javascript - Syntax of Closures -

function makeincreasebyfunction(increasebyamount) {   return function (numbertoincrease) {     return numbertoincrease + increasebyamount;   }; }  makeincreasebyfunction(3)(10); 

updated clarity can explain why (3)(10) written way is? understand 10 being supplied argument inner function, why syntactically notated this? if had 3 nested functions, arguments written in order in (3)(10)(20)?

with intermediate variable:

var increaseby3 = makeincreasebyfunction(3); var foo = increaseby3(10); 

without intermediate variable:

var foo = makeincreasebyfunction(3)(10); 

in both cases, first invokation passes argument 3 makeincreasebyfunction, , result returns inner function has closed on increasebyamount value of 3. whether create variable intermediate function returned makeincreasebyfunction, or invoke directly, same thing.

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can explain little bit more detail how in var foo = makeincreasebyfunction(3)(10); 10 getting inner function? looks syntactically different how arguments passed in javascript me. – ggg

makeincreasebyfunction(3) returns function, "inner function" defined inside makeincreasebyfunction. functions, call function ( arguments ) syntax. can write if makes more sense way:

( makeincreasebyfunction(3) )(10) 

what happens here makeincreasebyfunction(3) gets called first , returns ⟪inner function⟫, , call ⟪inner function⟫(10).

if evaluating hand (i think meant "syntactically"), think of happening step-by-step this:

// original invocation var foo = makeincreasebyfunction(3)(10);  // substitute definition of makeincreasebyfunction var foo = (function (increasebyamount) {   return function (numbertoincrease) {     return numbertoincrease + increasebyamount;   }; })(3)(10);   // apply parameter 3 var foo = (function (numbertoincrease) {   return numbertoincrease + 3; })(10);   // apply parameter 10 var foo = 10 + 3;  // final result var foo = 13; 

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note: if want technical, we're doing here 2 beta reductions—but unless have background lambda calculus confuse more you!