javascript - Incrementing the result of Map.get() -

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i following error when trying increment value obtained map in console in chrome (50.0.2661.86):

uncaught referenceerror: invalid left-hand side expression in postfix operation(…)

and similar in node (4.4.3):

referenceerror: invalid left-hand side expression in postfix operation     @ repl:1:3     @ replserver.defaulteval (repl.js:262:27)     @ bound (domain.js:287:14)     @ replserver.runbound [as eval] (domain.js:300:12)     @ replserver.<anonymous> (repl.js:431:12)     @ emitone (events.js:82:20)     @ replserver.emit (events.js:169:7)     @ replserver.interface._online (readline.js:211:10)     @ replserver.interface._line (readline.js:550:8)     @ replserver.interface._ttywrite (readline.js:827:14)

the offending code is:

var m = new map() m.set(1, 0) m.get(1) var n = m.get(1)++ // uncaught referenceerror: invalid left-hand side expression in postfix operation(…)

the following fail:

var n = ++m.get(1) var n = ++(m.get(1))

a bug in v8 maybe? or misunderstanding in what's happening syntax-wise ++ operator?

i'm not expert in js evaluation scheme, here's try @ explaining that.

in js, (1)++ invalid; can't change a value. can change value attribution name:

var = 1; a++;

in case value of 1 doesn't change; changes a points different value.

similarly, can't increment value returned .get(); need convert named expression before can change that.

in c++ terms ++ needs lvalue, function return rvalue.


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