I am wondering if there's a way to use the str.includes() function in JavaScript, but check at a certain index of the string, without changing the original string. For example:
var str = "this is a test";
str.includes("test"); //returns true
str.includes("test", 0) //returns false, as "test" is not at position 0
str.includes("test", 10) //returns true, as "test" is at position 10 in the string
I've been trying to find a way to do this, but haven't been able to figure it out. Could somebody please help me?
String.prototype.includes() has something close to this functionality, as argument 2 is taken as the start position for searching.
If you want to search at, not after, a specific index, you can write a function that takes the string, creates a slice of it, and checks if that slice matches
function substring_at(string, substring, position) {
let slice = string.slice(position, position + substring.length)
// Triple equals removes type coercion support, and is slightly faster
return slice === substring
}
I've tested it with your examples and all seems well.
Related
I want to access the first two digits of a number, and i have tried using substring, substr and slice but none of them work. It's throwing an error saying substring is not defined.
render() {
let trial123 = this.props.buildInfo["abc.version"];
var str = trial123.toString();
var strFirstThree = str.substring(0,3);
console.log(strFirstThree);
}
I have tried the above code
output of(above code)
trial123=19.0.0.1
I need only 19.0
How can i achieve this?
I would split it by dot and then take the first two elements:
const trial = "19.0.0.1"
console.log(trial.split(".").slice(0, 2).join("."))
// 19.0
You could just split and then join:
const [ first, second ] = trial123.split('.');
const result = [ first, second ].join('.');
I have added a code snippet of the work: (explanation comes after it, line by line).
function getFakePropValue(){
return Math.round(Math.random()) == 0 ? "19.0.0.1" : null;
}
let trial123 = getFakePropValue() || "";
//var str = trial123.toString();
// is the toString() really necessary? aren't you passing it along as a String already?
var strFirstThree = trial123.split('.');
//var strFirstThree = str.substring(0,3);
//I wouldn't use substring , what if the address 191.0.0.1 ?
if(strFirstThree.length >= 2)
console.log(strFirstThree.splice(0,2).join("."));
else
console.error("prop was empty");
Because you are using React, the props value was faked with the function getFakePropValue. The code inside is irrelevant, what I am doing is returning a String randomly, in case you have allowed in your React Component for the prop to be empty. This is to show how you an create minimal robust code to avoid having exceptions.
Moving on, the following is a safety net to make sure the variable trial123 always has a string value, even if it's "".
let trial123 = getFakePropValue() || "";
That means that if the function returns something like null , the boolean expression will execute the second apart, and return an empty string "" and that will be the value for trial123.
Moving on, the line where you convert to toString I have removed, I assume you are already getting the value in string format. Next.
var strFirstThree = trial123.split('.');
That creates an array where each position holds a part of the IP addrss. So 19.0.0.1 would become [19,0,0,1] that's thanks to the split by the delimiter . . Next.
if(strFirstThree.length >= 2)
console.log(strFirstThree.splice(0,2).join("."));
else
console.error("prop was empty");
This last piece of code uses the conditional if to make sure that my array has values before I try to splice it and join. The conditional is not to avoid an exception, since splice and join on empty arrays just returns an empty string. It's rather for you to be able to raise an error or something if needed. So if the array has values, I keep the first two positions with splice(0,2) and then join that array with a '.'. I recommend it more than the substr method you were going for because what if you get a number that's 191.0.0.1 then the substr would return the wrong string back, but with splice and join that would never happen.
Things to improve
I would strongly suggest using more human comprehensible variables (reflect their use in the code)
The right path for prop value checking is through Prop.Types, super easy to use, very helpful.
Happy coding!
I have a group of strings in Javascript and I need to write a function that detects if another specific string belongs to this group or not.
What is the fastest way to achieve this? Is it alright to put the group of values into an array, and then write a function that searches through the array?
I think if I keep the values sorted and do a binary search, it should work fast enough. Or is there some other smart way of doing this, which can work faster?
Use a hash table, and do this:
// Initialise the set
mySet = {};
// Add to the set
mySet["some string value"] = true;
...
// Test if a value is in the set:
if (testValue in mySet) {
alert(testValue + " is in the set");
} else {
alert(testValue + " is not in the set");
}
You can use an object like so:
// prepare a mock-up object
setOfValues = {};
for (var i = 0; i < 100; i++)
setOfValues["example value " + i] = true;
// check for existence
if (setOfValues["example value 99"]); // true
if (setOfValues["example value 101"]); // undefined, essentially: false
This takes advantage of the fact that objects are implemented as associative arrays. How fast that is depends on your data and the JavaScript engine implementation, but you can do some performance testing easily to compare against other variants of doing it.
If a value can occur more than once in your set and the "how often" is important to you, you can also use an incrementing number in place of the boolean I used for my example.
A comment to the above mentioned hash solutions.
Actually the {} creates an object (also mentioned above) which can lead to some side-effects.
One of them is that your "hash" is already pre-populated with the default object methods.
So "toString" in setOfValues will be true (at least in Firefox).
You can prepend another character e.g. "." to your strings to work around this problem or use the Hash object provided by the "prototype" library.
Stumbled across this and realized the answers are out of date. In this day and age, you should not be implementing sets using hashtables except in corner cases. You should use sets.
For example:
> let set = new Set();
> set.add('red')
> set.has('red')
true
> set.delete('red')
true
> set.has('red')
false
Refer to this SO post for more examples and discussion: Ways to create a Set in JavaScript?
A possible way, particularly efficient if the set is immutable, but is still usable with a variable set:
var haystack = "monday tuesday wednesday thursday friday saturday sunday";
var needle = "Friday";
if (haystack.indexOf(needle.toLowerCase()) >= 0) alert("Found!");
Of course, you might need to change the separator depending on the strings you have to put there...
A more robust variant can include bounds to ensure neither "day wed" nor "day" can match positively:
var haystack = "!monday!tuesday!wednesday!thursday!friday!saturday!sunday!";
var needle = "Friday";
if (haystack.indexOf('!' + needle.toLowerCase() + '!') >= 0) alert("Found!");
Might be not needed if the input is sure (eg. out of database, etc.).
I used that in a Greasemonkey script, with the advantage of using the haystack directly out of GM's storage.
Using a hash table might be a quicker option.
Whatever option you go for its definitely worth testing out its performance against the alternatives you consider.
Depends on how much values there are.
If there are a few values (less than 10 to 50), searching through the array may be ok. A hash table might be overkill.
If you have lots of values, a hash table is the best option. It requires less work than sorting the values and doing a binary search.
I know it is an old post. But to detect if a value is in a set of values we can manipulate through array indexOf() which searches and detects the present of the value
var myString="this is my large string set";
var myStr=myString.split(' ');
console.log('myStr contains "my" = '+ (myStr.indexOf('my')>=0));
console.log('myStr contains "your" = '+ (myStr.indexOf('your')>=0));
console.log('integer example : [1, 2, 5, 3] contains 5 = '+ ([1, 2, 5, 3].indexOf(5)>=0));
You can use ES6 includes.
var string = "The quick brown fox jumps over the lazy dog.",
substring = "lazy dog";
console.log(string.includes(substring));
This is the first part of my code in a function:
var pattern = new RegExp(myTypedString,"gi");
The goal is to populate results as I type in a text field. Everytime an input event is triggered I loop through an array of strings and check if my typed string exists in my array of strings using this code:
return pattern.test(myArrayString);
but luckily I've realised that one of the results was wrong and it was only happening to this particular case: when I type "t" or "T" I don't have a match in "Trafalgar Square" (a string from myArrayString), so I did a few debugging in the console and when I check the variable pattern doing console.log(pattern) it outputs this string: /t/gi, which i think it's where my problem relies... (later on this)
when I tried to replicate the problem typing:
var pattern = new RegExp('t',"gi")
and:
pattern.test('Trafalgar Square')
which gives me true at a first try, BUT... if I type this again:
pattern.test('Trafalgar Square')
the result will be false and if I continue, true, false, true, false,...
One of the first things that came into my mind is that the /t in the regex /t/gi might be a rule but I couldn't find anything about it.
I would like to know an explanation for this unexpected result but I'm also concerned that my function is not flawless, so I ask you how can I improve this function? thanks
It alternates between true and false because it matches the first time (true), then tries a second match, which fails because there’s only one T in Trafalgar Square. When you call it again, it loop back, thus the true, false, true, etc.
To fix that, update its cursor before matching:
var p = new Regexp('t', 'gi') // same as `p = /t/gi`
// ...
p.lastIndex = 0;
p.test("Trafalgar Square"); // true
// try again:
p.lastIndex = 0;
p.test("Trafalgar Square"); // true -> it works
Here is a relevant jsfiddle with a couple examples.
I do not know why it alternates between true and false (i can reproduce it, too). The /t/ should not be anything special in the regex. It is not a special character like { or \ or [ and should just be a literal character t.
But you could use another function to achieve more or less the same:
'Trafalgar Square'.match(pattern)
=> Array [ "T" ]
'lalala'.match(pattern)
=> null
match takes slightly more resources than test would, but this is usually not a problem. The result will be an Array for matches or null when nothing matches, so you can just return that and the rest of your code will probably work just fine. Array... will be a truthy value and null will be a falsy value.
I can't seem to find an example of anyone using RegEx matches to create an overlay in CodeMirror. The Moustaches example matching one thing at a time seems simple enough, but in the API, it says that the RegEx match returns the array of matches and I can't figure out what to do with it in the context of the structure in the moustaches example.
I have a regular expression which finds all the elements I need to highlight: I've tested it and it works.
Should I be loading up the array outside of the token function and then matching each one? Or is there a way to work with the array?
The other issue is that I want to apply different styling depending on the (biz|cms) option in the regex - one for 'biz' and another for 'cms'. There will be others but I'm trying to keep it simple.
This is as far as I have got. The comments show my confusion.
CodeMirror.defineMode("tbs", function(config, parserConfig) {
var tbsOverlay = {
token: function(stream, state) {
tbsArray = match("^<(biz|cms).([a-zA-Z0-9.]*)(\s)?(\/)?>");
if (tbsArray != null) {
for (i = 0; i < tbsArray.length; i++) {
var result = tbsArray[i];
//Do I need to stream.match each element now to get hold of each bit of text?
//Or is there some way to identify and tag all the matches?
}
}
//Obviously this bit won't work either now - even with regex
while (stream.next() != null && !stream.match("<biz.", false)) {}
return null;
}
};
return CodeMirror.overlayMode(CodeMirror.getMode(config, parserConfig.backdrop || "text/html"), tbsOverlay);
});
It returns the array as produced by RegExp.exec or String.prototype.match (see for example https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Global_Objects/String/match), so you probably don't want to iterate through it, but rather pick out specific elements the correspond to groups in your regexp (if (result[1] == "biz") ...)
Look at implementation of Code Mirror method match() and you'll see, that it processes method parameter for two types: string and RegExp.
Your constant in
stream.match("<biz.")
is of string type.
Define it in RegExp type:
tbsArray = /<biz./g
Thus, your stream will be matched with RegExp.
for (var i=a.length-1;i>0;i--) {
if (i!=a.indexOf(a.charAt(i))) {
a=a.substring(0,i)+a.substring(i+1);
}
}
I found this in a web app I'm auditing, it just baffles me why it's there.
I can't seem to see a case where i!=a.indexOf(a.charAt(i)) would be false.
The value the pass to it is:
a = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
There is no comment either //sigh
This would be true for repeated characters, since indexOf finds the first index of a string, and you're searching from the end. Example:
var a = "xyzxyz";
On first iteration, i === 4, a.charAt(4) === "x", and a.indexOf("x") === 0. So 4 !== 0.
It then sets a = a.substring(0, 4) + a.substring(5). Recalling at substring is inclusive in the first index but exclusive in the last index, that means in this case a = "xyz" + "yz", so we have removed the duplicate "x" from the string.
Since the loop traverses backward, this will continue to work even for characters repeated more than once; you can see that the portion a.substring(i + 1) will always have been covered by the algorithm already, i.e. not contain any duplicates.
As always when encountering this type of thing, applying the extract method refactoring would be a great way to make the code clearer. (Even better than commenting it!) So if you just pulled this out into a method, the code could become a = removeDuplicateChars(a), and everyone is much happier.