ReferenceError: "x" is not defined

訊息

ReferenceError: "x" is not defined

錯誤類型

ReferenceError.

哪裡錯了？

有個地方參照到不存在的變數了。這個變數需要宣告、或確定在目前腳本、或在 scope 裡可用。

备注： 如果要使用函式庫（例如 jQuery）的話，請確定在你使用諸如 $ 這樣的函式庫變數前，就已載入完畢。把載入函式庫的 <script> 標籤，放在你使用的程式碼之前。

實例

變數未宣告

js
foo.substring(1); // ReferenceError: foo is not defined

"foo" 變數在任何地方都沒被定義到。它需要字串使 String.prototype.substring() 得以運作。

js
var foo = "bar";
foo.substring(1); // "ar"

作用域錯誤

A variable need to be available in the current context of execution. Variables defined inside a function cannot be accessed from anywhere outside the function, because the variable is defined only in the scope of the function

js
function numbers() {
  var num1 = 2,
    num2 = 3;
  return num1 + num2;
}

console.log(num1); // ReferenceError num1 is not defined.

However, a function can access all variables and functions defined inside the scope in which it is defined. In other words, a function defined in the global scope can access all variables defined in the global scope.

js
var num1 = 2,
  num2 = 3;

function numbers() {
  return num1 + num2;
}

console.log(num1); // 2

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