JavaScriptの switch(true) で、複数のcaseにマッチする時の挙動がわからないという意見があったので書いた。

switch_true_with_fall_through.js

// switch(true) with fall through

var ary = [0, 1, 2, 3, 4, 5];

function compare(a, b) {
  console.log("compare " + a + " with " + b);
  return b - a;
}

console.log("==========");
console.log("with fall through");
ary.forEach(function(item) {
  console.log("----------");
  console.log("item : " + item);
  switch(true) {
  default                   : console.log("run : default"          );
  case compare(item, 0) >= 0: console.log("run : " + item + "/" + 0);
  case compare(item, 1) >= 0: console.log("run : " + item + "/" + 1);
  case compare(item, 2) >= 0: console.log("run : " + item + "/" + 2);
  case compare(item, 3) >= 0: console.log("run : " + item + "/" + 3);
  case compare(item, 4) >= 0: console.log("run : " + item + "/" + 4);
  }
});

// result
/*
==========
with fall through
----------
item : 0
compare 0 with 0
run : 0/0
run : 0/1
run : 0/2
run : 0/3
run : 0/4
----------
item : 1
compare 1 with 0
compare 1 with 1
run : 1/1
run : 1/2
run : 1/3
run : 1/4
----------
item : 2
compare 2 with 0
compare 2 with 1
compare 2 with 2
run : 2/2
run : 2/3
run : 2/4
----------
item : 3
compare 3 with 0
compare 3 with 1
compare 3 with 2
compare 3 with 3
run : 3/3
run : 3/4
----------
item : 4
compare 4 with 0
compare 4 with 1
compare 4 with 2
compare 4 with 3
compare 4 with 4
run : 4/4
----------
item : 5
compare 5 with 0
compare 5 with 1
compare 5 with 2
compare 5 with 3
compare 5 with 4
run : default
run : 5/0
run : 5/1
run : 5/2
run : 5/3
run : 5/4
*/

switch_true_without_fall_through.js

// switch(true) without fall through

var ary = [0, 1, 2, 3, 4, 5];

function compare(a, b) {
  console.log("compare " + a + " with " + b);
  return b - a;
}

console.log("==========");
console.log("without fall through");
ary.forEach(function(item) {
  console.log("----------");
  console.log("item : " + item);
  switch(true) {
  default                   : console.log("run : default"          ); break;
  case compare(item, 0) >= 0: console.log("run : " + item + "/" + 0); break;
  case compare(item, 1) >= 0: console.log("run : " + item + "/" + 1); break;
  case compare(item, 2) >= 0: console.log("run : " + item + "/" + 2); break;
  case compare(item, 3) >= 0: console.log("run : " + item + "/" + 3); break;
  case compare(item, 4) >= 0: console.log("run : " + item + "/" + 4); break;
  }
});

// result
/*
==========
without fall through
----------
item : 0
compare 0 with 0
run : 0/0
----------
item : 1
compare 1 with 0
compare 1 with 1
run : 1/1
----------
item : 2
compare 2 with 0
compare 2 with 1
compare 2 with 2
run : 2/2
----------
item : 3
compare 3 with 0
compare 3 with 1
compare 3 with 2
compare 3 with 3
run : 3/3
----------
item : 4
compare 4 with 0
compare 4 with 1
compare 4 with 2
compare 4 with 3
compare 4 with 4
run : 4/4
----------
item : 5
compare 5 with 0
compare 5 with 1
compare 5 with 2
compare 5 with 3
compare 5 with 4
run : default
*/