Multiple left-hand assignment with JavaScript

Question

var var1   1      var2   1      var3   1    This is equivalent to this   var var1   var2   var3   1    I m fairly certain this is the order the variables are defined  var3  var2  var1  which would be equivalent to this   var var3   1  var2   var3  var1   var2    Is there any way to confirm this in JavaScript   Using some profiler possibly

User · Answer

Try this   var var1 42  var var2   alert var2   var1     show result of assignment expression is assigned value alert var2      show assignment did occur    Note the single     in the first alert   This will show that the result of an assignment expression is the assigned value  and the 2nd alert will show you that assignment did occur   It follows logically that assignment must have chained from right to left   However  since this is all atomic to the javascript  there s no threading  a particular engine may choose to actually optimize it a little differently

User · Answer

It is clear by now  that they are not the same  The way to code that is  var var1  var2  var3 var1   var2   var3   1   And  what about let assigment  Exactly the same as var  don t let the let assigment confuse you because of block scope   let var1   var2   1    here var2 belong to the global scope   We could do the following   let v1  v2  v3 v1   v2   v3   2   Note  btw  I do not recommend use multiple assignments  not even multiple declarations in the same line

User · Answer

Assignment in javascript works from right to left  var var1   var2   var3   1     If the value of any of these variables is 1 after this statement  then logically it must have started from the right  otherwise the value or var1 and var2 would be undefined   You can think of it as equivalent to var var1    var2    var3   1    where the inner-most set of parenthesis is evaluated first

User · Answer

a    b    string is truthy       b gets string  a gets b  which is a primitive  copy  a    b     c   yes         they point to the same object  a     b  not a copy       a  amp  amp  b  is logically  a   b   a  and behaves like multiplication  eg    a     b    a    b  is logically  a   a   b  and behaves like addition  eg    a     b    a   0  b  is short for not caring if a is truthy  implicitly return b    a    b   0   amp  amp   nope  but a is 0 and b is 0      b is falsey   order of operations a    b    b is this string    amp  amp   a gets this string      b is truthy   order of ops   JavaScript Operator Precedence  Order of Operations   Note that the comma operator is actually the least privileged operator  but parenthesis are the most privileged  and they go hand-in-hand when constructing one-line expressions      Eventually  you may need  thunks  rather than hardcoded values  and to me  a thunk is both the function and the resultant value  the same  thing     const windowInnerHeight        gt  0 8   window innerHeight     a thunk  windowInnerHeight       a thunk

User · Answer

coffee-script can accomplish this with aplomb    for x in    a    b    c    then    x     true          a  true      b  true      c  true

User · Answer

var var1   1         var2   1         var3   1    In this case  var keyword is applicable to all the three variables   var var1   1      var2   1      var3   1    which is not equivalent to this      var var1   var2   var3   1    In this case behind the screens var keyword is only applicable to var1 due to variable hoisting and rest of the expression is evaluated normally so the variables var2  var3 are becoming globals   Javascript treats this code in this order      var 1 is local to the particular scope because of var keyword var2 and var3 will become globals because they ve used without var keyword     var var1      only variable declarations will be hoisted   var1  var2  var3   1

User · Answer

Actually   var var1   1  var2   1  var3   1    is not equivalent to   var var1   var2   var3   1    The difference is in scoping    x000D   x000D  function good     x000D    var var1   1  var2   1  var3   1  x000D    x000D   x000D  function bad     x000D    var var1   var2   var3   1  x000D    x000D   x000D  good    x000D  console log window var2      undefined x000D   x000D  bad    x000D  console log window var2      1  Aggh  x000D   x000D   x000D    Actually this shows that assignment are right associative  The bad example is equivalent to   var var1    window var2    window var3   1

[javascript] Multiple left-hand assignment with JavaScript

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