Recommended method of "null and undefined" check in javascript. about opentouryo

Verification result.

The results in my environment are as follows.
Target Browsers: IE 9, Chrome 42, Firefox 36.

	Property "aaa" is undefined	Property "aaa" is null
aaa === void 0	true	false
aaa == void 0	true	true
typeof aaa === 'undefined'	true	false
typeof aaa == 'undefined'	true	false
aaa === undefined	true	false
aaa == undefined	true	true
aaa === null	false	true
aaa == null	true	true

Proper use.

Is it necessary to distinguish "undefined" from "null" exactly?

• If necessary, you should use '==='.
• If not necessary, you should use '=='.

[daisukenishino]

Conclusion

Proper use.

Is it necessary to distinguish "undefined" from "null" exactly?

• If necessary, you should use '==='.
• If not necessary, you should use '=='.

Therefore, following the error.

// hiddenがnullになることがある・・・ 
if (childScreenType == null || childScreenType == undefined) { // This is redundance.
  return; 
} 

Properly, as follows.

// hiddenがnullになることがある・・・ 
if (childScreenType == undefined) { // Can check the undefined or null.
  return; 
}

Recommended check method of the attributes that is defined on the tag.

if (event.srcElement.attributes.xxxx == undefined) { // Can check the undefined or null.
  // xxxx属性が定義されていない場合、
  // ・・・
}
else {
  // xxxx属性が定義されている場合
  if (event.srcElement.attributes.xxxx.value == undefined) { // Can check the undefined or null.
    // xxxx属性がundefined or nullの場合、
    // ・・・
  }
  else if (event.srcElement.attributes.xxxx.value.toLowerCase() == "true") {
    // xxxx属性がtrueの場合、
    // ・・・
  }
  else {
    // xxxx属性が上記以外の場合、
    // ・・・
  }
}