How to check if a string StartsWith another string

Question

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

User · Answer

The best performant solution is to stop using library calls and just recognize that you're working with two arrays. A hand-rolled implementation is both short and also faster than every other solution I've seen here.

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;
}

For performance comparisons (success and failure), see http://jsperf.com/startswith2/4. (Make sure you check for later versions that may have trumped mine.)

User · Answer

I just learned about this string library   http   stringjs com   Include the js file and then use the S variable like this   S  hi there   endsWith  hi there     It can also be used in NodeJS by installing it   npm install string   Then requiring it as the S variable   var S   require  string      The web page also has links to alternate string libraries  if this one doesn t take your fancy

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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

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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

User · Answer

data substring 0  input length      input

User · Answer

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     gt  true

User · Answer

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  lt  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 - gt  Hello  Helium

User · Answer

The question is a bit old  but I wanted to write this answer to show you some benchmarks I made based on all the answers provided here and the jsperf shared by Jim Buck    I basically needed a fast way to find if a long needle is within a long haystack and they are very similar except for the last characters   Here s the code I have written which for each function  splice  substring  startsWith  etc   tests both when they return false and true against a haystack string  nestedString  of 1 000 0001 characters and a falsy or truthy needle string of 1 000 000 chars  testParentStringFalse and testParentStringTrue  respectively       nestedString is made of 1 000 001  1  repeated characters  var nestedString             testParentStringFalse is made of 1 000 000 characters     all characters are repeated  1   but the last one is  2      so for this string the test should return false  var testParentStringFalse             testParentStringTrue is made of 1 000 000  1  repeated characters     so for this string the test should return true  var testParentStringTrue             You can make these very long strings by running the following bash command    and edit each one as needed in your editor     NOTE  on OS X   pbcopy  copies the string to the clipboard buffer            on Linux  you would probably need to replace it with  xclip               printf  1  0s   1  1000000    pbcopy      function testString         let dateStart     let dateEnd     let avg     let count   100000     const falseResults          const trueResults              slice        console log           gt  slice       dateStart    new Date       var res     for  let j   0  j  lt  count  j              res   nestedString slice 0  testParentStringFalse length      testParentStringFalse           dateEnd    new Date       avg    dateEnd - dateStart  count     falseResults falseResults length              label   slice           avg           console log  testString   slice   false   res   avg      avg    ms        dateStart    new Date       var res     for  let j   0  j  lt  count  j              res   nestedString slice 0  testParentStringTrue length      testParentStringTrue           dateEnd    new Date       avg    dateEnd - dateStart  count     trueResults trueResults length              label   slice           avg           console log  testString   slice   true   res   avg      avg    ms       console log   lt          slice       console log            slice END            lastIndexOf        console log           gt  lastIndexOf       dateStart    new Date       var res     for  let j   0  j  lt  count  j              res   nestedString lastIndexOf testParentStringFalse  0      0           dateEnd    new Date       avg    dateEnd - dateStart  count     falseResults falseResults length              label   lastIndexOf           avg           console log  testString   lastIndexOf   false   res   avg      avg    ms        dateStart    new Date       var res     for  let j   0  j  lt  count  j              res   nestedString lastIndexOf testParentStringTrue  0      0           dateEnd    new Date       avg    dateEnd - dateStart  count     trueResults trueResults length              label   lastIndexOf           avg           console log  testString   lastIndexOf   true   res   avg      avg    ms       console log   lt          lastIndexOf       console log            lastIndexOf END            indexOf        console log           gt  indexOf       dateStart    new Date       var res     for  let j   0  j  lt  count  j              res   nestedString indexOf testParentStringFalse      0           dateEnd    new Date       avg    dateEnd - dateStart  count     falseResults falseResults length              label   indexOf           avg           console log  testString   indexOf   false   res   avg      avg    ms        dateStart    new Date       var res     for  let j   0  j  lt  count  j              res   nestedString indexOf testParentStringTrue      0           dateEnd    new Date       avg    dateEnd - dateStart  count     trueResults trueResults length              label   indexOf           avg           console log  testString   indexOf   true   res   avg      avg    ms       console log   lt          indexOf       console log            indexOf END            substring        console log           gt  substring       dateStart    new Date       var res     for  let j   0  j  lt  count  j              res   nestedString substring 0  testParentStringFalse length      testParentStringFalse           dateEnd    new Date       avg    dateEnd - dateStart  count     falseResults falseResults length              label   substring           avg           console log  testString   substring   false   res   avg      avg    ms        dateStart    new Date       var res     for  let j   0  j  lt  count  j              res   nestedString substring 0  testParentStringTrue length      testParentStringTrue           dateEnd    new Date       avg    dateEnd - dateStart  count     trueResults trueResults length              label   substring           avg           console log  testString   substring   true   res   avg      avg    ms       console log   lt          substring       console log            substring END            startsWith        console log           gt  startsWith       dateStart    new Date       var res     for  let j   0  j  lt  count  j              res   nestedString startsWith testParentStringFalse            dateEnd    new Date       avg    dateEnd - dateStart  count     falseResults falseResults length              label   startsWith           avg           console log  testString   startsWith   false   res   avg      avg    ms        dateStart    new Date       var res     for  let j   0  j  lt  count  j              res   nestedString startsWith testParentStringTrue            dateEnd    new Date       avg    dateEnd - dateStart  count     trueResults trueResults length              label   startsWith           avg           console log  testString   startsWith   true   res   avg      avg    ms       console log   lt          startsWith       console log            startsWith END         falseResults sort  a  b    gt  a avg - b avg      trueResults sort  a  b    gt  a avg - b avg       console log  false results from fastest to slowest avg    falseResults      console log  true results from fastest to slowest avg    trueResults       I runned this benchmark test on Chrome 75  Firefox 67  Safari 12 and Opera 62   I haven t included Edge and IE because I do not have them on this machine  but if someone of you wants to run the script against Edge and at least IE 9 and share the output here I would be very curious to see the results   Just remember that you need to recreate the 3 long strings and save the script in a file which you then open in your browser as copy paste on the browser s console will block it as each string s length is    1 000 000    Here are the outputs   Chrome 75  substring wins    false results from fastest to slowest avg  1     label   substring   avg  0 08271  2     label   slice   avg  0 08615  3     label   lastIndexOf   avg  0 77025  4     label   indexOf   avg  1 64375  5     label   startsWith   avg  3 5454   true results from fastest to slowest avg  1     label   substring   avg  0 08213  2     label   slice   avg  0 08342  3     label   lastIndexOf   avg  0 7831  4     label   indexOf   avg  0 88988  5     label   startsWith   avg  3 55448    Firefox 67  indexOf wins    false results from fastest to slowest avg 1     label   indexOf   avg  0 1807  2     label   startsWith   avg  0 74621  3     label   substring   avg  0 74898  4     label   slice   avg  0 78584  5     label   lastIndexOf   avg  0 79668   true results from fastest to slowest avg  1     label   indexOf   avg  0 09528  2     label   substring   avg  0 75468  3     label   startsWith   avg  0 76717  4     label   slice   avg  0 77222  5     label   lastIndexOf   avg  0 80527    Safari 12  slice wins for false results  startsWith wins for true results  also Safari is the fastest in terms of total time to to execute the whole test    false results from fastest to slowest avg  1      label     slice     avg   0 0362   2      label     startsWith     avg   0 1141   3      label     lastIndexOf     avg   0 11512   4      label     substring     avg   0 14751   5      label     indexOf     avg   0 23109    true results from fastest to slowest avg  1      label     startsWith     avg   0 11207   2      label     lastIndexOf     avg   0 12196   3      label     substring     avg   0 12495   4      label     indexOf     avg   0 33667   5      label     slice     avg   0 49923     Opera 62  substring wins  Results are similar to Chrome and I am not surprised as Opera is based on Chromium and Blink    false results from fastest to slowest avg    label   substring   avg  0 09321    label   slice   avg  0 09463    label   lastIndexOf   avg  0 95347    label   indexOf   avg  1 6337    label   startsWith   avg  3 61454   true results from fastest to slowest avg  1     label   substring   avg  0 08855  2     label   slice   avg  0 12227  3     label   indexOf   avg  0 79914  4     label   lastIndexOf   avg  1 05086  5     label   startsWith   avg  3 70808    It turns out every browser has its own implementation details  apart Opera  which is based on Chrome s Chromium and Blink    Of course  further test with different use cases could and should be performed  e g  when needle is really short compared to haystack  when haystack is shorter than needle  etc      but in my case I needed to compare very long strings and wanted to share it here

User · Answer

Best solution   function startsWith str  word        return str lastIndexOf word  0      0      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   With method    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

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var str    hol   var data    hola mundo   if  data length  gt   str length  amp  amp  data substring 0  str length     str      return true  else     return false

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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  amp  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

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You can use ECMAScript 6 s String prototype startsWith   method  but it s not yet supported in all browsers  You ll want to use a shim polyfill to add it on browsers that don t support it  Creating an implementation that complies with all the details laid out in the spec is a little complicated  If you want a faithful shim  use either   Matthias Bynens s String prototype startsWith shim  or The es6-shim  which shims as much of the ES6 spec as possible  including String prototype startsWith   Once you ve shimmed the method  or if you re only supporting browsers and JavaScript engines that already have it   you can use it like this   x000D   x000D     console log  Hello World   startsWith  He        true      var haystack    Hello world   var prefix    orl   console log haystack startsWith prefix       false x000D   x000D   x000D

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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

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Based on the answers here  this is the version I am now using  as it seems to give the best performance based on JSPerf testing  and is functionally complete as far as I can tell    if typeof String prototype startsWith     function        String prototype startsWith   function str           if str    null  return false          var i   str length          if this length  lt  i  return false          for --i   i  gt   0   amp  amp   this i      str i    --i  continue          return i  lt  0            This was based on startsWith2 from here  http   jsperf com startswith2 6  I added a small tweak for a tiny performance improvement  and have since also added a check for the comparison string being null or undefined  and converted it to add to the String prototype using the technique in CMS s answer   Note that this implementation doesn t support the  position  parameter which is mentioned in this Mozilla Developer Network page  but that doesn t seem to be part of the ECMAScript proposal anyway

User · Answer

I am not sure for javascript but in typescript i did something like   var str    something     lt String gt str  startsWith  some      I guess it should work on js too   I hope it helps

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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  lt  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  lt  prefix length      return false    for  var i   prefix length - 1   i  gt   0   amp  amp   str i      prefix i    --i      continue    return i  lt  0

User · Answer

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

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The string object has methods like startsWith  endsWith and includes methods   StartsWith  checks whether the given string starts at the beginning or not   endsWith  checks whether the given string is at the end or not   includes  checks whether the given string is present at any part or not    You can find the complete difference between these three in the bellow youtube video https   www youtube com watch v E-hyeSwg0PA

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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

[javascript] How to check if a string "StartsWith" another string?

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