TypeError: can't access property "x" of "y"

メッセージ

TypeError: Unable to get property {x} of undefined or null reference (Edge)
TypeError: can't access property {x} of {y} (Firefox)
TypeError: {y} is undefined, can't access property {x} of it (Firefox)
TypeError: {y} is null, can't access property {x} of it (Firefox)

Examples:
TypeError: x is undefined, can't access property "prop" of it
TypeError: x is null, can't access property "prop" of it
TypeError: can't access property "prop" of undefined
TypeError: can't access property "prop" of null

エラータイプ

TypeError。

何がうまくいかなかったのか？

undefined か null に対してプロパティアクセスを行いました。

例

無効なケース

// undefined and null cases on which the substring method won't work
var foo = undefined;
foo.substring(1); // TypeError: x is undefined, can't access property "substring" of it

var foo = null;
foo.substring(1); // TypeError: x is null, can't access property "substring" of it

問題解決

undefined か null の null pointer アクセスを修正するには、たとえば typeof 演算子を使用できます。

if (typeof foo !== 'undefined') {
  // Now we know that foo is defined, we are good to go.
}

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