how does Array prototype slice call work

Question

I know it is used to make arguments a real array  but I don t understand what happens when using Array prototype slice call arguments

User · Answer

Its because  as MDN notes     The arguments object is not an array  It is similar to an array  but   does not have any array properties except length  For example  it does   not have the pop method  However it can be converted to a real array    Here we are calling slice on the native object Array and not on its implementation and thats why the extra  prototype  var args   Array prototype slice call arguments

User · Answer

It uses the slice method arrays have and calls it with its this being the arguments object  This means it calls it as if you did arguments slice   assuming arguments had such a method   Creating a slice without any arguments will simply take all elements - so it simply copies the elements from arguments to an array

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when  slice   is called normally  this is an Array  and then it just iterates over that Array  and does its work      ARGUMENTS function func      console log arguments     1  2  3  4       var arrArguments   arguments slice     Uncaught TypeError  undefined is not a function   var arrArguments      slice call arguments    cp array with explicity THIS     arrArguments push  new      console log arrArguments    func 1 2 3 4    1  2  3  4   new

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We can apply  slice  from   Array prototype   Array prototype slice call        - gt         Since  slice  is available on an array s prototype chain   slice  in       - gt  true    slice     Array prototype slice    - gt  true         we can just invoke it directly     slice      - gt          arguments  has no  slice  method  slice  in arguments    - gt  false         but we can apply it the same way  Array prototype slice call arguments     - gt            In fact  though  slice  belongs to  Array prototype      it can operate on any array-like object  Array prototype slice call  0  1  length  1      - gt   1

User · Answer

What happens under the hood is that when  slice   is called normally  this is an Array  and then it just iterates over that Array  and does its work   How is this in the  slice   function an Array  Because when you do   object method         the object automatically becomes the value of this in the method    So with    1 2 3  slice        the  1 2 3  Array is set as the value of this in  slice       But what if you could substitute something else as the this value  As long as whatever you substitute has a numeric  length property  and a bunch of properties that are numeric indices  it should work  This type of object is often called an array-like object   The  call   and  apply   methods let you manually set the value of this in a function  So if we set the value of this in  slice   to an array-like object   slice   will just assume it s working with an Array  and will do its thing   Take this plain object as an example   var my object          0    zero        1    one        2    two        3    three        4    four       length  5      This is obviously not an Array  but if you can set it as the this value of  slice    then it will just work  because it looks enough like an Array for  slice   to work properly   var sliced   Array prototype slice call  my object  3      Example  http   jsfiddle net wSvkv   As you can see in the console  the result is what we expect     three   four      So this is what happens when you set an arguments object as the this value of  slice    Because arguments has a  length property and a bunch of numeric indices   slice   just goes about its work as if it were working on a real Array

User · Answer

I m just writing this to remind myself         Array prototype slice call arguments       Array prototype slice arguments 1   arguments 2   arguments 3              arguments 1   arguments 2   arguments 3           Or just use this handy function  A to turn most things into an array   function hasArrayNature a        return   a  amp  amp   typeof a     object     typeof a     function    amp  amp   length  in a  amp  amp     setInterval  in a   amp  amp   Object prototype toString call a        object Array       callee  in a     item  in a      function  A b        if   hasArrayNature b   return   b        if  b item            var a   b length  c   new Array a           while  a--  c a    b a           return c            return Array prototype slice call b       example usage     function test          A  arguments   forEach  function arg            console log  Argument      arg

User · Answer

Let s assume you have  function apply thisArg  argArray       The apply method invokes a function  passing in the object that will be bound to this   and an optional array of arguments    The slice   method selects a part of an array  and returns the new array   So when you call Array prototype slice apply arguments   0   the array slice method is invoked  bind  on arguments

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Maybe a bit late  but the answer to all of this mess is that call   is used in JS for inheritance  If we compare this to Python or PHP  for example  call is used respectively as super   init   or parent   construct     This is an example of its usage that clarifies all   function Teacher first  last  age  gender  interests  subject      Person call this  first  last  age  gender  interests      this subject   subject      Reference  https   developer mozilla org en-US docs Learn JavaScript Objects Inheritance

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Dont forget  that a low-level basics of this behaviour is the type-casting that integrated in JS-engine entirely   Slice just takes object  thanks to existing arguments length property  and returns array-object casted after doing all operations on that   The same logics you can test if you try to treat String-method with an INT-value   String prototype bold call 11       returns   lt b gt 11 lt  b gt     And that explains statement above

User · Answer

First  you should read how function invocation works in JavaScript  I suspect that alone is enough to answer your question  But here s a summary of what is happening   Array prototype slice extracts the slice method from Array s prototype  But calling it directly won t work  as it s a method  not a function  and therefore requires a context  a calling object  this   otherwise it would throw Uncaught TypeError  Array prototype slice called on null or undefined   The call   method allows you to specify a method s context  basically making these two calls equivalent   someObject slice 1  2   slice call someObject  1  2     Except the former requires the slice method to exist in someObject s prototype chain  as it does for Array   whereas the latter allows the context  someObject  to be manually passed to the method   Also  the latter is short for   var slice   Array prototype slice  slice call someObject  1  2     Which is the same as   Array prototype slice call someObject  1  2

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Array prototype slice call arguments  is the old-fashioned way to convert an arguments into an array   In ECMAScript 2015  you can use Array from or the spread operator   let args   Array from arguments    let args       arguments

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The arguments object is not actually an instance of an Array  and does not have any of the Array methods  So  arguments slice      will not work because the arguments object does not have the slice method   Arrays do have this method  and because the arguments object is very similar to an array  the two are compatible  This means that we can use array methods with the arguments object  And since array methods were built with arrays in mind  they will return arrays rather than other argument objects   So why use Array prototype  The Array is the object which we create new arrays from  new Array     and these new arrays are passed methods and properties  like slice  These methods are stored in the  Class  prototype object  So  for efficiency sake  instead of accessing the slice method by  new Array    slice call   or    slice call    we just get it straight from the prototype  This is so we don t have to initialise a new array   But why do we have to do this in the first place  Well  as you said  it converts an arguments object into an Array instance  The reason why we use slice  however  is more of a  hack  than anything  The slice method will take a  you guessed it  slice of an array and return that slice as a new array  Passing no arguments to it  besides the arguments object as its context  causes the slice method to take a complete chunk of the passed  array   in this case  the arguments object  and return it as a new array

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Array prototype slice function start end       let res         start start  0      end end  this length     for let i start i lt end i             res push this i             return res     when you do  Array prototype slice call arguments    arguments becomes the value of this in slice  and then slice returns an array

User · Answer

Normally  calling  var b   a slice      will copy the array a into b  However  we can t do  var a   arguments slice      because arguments isn t a real array  and doesn t have slice as a method  Array prototype slice is the slice function for arrays  and call runs the function with this set to arguments

[javascript] how does Array.prototype.slice.call() work?

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