現行版JavaScriptの仕様書は以下からPDFファイルをダウンロードしてください。

現行版っていうか、まあ、うん、現行版です。

演算子の仕様

==の場合

11.9.1 The Equals Operator ( == )

The production EqualityExpression : EqualityExpression == RelationalExpression is evaluated as follows:

  1. Let lref be the result of evaluating EqualityExpression.
  2. Let lval be GetValue(lref).
  3. Let rref be the result of evaluating RelationalExpression.
  4. Let rval be GetValue(rref).
  5. Return the result of performing abstract equality comparison rval == lval. (see 11.9.3).

===の場合

11.9.4 The Strict Equals Operator ( === )

The production EqualityExpression : EqualityExpression === RelationalExpression is evaluated as follows:

  1. Let lref be the result of evaluating EqualityExpression.
  2. Let lval be GetValue(lref).
  3. Let rref be the result of evaluating RelationalExpression.
  4. Let rval be GetValue(rref).
  5. Return the result of performing the strict equality comparison rval === lval. (See 11.9.6)

というわけで

演算子の直接の処理は、左右の値を評価してから別途内部的な==、===に渡しているだけで、ほとんど同じ処理。それぞれについて確認する。

内部で利用されている同値評価アルゴリズム

英語は読飛ばしても可。

抽象同値評価 (abstract equality comparison)

11.9.3 The Abstract Equality Comparison Algorithm

The comparison x == y, where x and y are values, produces true or false. Such a comparison is performed as follows:

  1. If Type(x) is the same as Type(y), then
    1. If Type(x) is Undefined, return true.
    2. If Type(x) is Null, return true.
    3. If Type(x) is Number, then
      1. If x is NaN, return false.
      2. If y is NaN, return false.
      3. If x is the same Number value as y, return true.
      4. If x is +0 and y is -0, return true.
      5. If x is -0 and y is +0, return true.
      6. Return false.
    4. If Type(x) is String, then return true if x and y are exactly the same sequence of characters (same length and same characters in corresponding positions). Otherwise, return false.
    5. If Type(x) is Boolean, return true if x and y are both true or both false. Otherwise, return false.
    6. Return true if x and y refer to the same object. Otherwise, return false.
  2. If x is null and y is undefined, return true.
  3. If x is undefined and y is null, return true.
  4. If Type(x) is Number and Type(y) is String, return the result of the comparison x == ToNumber(y).
  5. If Type(x) is String and Type(y) is Number, return the result of the comparison ToNumber(x) == y.
  6. If Type(x) is Boolean, return the result of the comparison ToNumber(x) == y.
  7. If Type(y) is Boolean, return the result of the comparison x == ToNumber(y).
  8. If Type(x) is either String or Number and Type(y) is Object, return the result of the comparison x == ToPrimitive(y).
  9. If Type(x) is Object and Type(y) is either String or Number, return the result of the comparison ToPrimitive(x) == y.
  10. Return false.

NOTE 1 Given the above definition of equality:

  • String comparison can be forced by: “” + a == “” + b.
  • Numeric comparison can be forced by: +a == +b.
  • Boolean comparison can be forced by: !a == !b.

NOTE 2 The equality operators maintain the following invariants:

  • A != B is equivalent to !(A == B).
  • A == B is equivalent to B == A, except in the order of evaluation of A and B.

NOTE 3 The equality operator is not always transitive. For example, there might be two distinct String objects, each representing the same String value; each String object would be considered equal to the String value by the == operator, but the two String objects would not be equal to each other. For Example:

  • new String(“a”) == “a” and “a” == new String(“a”)are both true.
  • new String(“a”) == new String(“a”) is false.

NOTE 4 Comparison of Strings uses a simple equality test on sequences of code unit values. There is no attempt to use the more complex, semantically oriented definitions of character or string equality and collating order defined in the Unicode specification. Therefore Strings values that are canonically equal according to the Unicode standard could test as unequal. In effect this algorithm assumes that both Strings are already in normalised form.

なっげえ!

完全同値評価 (strict equality comparison)

11.9.6 The Strict Equality Comparison Algorithm

The comparison x === y, where x and y are values, produces true or false. Such a comparison is performed as follows:

  1. If Type(x) is different from Type(y), return false.
  2. If Type(x) is Undefined, return true.
  3. If Type(x) is Null, return true.
  4. If Type(x) is Number, then
    1. If x is NaN, return false.
    2. If y is NaN, return false.
    3. If x is the same Number value as y, return true.
    4. If x is +0 and y is -0, return true.
    5. If x is -0 and y is +0, return true.
    6. Return false.
  5. If Type(x) is String, then return true if x and y are exactly the same sequence of characters (same length and same characters in corresponding positions); otherwise, return false.
  6. If Type(x) is Boolean, return true if x and y are both true or both false; otherwise, return false.
  7. Return true if x and y refer to the same object. Otherwise, return false. NOTE This algorithm differs from the SameValue Algorithm (9.12) in its treatment of signed zeroes and NaNs.

つまり?

==のアルゴリズムは===のそれにおける演算を全て含み、さらにそれ以外の演算を行っている。つまり、===より低速。

まあ英語を全部読むまでもなく、まあ後者===の方が短いんだから速いっていう解釈で良いと思います。

まとめ

==より===の方がやってる事が少ないので速い。

(まあ一般的には誤差として無視出来る程度の速度差だと思うけどさ。未検証。)