javascript - alternative - typescript string startswith




How to check if a string “StartsWith” another string? (12)

Also check out underscore.string.js. It comes with a bunch of useful string testing and manipulation methods, including a startsWith method. From the docs:

startsWith _.startsWith(string, starts)

This method checks whether string starts with starts.

_("image.gif").startsWith("image")
=> true

How would I write the equivalent of C#'s String.StartsWith in JavaScript?

var haystack = 'hello world';
var needle = 'he';

haystack.startsWith(needle) == true

Note: This is an old question, and as pointed out in the comments ECMAScript 2015 (ES6) introduced the .startsWith method. However, at the time of writing this update (2015) browser support is far from complete.


Another alternative with .lastIndexOf:

haystack.lastIndexOf(needle, 0) === 0

This looks backwards through haystack for an occurrence of needle starting from index 0 of haystack. In other words, it only checks if haystack starts with needle.

In principle, this should have performance advantages over some other approaches:

  • It doesn't search the entire haystack.
  • It doesn't create a new temporary string and then immediately discard it.

Best solution:

function startsWith(str, word) {
    return str.lastIndexOf(word, 0) === 0;
}

startsWith("aaa", "a")
true
startsWith("aaa", "ab")
false
startsWith("abc", "abc")
true
startsWith("abc", "c")
false
startsWith("abc", "a")
true
startsWith("abc", "ba")
false
startsWith("abc", "ab")
true

And here is endsWith if you need that too:

function endsWith(str, word) {
    return str.indexOf(word, str.length - word.length) !== -1;
}

For those that prefer to prototype it into String:

String.prototype.startsWith || (String.prototype.startsWith = function(word) {
    return this.lastIndexOf(word, 0) === 0;
});

String.prototype.endsWith   || (String.prototype.endsWith = function(word) {
    return this.indexOf(word, this.length - word.length) !== -1;
});

Usage:

"abc".startsWith("ab")
true
"c".ensdWith("c") 
true

Here is a minor improvement to CMS's solution:

if(!String.prototype.startsWith){
    String.prototype.startsWith = function (str) {
        return !this.indexOf(str);
    }
}

"Hello World!".startsWith("He"); // true

 var data = "Hello world";
 var input = 'He';
 data.startsWith(input); // true

Checking whether the function already exists in case a future browser implements it in native code or if it is implemented by another library. For example, the Prototype Library implements this function already.

Using ! is slightly faster and more concise than === 0 though not as readable.


I just wanted to add my opinion about this.

I think we can just use like this:

var haystack = 'hello world';
var needle = 'he';

if (haystack.indexOf(needle) == 0) {
  // Code if string starts with this substring
}

I recently asked myself the same question.
There are multiple possible solutions, here are 3 valid ones:

  • s.indexOf(starter) === 0
  • s.substr(0,starter.length) === starter
  • s.lastIndexOf(starter, 0) === 0 (added after seeing Mark Byers's answer)
  • using a loop:

    function startsWith(s,starter) {
      for (var i = 0,cur_c; i < starter.length; i++) {
        cur_c = starter[i];
        if (s[i] !== starter[i]) {
          return false;
        }
      }
      return true;
    }
    

I haven't come across the last solution which makes uses of a loop.
Surprisingly this solution outperforms the first 3 by a significant margin.
Here is the jsperf test I performed to reach this conclusion: http://jsperf.com/startswith2/2

Peace

ps: ecmascript 6 (harmony) introduces a native startsWith method for strings.
Just think how much time would have been saved if they had thought of including this much needed method in the initial version itself.

Update

As Steve pointed out (the first comment on this answer), the above custom function will throw an error if the given prefix is shorter than the whole string. He has fixed that and added a loop optimization which can be viewed at http://jsperf.com/startswith2/4.

Note that there are 2 loop optimizations which Steve included, the first of the two showed better performance, thus I will post that code below:

function startsWith2(str, prefix) {
  if (str.length < prefix.length)
    return false;
  for (var i = prefix.length - 1; (i >= 0) && (str[i] === prefix[i]); --i)
    continue;
  return i < 0;
}

If you are working with startsWith() and endsWith() then you have to be careful about leading spaces. Here is a complete example:

var str1 = " Your String Value Here.!! "; // Starts & ends with spaces    
if (str1.startsWith("Your")) { }  // returns FALSE due to the leading spaces…
if (str1.endsWith("Here.!!")) { } // returns FALSE due to trailing spaces…

var str2 = str1.trim(); // Removes all spaces (and other white-space) from start and end of `str1`.
if (str2.startsWith("Your")) { }  // returns TRUE
if (str2.endsWith("Here.!!")) { } // returns TRUE

Since this is so popular I think it is worth pointing out that there is an implementation for this method in ECMA 6 and in preparation for that one should use the 'official' polyfill in order to prevent future problems and tears.

Luckily the experts at Mozilla provide us with one:

https://developer.mozilla.org/de/docs/Web/JavaScript/Reference/Global_Objects/String/startsWith

if (!String.prototype.startsWith) {
    String.prototype.startsWith = function(searchString, position) {
        position = position || 0;
        return this.indexOf(searchString, position) === position;
    };
}

Please note that this has the advantage of getting gracefully ignored on transition to ECMA 6.


Without a helper function, just using regex's .test method:

/^He/.test('Hello world')

To do this with a dynamic string rather than a hardcoded one (assuming that the string will not contain any regexp control characters):

new RegExp('^' + needle).test(haystack)

You should check out Is there a RegExp.escape function in Javascript? if the possibility exists that regexp control characters appear in the string.


You can also return all members of an array that start with a string by creating your own prototype / extension to the the array prototype, aka

Array.prototype.mySearch = function (target) {
    if (typeof String.prototype.startsWith != 'function') {
        String.prototype.startsWith = function (str){
        return this.slice(0, str.length) == str;
      };
    }
    var retValues = [];
    for (var i = 0; i < this.length; i++) {
        if (this[i].startsWith(target)) { retValues.push(this[i]); }
    }
    return retValues;
};

And to use it:

var myArray = ['Hello', 'Helium', 'Hideout', 'Hamster'];
var myResult = myArray.mySearch('Hel');
// result -> Hello, Helium

data.substring(0, input.length) === input

var str = 'hol';
var data = 'hola mundo';
if (data.length >= str.length && data.substring(0, str.length) == str)
    return true;
else
    return false;




startswith