Why does return the string 10

Question

This is valid and returns the string  10  in JavaScript  more examples here     x000D   x000D  console log                    x000D   x000D   x000D    Why  What is happening here

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gt  1             0     0   1         gt   0    Then we have a string concatenation  1  0  toString     10

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This one evaluates to the same but a bit smaller                       - is an array is converted that is converted to 0 when you add or subtract from it  so hence       0     - evaluates to false  so hence      evaluates to true       - converts the true to a numeric value that evaluates to true  so in this case 1     - appends an empty string to the expression causing the number to be converted to string     - evaluates to 0   so is evaluates to    true          0  1        0  10    So now you got that  try this one

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evaluates to 0       then summing    operation  it with anything converts array content to its string representation consisting of elements joined with comma   Anything other like taking index of array  have grater priority than   operation  is ordinal and is nothing interesting

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Let   s make it simple                        10   var a              var b              so a       and b     0     a      then a    1 and b is still that array  0     when you sum the var a and an array  it will sum b as a string just like that   1    0     10

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The following is adapted from a blog post answering this question that I posted while this question was still closed  Links are to  an HTML copy of  the ECMAScript 3 spec  still the baseline for JavaScript in today s commonly used web browsers   First  a comment  this kind of expression is never going to show up in any  sane  production environment and is only of any use as an exercise in just how well the reader knows the dirty edges of JavaScript  The general principle that JavaScript operators implicitly convert between types is useful  as are some of the common conversions  but much of the detail in this case is not   The expression                   may initially look rather imposing and obscure  but is actually relatively easy break down into separate expressions  Below I   ve simply added parentheses for clarity  I can assure you they change nothing  but if you want to verify that then feel free to read up about the grouping operator  So  the expression can be more clearly written as                                Breaking this down  we can simplify by observing that     evaluates to 0  To satisfy yourself why this is true  check out the unary   operator and follow the slightly tortuous trail which ends up with ToPrimitive converting the empty array into an empty string  which is then finally converted to 0 by ToNumber  We can now substitute 0 for each instance of                0       0    Simpler already  As for        0   that   s a combination of the prefix increment operator       an array literal defining an array with single element that is itself an empty array        and a property accessor   0   called on the array defined by the array literal   So  we can simplify      0  to just    and we have       right  In fact  this is not the case because evaluating      throws an error  which may initially seem confusing  However  a little thought about the nature of    makes this clear  it   s used to increment a variable  e g    i  or an object property  e g    obj count   Not only does it evaluate to a value  it also stores that value somewhere  In the case of       it has nowhere to put the new value  whatever it may be  because there is no reference to an object property or variable to update  In spec terms  this is covered by the internal PutValue operation  which is called by the prefix increment operator   So then  what does        0  do  Well  by similar logic as      the inner array is converted to 0 and this value is incremented by 1 to give us a final value of 1  The value of property 0 in the outer array is updated to 1 and the whole expression evaluates to 1   This leaves us with  1    0        which is a simple use of the addition operator  Both operands are first converted to primitives and if either primitive value is a string  string concatenation is performed  otherwise numeric addition is performed   0  converts to  0   so string concatenation is used  producing  10    As a final aside  something that may not be immediately apparent is that overriding either one of the toString   or valueOf   methods of Array prototype will change the result of the expression  because both are checked and used if present when converting an object into a primitive value  For example  the following  Array prototype toString   function       return  foo                              produces  NaNfoo   Why this happens is left as an exercise for the reader

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Step by steps of that    turn value to a number and if you add to an empty array       as it s empty and is equal to 0  it will   So from there  now look into your code  it s                       And there is plus between them                      So these       will return  0  as they have an empty array which gets converted to 0 inside the other array     So as imagine  the first value is a 2-dimensional array with one array inside    so           will be equal to      0  which will return        And at the end    convert it and increase it to 1     So you can imagine  1    0  will be  10

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If we split it up  the mess is equal to                         In JavaScript  it is true that         0    converts something into a number  and in this case it will come down to     or 0  see specification details below    Therefore  we can simplify it     has precendence over             0     0    Because      0  means  get the first element from       it is true that        0  returns the inner array       Due to references it s wrong to say      0          but let s call the inner array A to avoid the wrong notation      before its operand means    increment by one and return the incremented result     So        0  is equivalent to Number A    1  or  A   1    Again  we can simplify the mess into something more legible  Let s substitute    back for A          1     0    Before     can coerce the array into the number 0  it needs to be coerced into a string first  which is     again  Finally  1 is added  which results in 1           1             1         1       0   1   0   1      1   Let s simplify it even more   1    0    Also  this is true in JavaScript   0      0   because it s joining an array with one element  Joining will concatenate the elements separated by    With one element  you can deduce that this logic will result in the first element itself   In this case    sees two operands  a number and an array  It   s now trying to coerce the two into the same type  First  the array is coerced into the string  0   next  the number is coerced into a string   1    Number   String     String    1     0       10     Yay      Specification details for       This is quite a maze  but to do      first it is being converted to a string because that s what   says      11 4 6 Unary   Operator      The unary   operator converts its operand to Number type       The production UnaryExpression     UnaryExpression is evaluated as follows          Let expr be the result of evaluating UnaryExpression    Return ToNumber GetValue expr         ToNumber   says      Object      Apply the following steps          Let primValue be ToPrimitive input argument  hint String     Return ToString primValue        ToPrimitive   says      Object      Return a default value for the Object  The default value of an object is retrieved by calling the   DefaultValue   internal method of the object  passing the optional hint PreferredType  The behaviour of the   DefaultValue   internal method is defined by this specification for all native ECMAScript objects in 8 12 8      DefaultValue   says      8 12 8   DefaultValue    hint       When the   DefaultValue   internal method of O is called with hint String  the following steps are taken          Let toString be the result of calling the   Get   internal method of object O with argument  toString     If IsCallable toString  is true then          a  Let str be the result of calling the   Call   internal method of toString  with O as the this value and an empty argument list       b  If str is a primitive value  return str    The  toString of an array says      15 4 4 2 Array prototype toString          When the toString method is called  the following steps are taken          Let array be the result of calling ToObject on the this value    Let func be the result of calling the   Get   internal method of array with argument  join     If IsCallable func  is false  then let func be the standard built-in method Object prototype toString  15 2 4 2     Return the result of calling the   Call   internal method of func providing array as the this value and an empty arguments list       So     comes down to      because    join            Again  the   is defined as      11 4 6 Unary   Operator      The unary   operator converts its operand to Number type       The production UnaryExpression     UnaryExpression is evaluated as follows          Let expr be the result of evaluating UnaryExpression    Return ToNumber GetValue expr         ToNumber is defined for    as      The MV of StringNumericLiteral      empty  is 0    So         0  and thus         0

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Perhaps the shortest possible ways to evaluate an expression into  10  without digits are                     10   -                 10                Explanation                           Converts  to 0   0 converts to true   true converts to 1  -      - -1  which is 1              Converts  to 0   0  is an array with a single element 0   Then JS evaluates the 1    0   thus Number   Array expression  Then the ECMA specification works    operator converts both operands to a string by calling the toString   valueOf   functions from the base Object prototype  It operates as an additive function if both operands of an expression are numbers only  The trick is that arrays easily convert their elements into a concatenated string representation   Some examples   1             1 object Object   1             1  1   new Date          1Wed Jun 19 2013 12 13 25 GMT 0400  Caucasus Standard Time     There s a nice exception that two Objects addition results in NaN                       1     2         12                  NaN  a 1     b 2            NaN  1         2             1  object Object 2  object Object

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Unary plus given string converts to number  Increment operator given string converts and increments by 1           Empty String     or      evaluates 0                       10         0     0    First part is gives zeroth element of the array which is empty string  1 0  10

[javascript] Why does ++[[]][+[]]+[+[]] return the string "10"?

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