special behaviors, but objects nevertheless.
15.4 Array Objects
However, if you start using non-array properties on an array, some implementations may change the underlying data structure to a hash. Then array operations might be slower.
In JavaScript, arrays, functions and objects are all objects. Arrays are objects created with Array constructor function.
E.g.
var a = new Array();
Or, using shortcut array literal,
var a = [];
Both are the same. They both create objects. However, special kind of object. It has a length property and numeric properties with corresponding values which are the array elements.
This object (array) has methods like push, pop etc. which you can use to manipulate the object.
When you add a non-numeric property to this array object, you do not affect its length. But, you do add a new property to the object.
So, if you have
var a = [1];
a.x = 'y';
console.log(Object.keys(a)); // outputs ["0", "x"]
console.log(a); // outputs [1];
I was working on a small javascript coding challenge that was simple enough, but ran into a odd bit of strange behavior that I couldn't find documented anywhere. Maybe someone could point me to where it states that this is expected behavior?
myIntegerArray = [1,2,3,4];
b = new Array();
for(var v in a)
{
b.push(v);
}
console.log(b); // returns ["1","2","3","4"]. Note String result
If I were to use the forEach() however I get an array of Numbers back:
a.forEach(function(element,index,ay)
{
b.push(element)
});
//a console.log(b) will return [1,2,3,4]
You're pushing the key name, not the value. You need to to do this:
b.push(a[v]);
This might help you understand:
for (var key in obj) {
var value = obj[key];
arr.push(value);
}
for(var v in a)
In JavaScript, Arrays are just like Objects. for..in loop will get the keys of the Array objects, which are the actual array indices. As we know that, JavaScript Object keys can only be Strings. So, what you are actually getting is, the Array indices in String format.
And another reason why for..in should not be used, is in MDN docs. Quoting from for..in
for..in should not be used to iterate over an Array where index order
is important. Array indexes are just enumerable properties with
integer names and are otherwise identical to general Object
properties. There is no guarantee that for...in will return the
indexes in any particular order and it will return all enumerable
properties, including those with non–integer names and those that are
inherited.
Because the order of iteration is implementation dependent, iterating
over an array may not visit elements in a consistent order. Therefore
it is better to use a for loop with a numeric index (or Array.forEach
or the non-standard for...of loop) when iterating over arrays where
the order of access is important.
So, you either use
for(var index = 0; index < array.length; index += 1) {
array[index];
}
Or the forEach which you have shown in the question itself.