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JavaScript undefined値の判定

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javaSciprt undefined値の判定

javaScriptでは初期化していない変数にはundefinedがセットされている
※undefinedはプリミティブ値を示す

javascirpt
var tmp;
console.log(tmp); //->undefined

当然文字列ではないので文字判定ではNG

javascirpt
var tmp;
if( tmp == "undefined"){
    console.log("true");
}else{
    console.log("false");
}
//->false

typeof 演算子を利用する

javascirpt
var tmp;
if( typeof tmp === 'undefined'){
    console.log("true");
}else{
    console.log("false");
}
//->true

MDN参考

厳格な等価評価

javascirpt
var tmp;
if( tmp === undefined){
    console.log("true");
}else{
    console.log("false");
}
//->true

undefined と厳格を行う事で評価することが出来るが、この方法は推奨されていない
MDN参考

void 0と比較

javascirpt
var tmp;
if( tmp === void 0){
    console.log("true");
}else{
    console.log("false");
}
//->true

voidはあらゆる値に作用し、常にundefinedを返す演算子でそれを利用する
MDN参考

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Help us understand the problem. What is going on with this article?